Cap job on Vibrosonic - pictorial. Update jun5.

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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 Stratelejazzuar » Sat Jun 05, 2010 7:36 am

my bassman head is finger-jointed. they look a little rough if you ask me...

Image

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

Post by Orang Goreng » Sat Jun 05, 2010 7:56 am

Ah cool. Somehow in my mind the cabs were plywood first and then changed to particle board all-over. The mind being a tricky thing.

Funny. When I had my first tube amp (a Carlsbro TR-100), I built a solid pine cab for it. Not to be fancy; I just had a solid pine book case that got dismantled, and part of it I worked into the cab. This one (I'm playing bass in the pic, the guitarist is playing my Vibro there, the one I'm working on now). You can just see the cab behind my grossly over-sized trousers ;). This is 1990-1991 or so. Shortly after I got my '73 JB (the one in the pic is a JV-series Squier) and a 135W Bassman top&cab from 1981 or so. I gave this amp and cab away to a friend, who within a year traded them in for an extremely horrible Epiphone bass (a bright white hair metal style bass). Broke my heart, because he has an original Stingray which is gorgeous, and I gave him this because he had no amp to go with it. Ah well, I'm of the firm opinion that when I give something away, the new owner has all say over it, so I shouldn't complain.

Image

The monitor shelf above my present desk is actually part of that same book case:

Image

Still need to paint it black or something. Maybe not.
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Re: Cap job on Vibrosonic - pictorial. Update jun5.

Post by Orang Goreng » Sat Jun 05, 2010 1:48 pm

OK:

Image

Flip over:

Image

Hmm, shit is considerably different compared to the other one. I need to print out the blueprint and check what's all different. Luckily, all the caps still have the same value. I'm staring to wonder if the silver ones in the other one were replacements alkready, because here it's all white Mallorys. Not anymore now:

Image

I guess this means I'm done :). For now, that is.
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