Cap job on Vibrosonic - pictorial. Update jun5.

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Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Thu Jun 03, 2010 9:03 am

I've been meaning to do this for a while, but shit always came up. Stratelejazzuar's bassman thread was as good as any a reason to finally start. We may already have pictorials on these, but it won't be the first time a thread is repeated on OSG ;).

What we have... (well, we... I) are two Fender Vibrosonic Reverbs from the mid-70s. 100 W master volume model. One I've owned since 1989/1990 or so, the other since last year. The one I've had for aeons was modded before I got it; it has no reverb but instead it has an extra gain stage. The other is more or less stock, save a horrid carpet upholstery job which will be tackled at some later date.
All 100W Fender guitar amps from this period are the same except for the face plate; the difference lies in the speaker combos. All but this one, I should add... the Vibrosonic has a different output transformer. But that should be the only difference. Anyway, this pictorial should be of equal use for a Twin, a Quad reverb, a super 6, etc.

Both my amps still have the original Mallory caps, and since they've been playing up over the past year, some servicing is in order, if not long overdue. Unfortunately, I've always been scared shitless of big tube amps due to horror stories about stored voltages. Brad was very effective in replacing my fear by some knowledge, to the extent I now know how to safely discharge these amps. I actually did that months ago already, so no pics of that. I'm the type of person to check whether I've left the oven on three times before I go out (and at least one time as soon as I'm out the door), so I checked the voltages on the caps anyway. They're empty :).

MIke (OffYourFace) was so kind to point me towards an eBay seller with great prices on sets of caps for this model. His shipping was insane though, so instead I paid Mike to buy me two sets and ship them to me. Yay for the Indo brotherhood ;).

OK, first the carpet amp.

Can removed, and here are the filter caps:

Image

Getting them out was quite easy. Just heat up the connections, and they fall out. The only tricky thing is not melting the mantle of the wires around them, but with some careful bending out of the way, that's not really hard. ONly the middle one is a bit tricky, since five different things connect to one of its eyelets.

OK, all out, first one in. Not very educational, is it?

Image

Brief intermezzo. I'm old-fashioned, so I like to put tin on everything before I put it in a circuit. Nothing as annoying as having to work with blobs of solder when you're fucking about in a circuit. So, I cut the legs to the appropriate size, and then put tin on them, over quite some length, in case I need to trim them further (don't want to cut off the tinned bit). This probably isn't necessary, it just makes it a tad easier.

Image

Solder one side, bend, and trim to put the other side in place. Of course, observe the correct polarity of these things, you don't want to put them in the wrong way.

Image

Since the 20 uF Sprague caps are a bit smaller than the Mallorys, I thought it smart to bend them in place in order to trim them to the correct length. The big 100 uF caps are roughly the same size as the Mallorys, so those I just trimmed them while holding the old Mallory cap next to it.

Image

All in place. A few good yanks on all the wires just to make sure everything is connected tightly. Final polarity check. Everything is as it should be. The can can go back on, but in general I find it smart to wait until more knowledgeable people have checked out what I did. You don't want to re-assemble your amp just so you can dismantle it again after someone points out something fatal while looking at your pics ;).

Image

Of course, next step is the remaining electrolytic caps on the main circuit board. Dinner first.

And oh yeah, a local tech asked € 300 to do this. I just saved about € 250.

---

And on we go to the main circuit board. There are a number of electrolytic caps there. They all look fine, and one has been replaced already, but hey. The kit was a complete set, so I might as well change them out all at once.

Most of them look like the silver one in the top of the image, the black one next to it is one I already installed before taking the pic. There's also one that looks milky white on the main board.

Image

Replaced:

Image

Then there's the big ones. The silver one is a replacement already, and it's been glued to the board. I'm sure that's very functional, but also quite annoying, as I had to cut it off with a scalpel.

