Cap job on Vibrosonic - pictorial. Update jun5.

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

Post by jimboyogi » Thu Jun 03, 2010 6:48 pm

Kinda related, I tried this on the first reservoir filter cap on the RhoneyTone. I have a big 100uF filter cap here, and I read about using a smaller cap in parallel to take out higher frequency noise. I listened very carefully just to the noise signal (hum), but I could not hear any difference at all with or without the smaller cap.
BUT... mine was a full recified voltage at 100Hz, and not the half rectified bias voltage situation above, so this might make some difference to the usefullness of the smaller cap.

FTR, my understanding of the reason that larger caps sometimes struggle to pass higher frequencies is because large electrolytic caps can have high ESR (Electrical Series Resistance), which makes them act as a bit of a low-pass filter, allowing the higher frequencies to remain on the power supply voltage. Different makes/brands/values of cap have differing ESR. If your cap has a low ESR then you probably don't need a smaller cap in parallel (probably what happened to me). If it has a high ESR then a paralleled smaller cap should reduce noise. The standard value of the smaller cap is usually 1/10th the capacitance of the big cap.

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

Post by Orang Goreng » Fri Jun 04, 2010 12:16 am

øøøøøøø wrote: 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.
Ah, this shows you how little I know indeed. I had assumed this is the function of the big filter caps, I haven't really thought about WHERE in the circuit the bias cap is placed ;D. So the full-wave rectifier next to the bias cap on the little circuit board is for different voltages in the amp?

I obviously need to read some more. Thanks all!
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Re: Cap job on two mid-70s Vibrosonics pictorial

Post by øøøøøøø » Fri Jun 04, 2010 2:56 am

yes-- the full-wave rectifier is for the B+ voltage, which then gets stepped down for all the other little voltage. The bias supply has its own dedicated half-wave rectifier.

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

Post by øøøøøøø » Fri Jun 04, 2010 3:02 am

jimboyogi wrote:Kinda related, I tried this on the first reservoir filter cap on the RhoneyTone. I have a big 100uF filter cap here, and I read about using a smaller cap in parallel to take out higher frequency noise. I listened very carefully just to the noise signal (hum), but I could not hear any difference at all with or without the smaller cap.
BUT... mine was a full recified voltage at 100Hz, and not the half rectified bias voltage situation above, so this might make some difference to the usefullness of the smaller cap.

FTR, my understanding of the reason that larger caps sometimes struggle to pass higher frequencies is because large electrolytic caps can have high ESR (Electrical Series Resistance), which makes them act as a bit of a low-pass filter, allowing the higher frequencies to remain on the power supply voltage. Different makes/brands/values of cap have differing ESR. If your cap has a low ESR then you probably don't need a smaller cap in parallel (probably what happened to me). If it has a high ESR then a paralleled smaller cap should reduce noise. The standard value of the smaller cap is usually 1/10th the capacitance of the big cap.
I think part of the deal is that the speakers in a guitar amp roll off pretty sharply after 7k or so (optimistically) anyway, so high-frequency noise isn't likely to be a huge issue. Using huge filter caps and bypassing them might in theory give the barest traces more clarity/headroom to the sound by preserving a tiniest trace of bandwidth or whatever, but I wouldn't hold my breath trying to hear it. The noise it would reduce would be so negligible it's not even funny, in that application, I would think.

The ESR thing makes sense-- I always thought it was about charge/discharge time. Electrolytic caps are not even remotely phase-linear, which is why they're frowned on for audio signal path applications, generally, if fidelity is the goal. Things get really weird/slow at high frequencies. Perhaps it's a combination of the two factors.

Which makes me wonder what it would sound like to use enormous film caps for the cathode bypass caps? Or to bypass the bypass caps with something like a 250pF or .0025µf styroflex or mica cap? Might sound slightly different. Not necessarily better, and certainly not a dramatic difference.

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

Post by jimboyogi » Fri Jun 04, 2010 2:39 pm

Good points Brad.

Electrolytics not only suck at high frequencies, they apparently have introduce time and phase distortions at low frequencies also. As cathode bypass caps, they supposedly cause low frequency distortion, and within the range of guitar and certainly bass frequencies. Not only that they don't last very long!

I have certainly read recomendations for using large film caps for cathode bypass, to give quicker transient response without distortion. Problem is that film caps just don't get that big a capacitance value, so invariably you get low frequency roll-off.
Another suggestion that I have read is instead to use bipolar electrolytics, like those in speaker crossovers. Because they do not require a polarising voltage across them to charge the dielectric, they apparently have a much faster transient response at low frequencies than regular electrolytics. I have tested bipolar v's polar cathode bypassing a 12AX7 gain stage in the RhoneyTone, and I was very impressed with the bipolar. Sounded to me that it had better high and low frequency attack. That's the cap that stayed in!

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

Post by øøøøøøø » Fri Jun 04, 2010 3:29 pm

jimboyogi wrote:Good points Brad.

