the IMPRO REVERB ... a pocket rocket of an amp

Make it loud here.
User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:14 am

PART ONE

Warning: those who know my style of build write ups know that I write a lot, I like to explain even the mundane because, to be perfectly frank, a lot of the issues I encountered ain’t fucking out there in the text books. Well actually they are there in the books but it is as if you have to already know about what ever it is to understand what is being written about. I have read a lot of the books (chapters in books) but it was only after I solved my problems and went back and re-read the books that I actually understood what the authors were trying to get across.

Amp building and the black magic behind it is quite intimidating to the novice amp builder. Yeah … if the amp works first time then get out the beers and celebrate. But in the majority of builds, even those from kits, there will be teething issues that need to be solved .In some builds (as in this one) issues arise that are problematic, are unknown to the builder, yet every “issue” is most certainly not unique to this amp build and have been worked through at some stage over the past 50 or 60 years.

When you are faced with a design/build problem you must follow a process of problem solving and root-cause analysis that allows you to solve the issue at hand and not introduce additional (unrelated) issues)

[In the car/manufacturing industry this is known as RED-X]

In short you have to be persistent, take things slowly, change one thing at a time and then see how the amp performs. No matter how many books by Gerald Webber you have they won’t really help if there is no logic to what you are doing from a fault/issue finding process. [to be honest I found no help in any of the Gerald Webber books I have - his books seem more from a perspective of wallpapering over a problem so you don’t notice it, rather than actually determining what the problem is and fixing it. But then again what do I know …..]

Another early piece of advice is to make sure you have someone you can bounce ideas off when you discover there is a problem with an amp you have just designed/built. People with a slightly different perspective can often see things you won’t … I talk from experience grasshopper …

For those who only want to see the amp pron, just page down to the photos and ignore the text…..

For those who like to read about build and have a spare 15 - 30 minutes … grab a beer , grab a cuppa tea, grab a coffee … what ever your poison … and follow me down the rabbit warren that is amp designing/building/fault finding.

The Genesis
About 12 months back I was in the middle of my Champ Reverb – on steroids (using an EL34 rather than a 6V6) build and I started to think about what was going to be my next amp project. Given that up until then the three builds I had undertaken were Single Ended amps, the next logical step was jumping head first into a Push-Pull amp.

As I thought about this next project a number of factors started to gel as to the specs of what I wanted the amp project to be. To a certain degree these have been influenced by the amps I already have and the fact that I wanted a design my nephew could build so he could have an amp that he could gig with in the future.

The initial specs were:

- power to be somewhere between 25 – 50 watts
- Single channel
- with reverb and tremolo
- small footprint
- must weigh less than a Deluxe Reverb (BFDR are 16kg, SFDR are 18kg)
- must have a BRIGHT switch and a MIDS switch
- switchable NFB
- must sound shit-hot
- must be a valve only amp

Cabinet and Chassis
I like Fender BF style amps from a visual perspective and which lead to the conclusion that a Princeton Reverb size amp may fit the bill, although it could be a tight squeeze getting everything in that chassis that I wanted.

I source my amp chassis from a “guy in Thailand”, who also sells on ebay. They are first class and fit the cabinets perfectly that I get from MOJO in the States.

Valves & circuitry
Power Amp section
I had decided on using two EL34 (6CA7) power valves. I have a medium size stash of NOS Mullard early 60s EL34s plus some late 60s RCA manufactured 6CA7s.

There are various methods of biasing designs that can be used for the power valves. I quite like the sound of a cathode biased valve amp. Even though cathode biased amps are said to have a slower attack than fixed biased amps, I have liked the sound they achieved in my previous SE amp builds. Cathode biasing also has the advantage in that you can stick in another (new) set of roughly matched power valves and the system will effectively self adjust with no need to change anything. (assuming that the second set of power valves are in the same ballpark as the first set)

I went with a Mercury Magnetics transformer. I had used MM transformers in my previous builds and have had no problems with them In fact I can highly recommend them. I was limited to what ever MM had in a PT that would fit into the transformer “cut out” in the PR chassis. Fortunately MM had PT would fit into the PR chassis which had a HT (B+) voltage of 360V (and was suitable for a 240V wall voltage.). It could deliver enough current to power the EL34s, GZ34s and a host of twin triode preamp valves.
Perfect.

Preamp
I went for the usual Fender BF/SF set up as far as the preamp. A 12AX7 as the pre-amp valve. What I did add was the addition of a bright switch (120pF cap) as seen in larger Fender amps and a MIDS switch.

Fender have used a “scooped mids” approach in the BF and SF amps to get that “Fender Amp Sound”. Most medium size BF and SF amps do not have a mid frequency control, but rely on a 6.8k resistor to “loose” these mid frequencies to ground, thus creating the “Fender Amp Sound”.

Even the larger Fender amps that do have a Mids control only have a 15k variable pot. [as a side note the Vibrochamp uses a 15k mids scoop resistor, which reduces the mids less than in the DR or PR etc. This allows the VC to sound a little more ballsy than if it had the standard 6.8k set up]

If you look at Marshall, Vox, or any number of modern day “heavy sounding” amps, they all use a mids scoop resistor of anywhere from 15 – 45k. With the majority some where between 18 – 30k.

I decided to have a mids switch that when off would be the normal Fender style 6.8k mid scoop resistor. But when switched on it would add in an additional 15k (making a total of 22 - 23k). This would bring up a level of the mids that was both noticeable, tactile but not overwhelming. (or so I hoped !!!).

Reverb Circuit
Having been very successful with the reverb circuit I designed (read … stole sections from other amp designs) for the Champ Reverb, I stayed with the usual 12AT7 for the input into the reverb tank and a full 12AX7 for the reverb recovery.

I picked up some 2 spring full size reverb tanks from Belton (who make Acoutronics Reverb Tanks) while on a business trip to South Korea. Word of warning … do not travel through an airports X-ray scanners with reverb tanks as your carry on luggage without the understanding that you will spend half an hour trying to explain to the idiot airport officials what exactly a reverb tank is.

Tremolo
I was however a little more uncertain about the tremolo circuitry. After doing some graphic manipulations to a photo of a PR chassis I realised I could add an additional 12AX7 size valve as well as an extra control pot/knob to the amp set up.
Image

I decided upon the VibroChamp style tremolo. I quite like the sound of it, it doesn’t have a “click” sound which is apparent in the designs that use a “cricket” and it doesn’t need any diodes or additional circuit board (as seen in many larger Fender amp designs).

The VC tremolo design is pretty simple and uses both triodes in a 12AX7. One of the issues that I was to find out later was that sourcing a potentiometer that is 25k Reverse Audio was near impossible, especially for someone living in Australia. I did have an old 25k RA pot that I had taken out of a 77 VC a couple of years back, but I did want to locate some new pots… so the search was on.

Phase Inverter (PI)
That then left me with a full 12AX7 (or 12AT7) for the PI.