Image

Easily replaced:

Image

Pardon my French, but doing this really is a piece of piss as soon as you're sure the amp has been discharged. If you can follow simple instructions and now how to handle a soldering iron, you can't really go wrong (unless you have dyslexia so severe that you cannot tell a + from a - reliably)..

OK, next step is turning the balance pot into a bias pot, as per Al's instructions.
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Pogue Mahone » Thu Jun 03, 2010 10:01 am

I love it! Nothing quite like "stickin' it to the man" and doing things your-self, something folks should do more often! :w00t:
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 10:54 am

Eek. So I've located all the caps on the circuit board, and they all correspond to the replacement ones, except two. There's one which according to the schematic should be 80 uF @ 75 V (that's what in the amp as well, an old Mallory), but the replacement one is 100 uF @ 100V. I'm sure the voltage rating is fine, but what is the consequence of the altered capacitance value? It's the one immediately to the left of the balance pot here. Then there's the other one on the rectifier board which also is supposed to be 80 uF. That one has been replaced already by a previous owner, there's a giant 100 uF @ 350V one there. So I'm guessing replacing that one won't do much harm. But I'd like to be sure anyway. And that's when we PM Brad ;).

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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Stratelejazzuar » Thu Jun 03, 2010 11:01 am

Well, that DOES look rather straightforward! Thanks for enabling me more to get some electrical tools! ;)

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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 11:14 am

That eBay seller is funny though. From the auction where Mike scored the caps for me:
All Sprague Atom highest quality available, fresh stock. US made. Do not be misled by others that claim "Premium" cap kits. Sprague Atom is the Premium cap kit so don't sit back and watch this auction, BUY IT.
From his current auction for a similar kit:
Why are some people wasting their money and spending 2-3 times more for Atoms? I haven't a clue.
In fairness, the kit is less than half the price now. But I'm still glad I got in before he ran out of Sprague Atoms, be they hype or not.
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 11:27 am

Stratelejazzuar wrote:Well, that DOES look rather straightforward! Thanks for enabling me more to get some electrical tools! ;)
All you need is a soldering iron and some wire clippers. And a screwdriver, but I know you have at least one ;).
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by al30 » Thu Jun 03, 2010 11:28 am

Replacing the 80uf with the 100uf won't hurt a thing. This is common - the 80uf caps are hard to come by. The voltage rating (350V) won't hurt anything either. Although it's probably a really big cap compared to what's supposed to be in there. I think that may qualify as the "it's what I had lying around" conversation I had here yesterday. :derp:

By the way, I think what you've got there is a balance control and not an actual Bias control. You may want to consider changing that. Here are some instructions.

http://www.el34world.com/charts/bias_conversions.htm" onclick="window.open(this.href);return false;

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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 11:42 am

al30 wrote: By the way, I think what you've got there is a balance control and not an actual Bias control. You may want to consider changing that. Here are some instructions.

http://www.el34world.com/charts/bias_conversions.htm" onclick="window.open(this.href);return false;

AL
Good point, I've been meaning to ask about that. It's supposed to be a balance control indeed, but then there's also this:

Image

Wired up to V2.

Image

In addition to this one, which corresponds to the schematic, but with shit added, I think:

Image

That second one is the one you're talking about, which is present on the schematic. That first one, however, does not occur on the blueprint. Eeks?
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by al30 » Thu Jun 03, 2010 12:06 pm

I really can't see too well what's going on in that pic. That's the Hum Balance Control pot correct? Wired to V2? What pins on V2? Off the top of my head (since there is already a mid control) the only thing I can come up with is maybe it's a variable resistor for the cathodes ???

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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 12:21 pm

al30 wrote:I really can't see too well what's going on in that pic. That's the Hum Balance Control pot correct? Wired to V2? What pins on V2? Off the top of my head (since there is already a mid control) the only thing I can come up with is maybe it's a variable resistor for the cathodes ???