Electrolytics not only suck at high frequencies, they apparently have introduce time and phase distortions at low frequencies also. As cathode bypass caps, they supposedly cause low frequency distortion, and within the range of guitar and certainly bass frequencies. Not only that they don't last very long!

I have certainly read recomendations for using large film caps for cathode bypass, to give quicker transient response without distortion. Problem is that film caps just don't get that big a capacitance value, so invariably you get low frequency roll-off.
Another suggestion that I have read is instead to use bipolar electrolytics, like those in speaker crossovers. Because they do not require a polarising voltage across them to charge the dielectric, they apparently have a much faster transient response at low frequencies than regular electrolytics. I have tested bipolar v's polar cathode bypassing a 12AX7 gain stage in the RhoneyTone, and I was very impressed with the bipolar. Sounded to me that it had better high and low frequency attack. That's the cap that stayed in!
You can get them that big-- they are just physically huge-- too huge.

Speaker crossovers use very large value film caps all the time. 25µf is not unheard of. Up to 15µf is commonplace, so worst-case scenario, two 12µf in parallel gives you the right value! But physically enormous, so highly impractical. hell, Solen fast caps are readily available from Parts Express in values up to 200µf!! And these are metallized film caps. A 24µf is 11 bucks and change though! And is 32mm by 53mm-- not exactly something you'd be fitting on an eyelet board in a Deluxe Reverb, that's for sure.

Also, bypassing with a film cap about 10% the value of the electrolytic solves a lot of problems. The non-polarized electrolytic is an intriguing compromise solution, too, and it might be the best compromise. Perhaps a NP electrolytic bypassed with a film cap 1% or 10% the value would be even "better." Of course, "better" is subjective. Distortion and a smoothed transient response is sometimes desirable in a guitar amp!

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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Sat Jun 05, 2010 5:31 am

Updated the first post; all the electrolytic caps have been replaced now.
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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Sat Jun 05, 2010 5:51 am

OK, I"m skipping the bias pot thing for now, let's do the other one first.

Question. In my mind one of the things that people don't like about the silverfaces is that they should be constructed of particle board. Did I get that wrong? Both mine have a particle board baffle, but the cabs are solid. Well, "solid" as in plywood.
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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Stratelejazzuar » Sat Jun 05, 2010 6:16 am

Orang Goreng wrote:OK, I"m skipping the bias pot thing for now, let's do the other one first.

Question. In my mind one of the things that people don't like about the silverfaces is that they should be constructed of particle board. Did I get that wrong? Both mine have a particle board baffle, but the cabs are solid. Well, "solid" as in plywood.
yeah, i think the particle board is frowned upon because it's weaker than plywood. is the cab ply or laminate?

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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Sat Jun 05, 2010 6:19 am

What's the difference between ply and laminate? The material of the layers?
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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Stratelejazzuar » Sat Jun 05, 2010 6:23 am

Orang Goreng wrote:What's the difference between ply and laminate? The material of the layers?
ply is thin, sandwiched together, like a pad of paper. laminate is side-by-side, like you'd see in a 5-piece MIM guitar body.

my bassman head is laminate, with a particle board front baffle. i've made the 15" speaker baffle out of ply.

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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Sat Jun 05, 2010 6:38 am

Eh... both? It's layers, but there's also side-by-side bits. Hard to explain.

OK. Am I really gonna document this one as well? Yes, I am!

This my old one, I"ve had it for 20 years. It came this way. I've changed the output tubes ONCE, in the early 90s. That's it. The speaker got blown a couple of years ago, and it has been replaced by a nice older JBLD130F I got from Brad. Grey frame, not orange like the original speaker.

OK, top back panel off, power cord clip unscrewed.

Image

Chassis unscrewed, using the cab as a workbench. Voltmeter between pin1 of V1 and chassis; flip power and standby to "on" position (of course with the thing unplugged!). No voltage worth mentioning.

Image

Flip over, remove can. Do I detect some races of leaked electrolyte in the can? There's two suspicious white spots there.

Image

Does this remind anyone else of muscle car owners tanning on the beach together? No? Look at the second and third cap from the left. Tiny bulges! Can't be good.

Image
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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by øøøøøøø » Sat Jun 05, 2010 7:16 am

You're in business!

Here's the story with cab material. They went to particle board baffles sometime during the BF era-- I think after CBS took over, maybe? But don't quote me on that. I have a '65 "Fender Electric" Deluxe that has a plywood baffle, and I had a '67 "Fender Musical" Deluxe that had a particle board baffle.

Plywood baffles are better-thought-of, typically than the particle board ones for reliability's sake if nothing else.

Older fender amp cabs are usually finger-jointed solid pine. Then I think they went to lap joints sometime in the 70s, and they may or may not have gone to plywood (don't remember offhand-- been awhile since I've owned a SF amp and taken it down to bare wood!). Solid pine is real lightweight and resonant and sounds good for a guitar cab. Plywood makes a better "textbook" cab-- it's more inert, but it's heavier. Finger joints are more durable, and true dovetail (quite rare in amps) is the best of all.

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