While looking at the various schematics I have of BF and SF Fender amps, it was apparent that in the majority they all used “Long Tailed” PI. The Long Tailed PI requires 12 components and a full 12AT7 and does have a level of gain to the amp Then I had a look at the Princeton Reverb circuit and it uses a Cathodyne style PI. This only needs 10 components and half a 12AT7 (or 12AX7). But on the negative side the Cathodyne style PI has no gain or even a very slight reduction in gain.

After doing some research there seemed to be a lot of negative comments about the cathodyne PI. More than one would expect. But it is used successfully in the PR (an amp I love) and I kept on researching this method of PI.

Eventually I decided to have a read of the book I bought from “The Valve Wizard” (http://www.freewebs.com/valvewizard/" onclick="window.open(this.href);return false;) called “Designing Tube Preamps for Guitar and Bass”. I truly recommend this book and the author’s website. Forget any of the Webber books … if you want to buy a book to help you design/build a guitar amp, this is the one to start with.

Merlin had some good comments about the Cathodyne PI and the calculation/design example he used in his text book utilised EL34s with a HT (B+) voltage roughly where I thought I’d want to be. Also Merlin’s example used a 12AX7 as the PI valve, rather than a 12AT7 as used in the PR. I did notice a difference between the Fender PR cathodyne PI set up and that preferred by Merlin. I went with Merlin’s option.
Image


So it was decided, I’d go with the Cathodyne PI which left half a 12AX7 un used. Easily fixed …. Just have another gain stage before the PI. Thus overall I would get additional gain using the Cathodyne style PI with a gain stage proceeding it, when compared to the the Long Tailed PI. A win-win as far as I was concerned.

During my investigations about PIs I had read that in the PR there is also a gain stage before the Cathodyne PI, but this is used more to get the combined “wet” and “dry” signal back to a half decent level, after the dry signal had been reduced so it wouldn’t swamp the wet (reverb) signal. (this applies to most BF/SF amps with reverb)

In my design I have an extra gain stage for the reverb signal, so the dry signal doesn’t need to be reduced before they are combined. This means that my gain stage before the PI can be a real gain stage (rather than a gain recovery) which will help drive the EL34s. Or so I hoped…. Remember I know jack-shit about designing amps !!!!

Filtering Caps and Rectifier
I had already decided upon a GZ34 rectifier. I have a nice stash of NOS Mullard GZ34s. My initial notion was to use a Cap-can for the filtering capacitors, as happens in the standard PR amp. There is a hole cut in the chassis of the PR for a cap-can. Cap-cans have just become available again and I notionally would use a 30/20/20/20 uF Cap-can. (I had used this on the successful CR03 build.)

So on paper (or at least in my mind) I had the rough design. Next I needed to draw up the schematic and start ordering parts that I knew may take a while.


I don’t know when it was but James (Jimboyogi) posted a thread about the Rhoney Tone Senior he was designing/building. It seems that James had the same sort of idea to build a Push-Pull amp which can be used to gig with, but James is about 6 months ahead of me. This was very fortunate for me … James has been a great source of help and advice through out this entire project. Let me say it right here … James … thanks mate for the help you provided over the past year or so, for a Collingwood supporter you’re still a half decent chap… :D


My first order was for the cabinet from MOJO. It is a standard Princeton Reverb type cabinet, which uses a 10” speaker. I went for red tolex this time. MOJO do great cabinets … never had a problem with any of their product. (although I wish they still had seafoam green tolex available)
Image

The PR chassis arrived soon after: I needed to drill in an additional hole for the extra valve.
Image

Given that the amp was to be anywhere between 35 and 45watts I needed at least a 50 watt speaker and decided upon a vintage Jensen C10N. I have really liked the 1960s Jensen ceramic C8R speakers I used in my Champ Reverb builds and had heard great things about the old 50s, 60s & 70s Jensen ceramic C10N. I managed to locate a early 60s C10N on ebay and purchased it. This is how the speaker arrived.
Image

The idiot ebay seller had decided to place foam beads all through out the box and then pressed down hard when he closed the box up and sealed it.

Over the next few months I successfully located a couple of early 60s Jensen C10Ns that had been in home organs (as apparent by the two pin connector plug on the wiring, typical for organs of that era).
relaxing alternative to doing actual work ...

User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:17 am

PART TWO

I started working on the schematic. Looked up a lot of amp designs on the web and saw that the Matchless Chieftain and the Bad Cat 30 amps were similar to what I was working on in as much as they both ran cathode biased EL34 Push-Pull set ups. Mind you there is so much talk on the net about these two designs (especially the Matchless) burning through power valves I was a little concerned. I did not have the intention of building an amp that would require a new set of EL34s every 3 or 4 months !!!

But when you start to look at the Matchless Chieftain and the Bad Cat 30 designs you see that they are biasing the EL34s at 110 or 120% of their maximum current load. No wonder modern EL34s are dying in these amps. If you design the amp correctly with the correct sized cathode resistors this problem should not occur.

With lots of advice from James I came up with my schematic. Six months on … it looks pretty simple and understandable, but at the time I can tell you it was tough going.

Essentially every amp is the same: Preamp to a gain stage to phase inverter to power amp stage. Plus the effects (like reverb, trem, presence) “hang off” the side. If you think of amp design that way, it is pretty simple to understand.

I also started working on the artwork for the face plate and back plate for the amp. But what to call the amp? Various ideas came to mind “Jade Reverb” naaah already an amp called “Jade” somewhere in the world. Name after name I came up with was soon shown to already exist. Then I thought about “Vamp Reverb” as in vamping to a musical groove…. But that is too much like V-Amp. So had a look at a thesaurus and saw the word “Improvisation” as a synonym for “vamping”. So I went with the name of “Impro Reverb”. [If this names exists elsewhere… don’t tell me !!!]

Yannis (Zhivago) helped me out with the artwork for the face plate.
Thanks so much Yannis.
Image
I got the man in Thailand who does the chassis to also manufacture the face plate.

Then I developed the component layout drawing at placed in a chassis to see what space I had.
Image
Things were tight but not impossible. All is good so far.

But then I realised that what I had not shown on the component layout diagram was the EL34 cathode capacitors. These were 220uF caps – 2”x1” (50mm x 25mm). No matter how I aligned these caps inside the chassis there just was not enough space to fit them in. Fuck fuck fuck.

That evening I went to bed rather annoyed. The next day at work I was doing a bit of WWW surfing and saw a design for an stereo (non guitar) amp that used a cap-can for the power valve cathode capacitors. What a great idea. Surely there would be cap cans with two 220uF caps in them.

Well don’t ask me to write the happy endings of movie love stories. Yes there are plenty of audiofile style capcans around (some even costing $500 each or more) but nowhere could I find anything close to a capcan with with two 220uF caps. Then for what ever reason I looked at a local Queensland electronics/amp suppliers website and they had some LCR brand capcans each with two 200uF caps. 200uF is the same as 220uF… wtf … I ordered them immediately … problem solved .. I’m a fucking hero… this “design on the run” lark is easy… throw a problem my way I can solve it.