AL
There's weird shit going on. The black wire on the pot comes from the bottom outer lug of the pot. It goes to pin 9 (just to make sure I got the numbering right: the first position counterclockwise of the position without a lug on the tube socket) on V2, which is connected to pin 9 on V1 via another wire. There's a wire coming from that position to pin 9 on V3, which is twisted with the heater wire and from there both this wire and the heater wire converge to the heater position on V4, and it doesn't end there.
Anyway, the center lug on the pot is grounded through the red wire, and something tells me the white wire coming from the top outer lug from the pot does exactly the opposite of the black wire ;).

What I'll do is print out the schematic and add these wires to them in colour. That probably is a lot more clear than my ramblings.
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 12:23 pm

Oh, BTW, yeah, it IS the hum balance pot. Lemme guess... the hum balance pot does something else than the balance pot? That latter one is a bias thing but not as good as a bias pot or something? Ah yes, that's what your link is abpout, never mind ;D.

Funny. My other vibrosonic has a line-out added though that hole in the chassis (yeah, it's been added. I know the later ones supposedly had one stock, but this one has "line out" in Dymo tape added to it, and IIRC the place of the balance pot is where its hum balance pot is... this is going to be freaky!)
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 12:31 pm

Interesting. The hum balance pot is on the later (135 W UL) model schematic, and it seems to be connected the way I attempted to describe (to all heater wires. Now that's concise ;)). I guess mine is a transitional model?
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 12:42 pm

al30 wrote:Replacing the 80uf with the 100uf won't hurt a thing. This is common - the 80uf caps are hard to come by. The voltage rating (350V) won't hurt anything either. Although it's probably a really big cap compared to what's supposed to be in there. I think that may qualify as the "it's what I had lying around" conversation I had here yesterday. :derp:
BTW, this also means I now know which one the bias cap is ;). I appear to have misread the blueprint. It calls for a 50 uF value, not 80 uF. What is in there is a 100 uF one, as is my replacement. I realise it's a safe replacement, but can one make generalisations about the difference between two two values when it comes to sound? Maybe I simply misunderstand what it does.
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by Orang Goreng » Thu Jun 03, 2010 5:00 pm

øøøøøøø wrote:Hey-- this bias filter cap tube isn't going to change the basic sound of the amp. It's going to be a matter of more/less noise/hum and the character of the noise/hum.
Right. If I understand it correctly, its only role is to bleed off everything non-DC from the bias supply, right?

Thanks, guys :).

BTW, weird, I'm fixing an amp and my toilet at the same time. Good thing I'm sober.
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by øøøøøøø » Thu Jun 03, 2010 5:54 pm

Kinda sorta! But not really!

What comes out of your wall is AC. Bias voltage needs DC. This much you know!

The diode is what converts it to DC-- it's what's called a "half-wave rectifier." But the way it does that is that it just chops off the voltage every time it swings negative-- in other words, only the positive bits remain after passing through the diode. Like this:

Image

So what you have left is not smooth battery-like DC, but rather pulsing DC. What's worse is that since it's half-wave rectified, it pulses only 50 times a second in Europe, or 60 times a second in the US. In other words, there's a big "gap" in between each pulse.

Predictably, if you didn't do anything to this, it would hum at 50 (or 60) Hz. Note that the waveform you have left is "sine" on one side, and "square" on the other (where the rectifier lopped it off). This means that in addition to the basic 50 or 60 Hz tone, you'll have lots of upper harmonics present, too (square waves have tons and tons of harmonics--- fourier's theorem states that any periodic waveform can be mathematically represented as the sum of several sine waves).

Your filter cap's job is to charge on the pulses, and discharge in the "gaps." This serves to "even out" the DC so that it's more like battery DC. This is why they're sometimes called "smoothing capacitors..." they take the "lumpy" input signal and even it out so that it's more like a straight line.

If you installed an extra small-value film bypass cap, it would be able to discharge and charge very quickly-- and ideally be 'fast' enough to deal with the upper harmonics imparted by the square half of the waveform.

blah blah blah!

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