Three or four days later the Capcans arrived in the post. I pulled one out and placed it on the chassis with the normal power supply cap-can in place. (the transformers still had not arrived from the states at stage , but I had a rough idea or so I thought of their size). . No matter where I placed the second capcan, there just was not enough space to fit it. I had bought a large 33mm step drill so I could drill a mounting hole anywhere I liked. The problem was that there was nowhere to mount it successfully. It also would not fit inside the chassis and I did not want to mount it horizontally outside the chassis as that would just look shit.

I had grasped defeat from the jaws of victory and had wasted $110. (anyone need some NOS capcans each with two 200uF caps ? I have some for sale !!!)

This project was seeming to be a little more complicated than the Champ Reverb builds were….

It wasn’t till a couple of days later that the solution hit me… a bloody obvious solution. Fit a dog-house. Many of the larger amps have a capacitor pan (aka dog-house) under the chassis that accommodates the power supply filter caps. So I searched and searched the main amp supply websites for a capacitor pan. Do you think anyone sells them ? It is almost like Fender doesn’t want anyone to manufacture/build their own home built amps. Surely not.

About two weeks later after I had contemplated using die cast electronic boxes or even plastic versions of the same glued to the underside of the chassis that I saw that an ebay seller had some capacitor pans for sale. Large ones for the Fender 65' Twin Reverb (7" X 4") small sized ones for the Vibroverb (5.75” x 3.5”).
I immediately bought two small ones.
Image


Yet another problem solved. I felt on top of this project. How naïve I was …

By about this stage the transformers had arrived from Mercury Magnetics. I have been fortunate over the past year or so that the US$ has been weak against the AU$, so things like cabinets, transformers and electronic components in general have been only 60 – 70 % of what I had previously been paying. This is why I bought an extra 2 sets of transformers, one for the build my nephew will do and an “extra set”. Just in case…

When I had ordered the transformers, the sales guy Paul (who is most helpful and patient) let me know what was the best PT they had that would fit the cut out in the PR chassis. It was their “Tone clone + FBFPP-S/E”. [Note: the FBFPP-S/E has been replaced/superseded in the past few months by the FBFPP-S/DP, which has 100, 120, 220, 240V inputs]

Paul also offered two different output transformers. One OT was pretty good, but he said that the 50s Bassman style OT would rock the house. Always wanting to rock the house I went with their “Toneclone + FO50BM-2M” OT. [ I had mentioned to Paul that I had lots of spare space to mount the OT.] This OT had a primary impedance of approximate 3700 ohm when used with the 4 & 8 ohm secondary taps. Again perfect.

But when the trannys arrived I was somewhat shocked at how large the Bassman style FO50BM-2M OT actually was. With previous amps build I had worked on the OT was secured to the chassis by self tappers. This OT was so large it was going to need actual nuts and bolts to secure it.
The OT was larger than the PT !

And then it hit me … the OT was so large that if placed in the usual position, next to the PT, it would cover the hole (and space) for the power supply filter cap-can. I quoted Shakespeare again… fuck fuck fucking fuck.

Before the dog-houses arrived from the States I tried different locations for the OT. None really worked. Plus there is a requirement that the orientation of the OT be 90 degrees off the orientation of the PT.
Image
None of the alternate locations worked. I had to mount the OT next to the PT as per normal processes.

But now what do I do ? I had already determined that the (that a) capcan could not be placed anywhere else. In the end there was only one solution. Fit the power supply filter caps in the dog-house as well. Luckily the cap pans I ordered were just large enough to accommodate both the four power supply filter caps and the two EL34 cathode capacitors.

But I was heading down a path that I had not been and James had not either. A Push-Pull amp with caps located away from the main circuit board in a doghouse on the underside of the chassis.
relaxing alternative to doing actual work ...

User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:26 am

PART THREE

By early September a large portion of the components and parts had arrived, even the face plate. So I did a mock to see how the amp would look like and was quite please with it.
Image

Had decided on the locations of the trannys but needed to cut/shorted one of the speaker mounting screws to allow the OT to fit in.
Image

Then I made up the circuit board… using layout ideas that I had used for the Champ Reverb. Little did I know that some of these ideas were just wrong for a P-P amp, especially one like I was building. But I am getting ahead of myself a little.
Image

The amp coming together.
At this stage with the circuit board populated it is now the time to start to mount everything into the chassis. As mentioned above the OT required quite substantial bolts/nuts”
Image

I had already drilled the holes into the chassis for the extra 12AX7 and the two slider switches for the BRIGHT and MIDS controls. The dog-house circuit board was populated. It was a tight fit. (note the 470k resistor … I’ll come back to this later)
Image

Because I had added an extra control pot to the set (compared to the standard PR) I needed to extend the brass plate used for the circuit earths. I had ordered an extra brass plate (for a Champ) and used a section of it to extend the PR size brass earth plate.
Image

The reverb tranny had been relocated to make way for the Dog-house.
Image
Image

At this point, early September, I was sent on another business trip to South Korea for a week. … and I hadn’t even done anything wrong… why am I being punished ?

Once back home to a land with real food and a society that doesn’t expect even the cleaner to have the word “manager” in his title, I was able to continue the build.
Image
Image

Remember I said I needed to locate a source of 25k Reverse Audio pots. Eventually I found a place in New York which sold them (more for industrial purposes though – I had to sign on line before I bought them a declaration that they would not be re=exported from Australia to “terrorist countries” WTF?) The place that I had been advised to go to by everybody at OSG couldn’t help me as the website kept on reverting to the Hong Kong version rather than staying with the US site. The HK site didn’t have the parts available.

With exorbitant shipping costs each pot ended up being about $15 each. ( bought six. Ouch! The tremolo better sound good. When the pots arrived, the shaft was much longer than the standard pots used on Fender amps, so I had to cut them back.
Image

The build seemed to go smoothly
Image

The cap for the BRIGHT switch was added
Image

I had used some turrets which allowed me to change the dropping resistor value into the dwell pot for the Reverb circuit. Here the resistor is just temporarily located.
Image

My set up for the MIDS boost switch
Image

By the middle of October as they might have said in “The Big Lebowski”, it was all pulling together … got out some NOS 60s Mullard EL34s reading to fit to the chassis
Image
Image
Image

It was starting to get a little packed inside that chassis !!

So at some stage in late October I plugged everything in, valves, an 8 ohm speaker cab I had lying around (I didn’t want to risk the vintage Jensen C10N), guitar and finally the power chord. Turned it on and waited for everything to warm up.

This is was the point in the space-time continuum where my life, my amp building life, went from “dreamtime” to “nightmare”.

Almost immediately the amp had warmed up uncontrollable high pitched feedback occurred. Deafening feedback. I had forgotten how loud 30 - 40 watts really is. I slapped the on/off switch off, but infact missed and hit the NFB switch to off. The screeching feedback stopped. Ahhhh I know what is going on…. I need to swap around the two leads from the OT to pin 3 of each of the EL34s. This is a well known occurrence.

Went to swap over the leads and touched the top of the on/off switch and shorted out the two leads to the power switch …. Mmmm 240V shock up my left arm… I am getting electrocuted …. thank goodness for electrical safety switches on the house power board.

The entire house electricity circuits were shut down…and I am alive…. Yes that safety switch has paid for itself !!! [Mind you it paid for itself at least another 8 times over the next 2 months !]

Swapped the leads from the OT, turned on the amp and the piecing feedback was gone. But what I then could here was an equally deafening hailstorm of noise, of 50Hz hum, of 100Hz hum, of crackling, of feedback.. you name it I could hear it. This didn’t bode well for a easy/clean/successful build.

At this point I should have used my scientific training, my fault finding experience… I should have remembered Ockham's razor : which says "simpler explanations are, other things being equal, generally better than more complex ones."

If I was to step back and listen to this hail storm of noise, even worse when a guitar was played or the tremolo and/or reverb controls engaged, I would have realised it was essentially earth/grounding noise. But because I was thinking along the lines of “what is different from this build to the other amp build I have completed?”, I started down paths that, although were useful, they were not the main issue at hand.

Here starts the investigation into what was fucked with my amp build/design……

Now I have to try and explain how loud this noise was… with a strat plugged in and an E chord played on the bridge pup, the initial chord could be heard but then almost immediately it was swamped by hum, by noise , by oscillations. The entire amp became microphonic … seriously…. I would barely touch a component on the circuit board with a wooden chop stick and it was sound like a snare drum through the speaker. The preamp valves themselves sounded like they were about to explode each time they were tapped. Just scraping one’s fingers (lightly) along the edge of the chassis sounded like gigantic sandpaper being used.

None of this made any sense.

When I turned on the tremolo, all weird shit would happen, the amp would loose power then get scratchy then no sound at all then as if 25 mother-in-laws were chasing you… I am not an amp expert, but I can track down repeatable problems. This issue seemed almost random . Randomly bad.

I also had issues with the reverb circuit which was a surprise because I thought my reverb circuit design had worked well in previous builds.

The reverb had this hum … 100Hz, but coupled with the hum problem the reverb wasn’t working properly either … it was doing stuff that sounded like a scratchy LP feed through a dull vintage radio.

I’d get scans on the scope like this with not even a guitar plugged in.
Image
Where in earth is the saw tooth wave signal coming from? Shhheeeeeezzzzz
Now I remember why I hated electronics at university …

I contacted James and he came back with a couple of good suggestions….

Firstly I looked at the heater wiring and saw that I had done it incorrectly. I had earthed one side of the heater circuit rather than utilise the centre tapping from the PT to isolate the heater circuit. I reconnected the centre tapping and hoped for the best
Image

After turning on the amp again, it was at this point I realised I had a massive issue to deal with. Because … yes the change of the heater circuit did remove the 50Hz hum, and I could see that on the scope, but there was still effectively the same level of noise/hum and issues with the amp. Something was really wrong and I was drowning.

By this stage I was discussing the build, the noise issue, with my manager at work each day. He is an electronics engineer and builds his own amps … mainly transistor based amps, but does have far more knowledge of valve amp design than I do.

So this would be the process… we would discuss something at work, I’d go home that evening and make the change, maybe try something else as well and then the next day or so we’d evaluate the results. Because I wasn’t “bread-boarding” the design, but working on essentially a completed amp, changing something, something even as simple on paper as the routing of wires from a pot to a pre-amp valve, took a long time to modify/change evening. I had to ensure that changing something didn’t actually introduce additional issues not related to the existing issues. Sounds easy … I can tell you it ain’t….

One of the things I first did was assume incorrectly that the tremolo circuit was causing all the problems. So I pulled out components, replaced them with new components… each time a component was replaced turn on the scope, plug in a guitar , on with the amp.. . listen .. take measurements, tap the circuit board, try different valves… on and on each evening and the weekends. [for the past 2 – 3 months I basically haven’t done anything else but attempt to fault find this amp design]

As I was checking the circuit, each time to make sure I was replacing the correct component into the correct location, I noticed that my layout drawing which I had created some months back was wrong. I had the output of the tremolo circuit feeding back into itself (pin two of the tremolo 12AX7) rather than into the next gain stage before the PI. Boy was I excited, I had solved the problem… No I hadn’t … it was just one of many false dawns.

I moved the output of the tremolo circuit to the correct locations and low and behold the tremolo started to work correctly. In fact I was really impressed with the tremolo, but the noise and microphonics were still there.

So I had replaced the entire tremolo circuitry, moved the power supply for it to various locations and finally wired it up correctly, producing a tremolo sound exactly like that in the VibroChamp, but the noise was still there.. For me that tells me at first blush that the tremolo circuit was not the issue.

One of the first comments people make when an amp is microphonic is whether the issue is a microphonic valve or three … so even though I knew they were OK, I did decide to change over some of the preamp valves. Tried NOS 1960s RCA, some amperex, modern groove tube valves … all produced the same barrage of noise. Cross that possible solution off the list.

But one evening after getting home from work and just going into the amp room, I turned on the amp and suddenly there was only one hum ,,, maybe two hums of different frequencies … it was bloody loud, but the cacophony of noises and hums and microphonics was gone…. but to my annoyance, when I bothered to look, the reverb valves and the tremolo valves were not in the amp.

I had not put them back the previous evening. This did tell me something .. some of the noise was coming from the FX circuits and some from the standard amp circuit. I would have to solve the amp issue(s) first and then and only then start on the noise/hums associated with the tremolo and the reverb circuit.

Not to be shy, I ripped out (unsoldered) all of the components, valves & pots for the reverb and tremolo. Noise still there. I did however discover that the amp sounded awesome.

I had used as my first guess 270 ohm cathode resistors for the EL34. (Matchless uses 220 Ohm - later I would change these to 390R) I can tell you that those 270 R resistors were damn hot to the touch… burning hot after about 5 – 10 seconds of touching. But the amp sounded like I wanted it … thick, strong, articulate .. rock and roll … I tested out the BRIGHT and MIDS switches and to my joy they were better than I expected.

The MIDS switch when “on” can make a Strat (with SC pups) sound like it has hambackers and it is a Marshall or Vox. Even the NFB switch worked wonders. I had started with the NFB resistor of 2.7k and initially I thought it wasn’t large enough (i.e. too much NFB is getting through) but when the amp is turned up loud there is plenty of growl. And if you want over the top… just turn the NFB off.

I started to download photos of the inside of larger amps from the web, spend hours looking at Fender, Marshall, Vox, Matchless, ceriatone etc schematics and layout drawings. Endless discussions at lunchtime with with my manager as to the next step to take, the next component to replace. I had even replaced the reverb transformer early on thinking that was the cause of all the problems… but still I had forgotten Ockham's razor : "simpler explanations are, other things being equal, generally better than more complex ones."

I moved HT power supply leads from the preamp section closer to the middle of the circuit board (multiple points were connected under the board which required the same HT voltage). No change. Replaced normal leads with shielded leads. Nothing worked, nothing solved the noise/hum problems.

Remember at this point I only had the Preamp/Gain stage/PI/main amp stage wired up. Yet there was 100Hz hum everywhere.

It was obvious (although it took a while for us to come to that conclusion haha) it was some sort of earth/grounding issue associated with the power supply filtering circuit. You see 100Hz then that is what you should first look to. Yet the filtering circuits on paper were no different to any number of other classic and newer amp designs. My manager suggested adding in another 20 or 30uF of filtering, but I was hesitant because (a) what was there already (30/20/20/20) on paper should work and (b) I had no space to put any extra caps/resistors.

I then started to twist various leads together… which meant that they had to be unsoldered, pulled out from behind “half the amp circuitry”, twisted and then placed back and re-soldered. Not a quick job. Especially when some of the leads went into and back from the dog-house. My manager suggested twisting the lead from the first filter cap coming out of the doghouse with the two leads going into the doghouse for the EL34 cathode caps. Just making that change alone took an evening.

But when I turned the amp on …. The level of hum had dropped… the level was still annoying, but there was a definite drop/change. Wow the first “win” in four weeks.
relaxing alternative to doing actual work ...

User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:31 am

PART FOUR

I spoke with James and he gave me further ideas to try (all valid) but after another week of trying this, or that. I bit the bullet again and decided to removed the entire circuit board, rebuild another (incorporating some design changes , mainly around placement of the EL34 cathode resistors – giving them more “air” to cool down in) and hope that the issues/hum was associated with bad soldering of mine somewhere. At this stage I was clutching at straws and half jokingly mentioned to James that I felt like throwing the amp against the wall and walking away.

The old with the new design(on paper)
Image

The old ripped out
Image

The new design/built circuit board in place
Image

Then with great hopes I turned on the amp… waited … listened … all I could hear was crackling, not good crackling … then I smelt burning and just as I am reaching to pull the 240V power lead from the power board my entire work room fills up with burnt resistor smell/smoke. Fuck fuck fuck fuck fuck fuck fuck fuck

Once I had turned off the smoke alarm and had a good look at the amp, I could not see what component had decided to self immolate. Upon opening up the dog house the rush of toxic smells told me I was getting warmer, so to speak. This is what I found:
Image

I am still amazed how much smoke can come from such a small component. It appears that when I had just rebuilt the amp, I had decided to replace some of the resistors (just in case … no logic at all to that decision) and had grabbed a 470k resistor from the draw containing the 470k resistors. Only problem was that a stray 470R had found its way into the 470K draw….. effectively a 400V short directly to earth, No wonder the little blighter committed suicide.

You see this is what happens when problems/issues arise that are way beyond the solving power of the amp builder. When trying to solve the problem at hand other problems occur. I haven’t even gone into the dry joint on one of the 12AX7 which resulted in 170V across the 25uF cap (only rated at 50V) which took 2 days to track down. Or a 1.5K resistor that suddenly became 8.8k or even a 250k pot that decided to die. And because there had been so many changes to layout and components, the amp was no longer looking like a new pristine build, but rather like a prototype. I decided (rightly or wrongly) that I needed to replace some of the preamp valve seats just to ensure that .. well to make sure solder joints were clean and not causing the problems or at the very least contributing to them. You name the problem, it’s raised its ugly head at least once during this project.

Replaced the dead resistor with a correctly sized 2 watt 470k, turned on the amp and the hum/buzz was still there. I had had enough. Turned off the amp, closed the door and went to bed (it was only 1.30AM) an early one this evening.

Didn’t discuss it all with anyone for a couple of days, and then I was looking through some schematics that I have at my desk at work …. surely everyone has Fender amp schematics at the place of employment …. and I noticed something. On the layout drawings for the larger Fender amps which have dog-houses, there was an extra outlet from the Doghouse away from the PT/rectifier end, close to the preamp end. This extra outlet appeared to have the HT lead from the final filter cap associated with the preamp. More importantly the earth for this filter cap was also shown there. This meant that the final filter cap’s earth was not the same (location or wiring) as the first 3 filter caps. I knew this was important, I ran it past my manager, he agreed and I felt there was a glimmer of hope. Here is an example of a Fender layout drawing showing the Dog-house entry point and in blue the take off point for the preamp filter cap HT and earth.
Image


The mod to the preamp (final) filter cap HT and earth position took all evening. I almost didn’t want to turn on the amp as again it was into the small o\hours of the morning. But I decided to all the same. Removed the 12AX7s and 12AT7 valves that were associated with the tremolo and reverb circuits (remember that with the rebuild of the circuit board all the components for the tremolo and reverb circuits were back in place.) Turned the amp on and .. and and nothing …. basically all the hum was gone … or at least the hum had been reduced to a level that I could live with. I have to admit there was the occasional raised fist into the air and maybe even the word “alright!” was shouted out.

This was a big win…. persistence with a large dollop of dumb luck had lead me to this point. The next day my manager was also excited with the outcome. We also discussed the fact that once you know about this it is obvious, but the text books don’t seem to say things like: the preamp filter cap needs to be earthed separately from the other filter caps otherwise the amp will be useless as there will be a blizzard of 100Hz hum and you might as well use the amp as a door stop …. ….

Yeah a few of the texts mention that the preamp earth should be or can be located separately from the rest of the filter caps, but it is stated in such a nonchalant manner that one can read it without really understanding the implications. Haven’t these authors ever heard of exclamation marks ?

The following evening, with all the valves back in the amp, it was turned on. Yes the 100Hz hum associated with the preamp HT filter cap was gone but there was still a glad bag of noise, instability, hum, crackling, microphonics.

I had managed to track down the heater hum, removed the hum associated with the leads from the 2nd and 3rd filter caps into the dog-house and the preamp HT filter cap hum, but my journey to become an Amp-Jedi was just starting.

The two things that kept me going were (a) the amp with out reverb and tremolo sounded awesome and (b) this ain’t rocket science… people have been solving these issues for the past 50 or 60 years, so I should be able to.

James had mentioned in passing a number of times the issues he had with noise in his Rhoney Tone Senior amp design and how he needed to separate earths and use the concept of “Star Grounding”.. “Star Grounding” being the process of bringing back all earths to a single point (or points) on the chassis, rather than having multiple grounding points for common areas of the circuit. That is … all the preamp earths are grounded at a star point, all the power amp section are grounded at a star point and so on.

I had also been looking at some of the photos on the Ceriatone website. In their build photos they have used a long brass (?) rod as a common ground point and then the rod itself is grounded back to a start point. So it is effectively a combination of an earth rail and a star ground.

I went away down the coast for 3 days for a mates 50th birthday and only thought about food and booze but on return decided to rip out the existing earth leads which connected the circuit board and the pots to the standard Fender style bent brass plate that sits behind the pots. (I had to leave the brass plate there as it would have been virtually impossible to remove it)

I found a local supplier of brass rod and bought some (the place I got it from called it brass wire, as to them 3mm rod is wire) and a day or so later had a new grounding system for the amp
Image
Image

I must say that there was a definite reduction in hum and noise (again remember that the reverb and tremolo valves are not fitted). I am so impressed with this that in any future builds I will not use the standard Fender style bent brass plate, but will stick with this “earth rail” and star grounding concept .

Maybe only a small win in the fight against noise and hum, but still a win.

A couple of days later I was reading one of the latter chapters in the Valve Wizard’s book “Designing Tube Preamps for Guitar and Bass”, specifically on the lead dress of heater wires. He was concluding that it is good practice to not only twist tightly the heater leads from valve to valve, but also the small sections of leads that drop down to the valves. Now while my heater lead dress is not a 10/10, they’re not bad either… but given that I have looked at everything else under the sun in this quest to solve noise and hum issues, I decided to redo the heater leads.

At about that time I had bought off ebay some silver plated Teflon coated solid core 20AWG wire from a guy in the Czech Republic (more than likely ex-Eastern block military wire…. I have as the NFB switch a standard 1960s Russian military toggle switch … also from ebay). This Teflon cover wire is brilliant to use in amps and the heater lead dress turned out better than I expected.
Image

Did the re-wiring and better wire dress of the heater wires do anything ? Yeah sure did. I could not have heard the change if I had done the heater lead dress this way 6 or 8 weeks back, because of all the other noise and hum that was there. But once the noise from the preamp HT filter cap, the other filter caps and the multiple grounds were reduced, there has been a noticeable improvement. Just to confirm the improvement was real, I could see a huge drop in the 50Hz when looking with the scope.

So where was I ? … at this stage I had successfully reduced the noise and hum from the amp … the amp section alone, the reverb and tremolo sections had their valves removed and the links to the HT desoldered. The amp was quiet, noise and hum levels well below what would be the maximum acceptable. I was pretty happy.

Something I haven’t really spoken about up till now is the Cathodyne Phase Inverter. This is just brilliant, I used the same set up as Merlin had suggested in his book and it just works like it should. There is no harshness, there are no noise issues, no distortion (emanating from the PI), I would recommend this form of PI for any medium small to medium size Push-Pull amp build.

The next thing to do was stick the effects sections of the circuit back in. I started with the tremolo… remember the tremolo was one of the sections of the design I was concerned about. Reconnected everything, turned on the amp, adjusted the speed and intensity and woooowoooowoooo … what a great tremolo.

No new noises, no new hums to worry about. Just a great sounding tremolo. The only thing which I could comment on is that when no music is being played one can hear the falling and rising of the background noise of the guitar/amp combination as the tremolo system operates … but that is inherent in the design of the tremolo circuitry anyway.

Seemed like I was on a roll ….. but something was waiting for me with a big stick …

I connected back the reverb HT and fitted the 12AT7 and 12AX7 valves. Turned on the amp and before I had a chance to even play anything, noise, microphonics, hum, instability, feedback … you name it … it was there. It obviously wasn’t as bad as when I first turned on the amp some 6 or 8 weeks back, but it was bad. But there was more … when I adjusted the reverb pot I could tune in a local AM radio station… in fact one evening I sat there and listened to the 8pm news bulletin, coming through the amp circuitry. I knew the radio station, its frequency being 774kHz.

What was also worrying was that I was seeing on the scope high levels of frequencies between 25kHz – 40kHz which could only mean ultrasonic oscillations.

How can I be so close and then fall over with the finish line in view ?

Removed the 12AT7 valve from the reverb circuit …. The issues were still there
Removed the 12AX7 valve from the reverb circuit …. The noise and feedback and everything vanished.
Removed the 12AT7 and the 12AX7 valve from the reverb circuit …. It was the same as when only the 12AX7 was removed.
Put back the 12AX7 and 12AT7 valves and shorted out the point where the wet and dry signal reach the gain stage prior to the PI. The amp was so quiet I had to move my head closer to the cabinet to “hear” the sound of the amp being “on”.

From the above I have basically left out a weeks worth of replacing components, re-flowing existing solder points, moving wires, adding and then removing shielded wiring and much more…. And each time getting the scope and checking each point in the reverb circuit “seeing” what was there, was there hum, had hum gone or had hum suddenly appeared…. Trying to get a feel of what was going on and by how much things changed.

The fact that when I short out the input to the gain stage prior to the PI the amp was quiet showed that the noise wasn’t coming from the preamp section and also showed that the power amp section of the amp was quiet. The tremolo section was cool…. What was the villain now was the reverb circuit and more than likely the reverb recovery 12AX7.
relaxing alternative to doing actual work ...

User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:36 am

PART FIVE

Tackling the hum/noise/microphonics in the Reverb circuitry
As I have already started, the reverb circuit I use employs the standard 12AT7 circuit that Fender always has used (with the addition of a dwell pot up stream of the 12AT7). The reverb recovery circuit uses both halves of a 12AX7, rather than the single half used by Fender BF/SF designs. However if you look closely at the Fender designs, there is the second half of a 12AX7 used as a gain stage for the combined wet/dry signal. So my design is very similar to Fender (hey .. I stole bits from here, from there, all from Fender anyway)

The reverb recovery 12AX7 only uses one cathode cap/resistor for both of the 12AX7 cathodes and has a “reverb” pot located in the circuit between each half of the 12AX7. This circuit has worked wonderfully in my previous builds,namely my CR2 and CR3 builds. James has heard how great it sounds and others have built the CR2 design also have commented on how much better the reverb sounds.

But why wasn’t it working now. … now in this amp I was working on ?

The problem also was that there was no consistency with the noises/hums I was hearing (seeing on the scope). Sometimes I’d change something, solve a problem thinking that was it, then 24 hours later the same problem is back or something worse was there.

It got to the stage that I wasn’t trusting the reverb circuit design that I knew had worked in other amps I had built. So with no answers I started to pull stuff out and add stuff in. James (along with my manager) was concerned about the fact that the 12AX7 had a single cathode resistor/capacitor, Maybe it was feeding back and hum was being amplified, ultrasonic oscillation did appear to be occurring, especially if the reverb pot was rotated away from zero.

I placed the scope at various points in the reverb recovery circuitry and did see high levels of noise and hum around pin 2…100Hz, 350Hz, 25kHz, 31kHz … you name it, I could see it. So I added a second cathode cap and resistor … using my idea of a bread board
Image

No change … of course … the solution could not be that simple.

Was it the leads from the dwell and reverb pots picking up noise/hum from other leads ? So I then extended the leads and moved them around to see of any change … any change good or bad … happened. Nothing consistent, nothing repeatable.
Image

Bit the bullet yet again and ripped out all of the reverb circuitry. Without it, the amp was perfect. No noise , nothing, just sweet EL34 distortion at ear and nose bleeding audio levels.
Image

Then started thinking about redesigning the reverb circuit. (in hindsight a stupid idea, but when you are drowning being a witch doesn’t help …. )

To remove any possibility of weird things occurring because of the reverb pot being in between the two halves of the 12AX7 I moved the pot to a position after the 12AX7. This means the entire reverb signal would be amplified by both halves then the reverb pot would be used to modulate the volume. On paper it may work. On the bread board ? … well who knows ?
Image
Image

Turned the amp on, nothing blew up … lots of noise still there, but I wanted to hear what was different, or what had stayed the same. Plugged in a guitar , started to play something and …. What ? what the fuck ? The guitar notes would suddenly disappear, or they sounded like farting, they sounded scratchy, the volume would suddenly all disappear even with the amp turned to 11. It was the worst sound I had heard … gone was the great overdriven EL34s, this sounded like the cheapest IC amp you could build for $10. It was crap.

What had I done ? what hadn’t I done.

Plus that wasn’t all (steak knives came later) … the amp would motor boat uncontrollably when there was no guitar input . Yeah … sounded like a bloody two stroke motor boat. Could not believe it. I had just added two more major issues.

It took me a couple of days to work out what was causing the farting/raspberry/loss of power. Blocking distortion at its finest.

In my redesign I had left out the 470k resistor between the two halves of the 12AX7. There was no grid stopper and sure enough all of the issues/problems associated with blocking distortion I experienced. In one way I was happy that I had made the mistake because I got to experience the entire amp sound shit and loose power, just because of a missing grid stopper on an FX section of the amp circuitry…. Just like the technical books predicted.

Five minutes later after soldering in a 470k resistor and all the blocking distortion and its symptoms were gone. It was like a high school physics experiment, there one moment gone the next. Grid stoppers are essential. Full stop.

Started trying different reverb recovery designs, different components, different size components, different layouts, shielded wire .. and so on…
Image
Image

Even moved the reverb and dwell pots away from the amp to see if the leads that crossed the amp were picking up something, anything….
Image

But the noise and hum (and microphonics) associated with the reverb 12AX7 was still there, plus I had the added pleasure of motor-boating.

It was at this point I started to do lots of searches on the net about motor-boating. In the majority the comments/solutions were either (a) earth (b) lead dress. Lots of people/websites said “oh I just moved lead XYZ 1/8 an inch to the left and all my problems went away”. Not wishing to seem too disingenuous, but fuckoff, give me a real answer not this magical land of amp fairies.

This is also where I stared to loose whatever respect I may have had for Gerald Webber’s books. You look up motor boating and he has solutions like placing a 10pF cap from pin 2 of the 12AX7 to earth. When I did this the amp almost blew up in about half a second … luckily I had my hand already on the power cord ready to yank it out. I want a solution or a reason why something occurs , not black magic to wall paper over a design fault.

While all this was going on over a couple of days, it became apparent to me that the current reverb recovery circuit (full 12AX7 then the reverb pot) was just not working … besides the motor-boating issue. The reverb sound was harsh, really over driven and I could only turn the reverb put up to about 2 before the whole amp went into feedback oscillation.

I would have to go back to my original reverb recovery circuit design. For better or for worse. By this stage I had decided to keep the two separate cathode resistors, as there was no logical reason not to have them. James suggested that the cathode for the first half not have a Ck, just to lower the gain a little. So I went with that idea.
relaxing alternative to doing actual work ...

User avatar
sookwinder
Mods
Mods
Posts: 10862
Joined: Mon Sep 11, 2006 6:47 pm
Location: Melbourne Australia

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by sookwinder » Sun Jan 08, 2012 4:46 am

PART SIX

Back with the original design (well almost) and the motor-boating is still there. I was really considering just walking away from the whole project…. Really considering it, but given that I wanted to do this amp and get the design correct so I could get my nephew to build one for his xmas gift , I stuck at it.

My desperate emails prompted James to call and we talked for a good hour about what direction I should go and the solutions he came up with for his “Rhoney Tone Senior” build.

James suggested that I needed to not just have one star earth, but maybe various star earths for various sections of the amp, all corresponding to the various HT filter caps.

So with James’ help the premap section and the reverb recovery section, all coming off the last HT filter cap needed to be earthed together, separate from the other earths.

Got out the Dremel and cut the earth rail !
Image
Half an hour later, some wires soldered here and there and wow !! … the motor-boating is totally gone, plus the overall level of the noise/hum is down… down a lot.

There was still a level of hum, when the reverb pot was turned up and there was also a level of scratchiness, but it was way way better than at any point before.

What I had done was route the earths for the preamp section (from the little section of earth rail at the front of the amp) around to the back of the amp where the earth for the final HT filter cap was.

This is how the amp stayed for a few days… not quite right, but way better than it had ever been. I was happy .. the amp wasn’t fully correct , but it was WAY better than it had ever been. Work was busy and for a couple of days I didn’t even think about the amp and then solutions start to pop up.

Maybe the long earth (joining) lead from the preamp earth to the final HT filter cap earth was picking up “crap and noise” ?

I removed the joining earth lead and just earthed the preamp section close by on the chassis floor near the guitar inputs and suddenly all the scratchiness vanished. Wow we are getting somewhere now….

Then removed the earth connection for the reverb pot off the main earth rail and routed it to the pre-amp section as discussed above. More noise/hum vanishes.

Likewise moved the earth connection for the Bass/Mids control to the peamp section earth. This didn’t do anything good or bad.
Image


At this point I had an amp that was working, yes a little noisy (or so I thought) but I was almost happy and for what ever reason the microphonics had all but disappeared. I did feel that the reverb sound was a little thin and a day or so later changed the second 0.003uF cap back to the original 0.1uF value. On paper it should not have really done anything, but it did, the reverb now was much warmer and silkier.

I have thought about this hum and noise issue emanating from the reverb recovery stage quite a bit now that the amp is completed. I suspect that to a certain degree I was just experiencing what it is… a valve amp with noise and a reasonable amount of gain.

Previous amps I have built have been 6 – 14 watts and they have had their own noise floor. This amp with no FX wired in is way way quiet…(obviously once the earth issues were solved) When the FX are added (reverb, tremolo) they introduce their own additional noise and this is then again amplified by the gain stages and finally the 2 x EL34s. Yes there will be noise, but the actual signal to noise ratio is way better than my HRD or SFDR.. amps of comparable wattage output. Once I realised this I was even happier.

Now the amp was basically what I wanted … a pocket rocket somewhere between 35 – 45 watts, weighing exactly 15kg (33lbs), awesome sounding, with reverb and tremolo. Just a thing a 14 year old would love (and his unsuspecting mother won’t … but we’ll cross that bridge later !!)

The date was around the 19th December, I had come up with a design, a layout and a build that worked, with 6 days to go before my deadline of xmas day. My nephew would get the chance to build his own amp over the summer/xmas vacation.

I then needed to put the amp together… but before I could the guitarist I record with couldn’t wait so we used it as it was…. I still had the early 70s RCA 6CA7 valve and not the early 60s Mullard EL34s. The preamp values were 60s RCA and not the black plate 50s RCA 12AX7s I has put away for this amp. It still sounded like the devil escaping from hell when turned up - fantastic, but with a nice chime and shimmer when turned down to a lower level.
Image

Stuck in the reverb tank.
Image

The valve chart
Image

The back face plate was ordered late than everything else and my man in Thailand had it ready by just after xmas. While playing the amp with the back panels off I was quite taken by how much heat the EL34s give off and decided to add an extra tie down point for the power lead to ensure it didn’t come in contact with either the GZ34 or the EL34s.

Image

The back faceplate arrived, the quality as good as ever (and thanks again to Yannis – Zhivago)

The IMPRO REVERB
Image
Image
Image
Image
Image
Image
Image
Image
Image
Image

Final thoughts … in no particular order.
It’s now the second week of January 2012, about twelve months since I first had an inkling that I was going to attempt a project like this. In one way I am sad it’s finished (well more on that in a minute) but I am also ecstatic that the project in the end exceeded any preconceived ideas of what the amp would be. Everything worked that I tried, some things (like the MIDS switch) way better than predicted.

The amp is way loud, has a great clean tone and can also get dirty, but not too dirty. Every guitar I have sounds great through it and it seems to take the pedals I have in its stride.

Spent way more than I should have on it, partially because of stuff ups here and there, but in the majority because I did not know what I was doing in the first half of the project. While the idea of using a cap can for the HT filter caps was good it was also naïve. Besides not having enough space to fit the cap-can in the end, there is a reason they’re not used in larger more powerful guitar amps. That reason is Earth loops and hums. The size of the output transformer that I chose was not what I expected but luckily everything went OK.

EL34s are an awesome power valve and even though I am a Beatle freak I don’t enjoy playing “British sound” amps … Just too dull more my liking… maybe it’s the Blues Physicist in me coming out … who knows. But the combination of the Fender style tone stack with the British style EL34 is a perfect match.

Apart from a couple of books and a couple of websites I am totally under whelmed by the quality of information out there for the novice amp builder. I have said it before, but everything appears to be written in these books and online sites with the premise that one already knows a shit load about electronics before you even start a project. Which in my situation was not the case...

Lastly ... I have learnt so much over the last 4 or 5 months that I am actually starting to understand this electronic sruff - well sort of ...
=============

Last week we used the amp in a real recording session, rather than just fucking around, and it was mouth wateringly beautiful. Suddenly sounds not too dissimilar to Peter Green’s or Snowy White’a late 70s / early 80s albums came alive. Just that alone is enough to justify the time and money spent.

There were some dark times in this project, last October / November were bad, This is where you need to have someone to throw ideas at, rather than throw the amp against the wall. My manger at work and James helped me there.

So what’s now happening ? I am going to build another one !! This time using the teflon covered wire I recently got and incorporating some slight design/layout changes including a larger DOGHOUSE. Already finished the main circuit board and will build in parallel with my nephew as he build his version of the amp.

When I finish this second version I will then fit the early 1960s Mullard EL34s and the RCA long black plate 1950s 12AX7s.

The current early 70s RCA 6CA7s and 60s RCA grey plate 12AX7s sound superb so they can stay there for the moment.

On the next version I will also do a proper determination of the output power, using my scope and tone generator, as well as update the schematic drawing below with DC voltages.

These drawings are updated June 2012 versions
IR01 layout
IR01 schematic
IR01 main fibreboard
IR01 DOGHOUSE fibreboard
IR01 DOGHOUSE layout


Over the next week or so I will get some recording samples with different settings and post them in here.

As they say … it’s been a hoot ….

================

Here are some audio samples - just fucking around, nothing serious.
All played with a 1996 AmStd strat, single coil pickups, plugged directly into the amp.
Treble and Bass settings as per "normal" i.e. nothing weird or strange.
Small level of Reverb dialed in.

Clean quiet, volume at about 4
Louder, volume at about 7
Louder with MIDS switch on, volume at about 7
Louder with Negative Feedback switch off, volume at about 7
relaxing alternative to doing actual work ...

User avatar
vastrange
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 1087
Joined: Sun Jan 02, 2011 1:39 am
Location: Western Suburbs Athens, Greece

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by vastrange » Sun Jan 08, 2012 5:50 am

whoa!!!! what an awesome thread.
thank you thank you thank you!!!
you have my total respect. :)
OSG rules!
Common sense is the collection of prejudices acquired by age eighteen.

User avatar
zhivago
Mods
Mods
Posts: 20981
Joined: Tue Sep 12, 2006 6:18 am
Location: London, UK

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by zhivago » Sun Jan 08, 2012 6:40 am

looks fantastic, David! :)
Resident Spartan.

User avatar
Jonesie
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 3037
Joined: Sat Aug 09, 2008 7:00 am
Location: Massachusetts

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by Jonesie » Sun Jan 08, 2012 7:21 am

Seriously, what an incredible thread!

So is this kinda like an EL34 version of the early Mesa Boogies? I know those were initially beefed up princetons with huge transformers. I'm just curious if there's any similarity.
"Cosmo! Call me a cab!"
"Alright, you're a cab!"

secndshft

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by secndshft » Sun Jan 08, 2012 7:26 am

WOW WOW WOW!!!!

I skimmed over this (honestly I don't understand any of it) but I love your in-depth approach to the post/build. I plan on reading over it a few more times. Your posts make me want to learn about amp building/repairs/etc.

I can't wait to hear this thing.

User avatar
StevenO
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 17747
Joined: Wed Oct 18, 2006 7:06 pm

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by StevenO » Sun Jan 08, 2012 7:31 am

What an incredible read! (Though I understood only 10% of it) So daunting, yet I'm glad everything worked out. I can't wait for clips...

Beautiful amp, sir!

User avatar
ryland
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 353
Joined: Fri Apr 15, 2011 7:44 pm
Location: Arizona

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by ryland » Sun Jan 08, 2012 7:34 am

WOW, Thank you so much for this write up! I have a 60's Silvertone Twin Twelve combo that I have been slowly working the hum out of (slowly being over the last 10 years). While I have it down to a minimally acceptable level after recapping, replacing far out of spec resistors, and replacing all of the non-grounding input jacks, I think your points and methods of routing ground may resolve more of the issues.

Fantastic stuff!

User avatar
stevejamsecono
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 3885
Joined: Tue Oct 17, 2006 10:55 am
Location: Brooklyn, NYC
Contact:

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by stevejamsecono » Sun Jan 08, 2012 7:43 am

Gorgeous, and a very interesting read to boot. I'll take one :)
And you find out life isn't like that
It's so hard to understand
Why the world is your oyster but your future's a clam

User avatar
Hoops
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 955
Joined: Mon Feb 07, 2011 11:23 am
Location: Cambridge, UK

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by Hoops » Sun Jan 08, 2012 7:57 am

That was a fantastic read, thanks so much for taking the time to document all of this :)

User avatar
Telliot
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 10529
Joined: Thu Mar 17, 2011 6:38 pm
Location: Orange County, CA

Re: the IMPRO REVERB ... a pocket rocket of an amp

Post by Telliot » Sun Jan 08, 2012 9:51 am

What an amazing project and thread. I didn't understand the vast majority of it, but I lived it. I was starting to build up the courage to try something like this, but reading this has been a nice dose of reality. I think I'll leave it to the pros. :blush:
The cool thing about fretless is you can hit a note...and then renegotiate.

Post Reply