Specific Filter Cap Replacement - Guild Bass Amp - diyAudio
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Old 4th April 2014, 03:48 AM   #1
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Default Specific Filter Cap Replacement - Guild Bass Amp

I was wondering if the 4 40uf caps in the filter section that are filtering the different B+ voltages could be changed without changing the B+ voltages? Should I really shell out the cash to replace them with 40uf caps? They are much more expensive than 47uf caps...

Also, most of the caps in the circuit (the coupling caps in the preamp and input sections) are electrolytics. It's recommended that I go ahead and replace those with a good polypropylene cap like an orange drop, correct?

Thank you!
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Old 4th April 2014, 05:49 AM   #2
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Any difference in voltage detected would be mains fluctuation, or replacing dying caps with good ones. The only caps in the preamp likely to be electrolytics are the cathode bypasses - despite the symbols used on the schematic.
Good luck with the amp.
Cheers
JimG
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Old 4th April 2014, 12:19 PM   #3
Enzo is offline Enzo  United States
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40-50 years ago it was common for caps like that to have a stated tolerance of -20/+80%. What that means is that whatever value is printed on it is only a rough estimate.

As you have found, 40uf is not a standard value. You will find 40uf mainly in the retro market for guys trying to be oh so exact rebuilding old stuff. For a 40uf cap, 47uf is within 20%, more than good enough for an old guitar amp. (39uf might also be easier to find than 40, but why go down when you can go up?)

But even if you doubled them, say up to 100uf, it won;t change the voltage on them. The power supp[ly will charge up to the rectified peaks regardless. What you MIGHT hear if you did that would be the bottom end maybe getting tighter, sag would be less, the amp might sustain a hair less. But the voltages would be about the same.

I agree with Jim, your coupling caps are not electrolytics, other than the two 30uf cathode bypass caps. Yes, 30uf is not a standard value. 22uf would be common there, and a standard value. Or 33uf.

Those coupling caps like the 0.005uf are paper or film caps, the small ones - the ones marked in picofarads - are likely ceramics or maybe mica. The thing to do is get out the volt meter and find out if they are leaking DC. Some caps of that era leak like a screen door, and others are still working fine. If they are working, leave them alone. I'd change out the electrlytics just for age myself, but the film caps I'd only change for reason.

Lytics were much larger then, the technology has improved since. You mentioned money. They used can caps then because they were common and took up very little room under the chassis. Nowdays can caps are relatively rare specialty items. And not cheap. When you have to replace a four section cap, look at how it is connected. There will be four wires (or maybe a couple more) leading from the cap can over to various points in the circuits. Nothing wrong with installing four individual caps in its place. You can usually wire them directly to wherever those wires went. They will take up less room than 40 years ago. Leave the old can there for appearance, just all disconnected. On the other hand, sometimes I simply mount a small terminal strip in the chassis, and wire the caps to it to take the place of the can terminals.
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Old 4th April 2014, 12:21 PM   #4
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Quote:
Originally Posted by shillings55 View Post
I was wondering if the 4 40uf caps in the filter section that are filtering the different B+ voltages could be changed without changing the B+ voltages? Should I really shell out the cash to replace them with 40uf caps? They are much more expensive than 47uf caps...
Use 47uF, 40 would be completely pointless - you might consider the tolerances on such caps, the originals were probably -25%/+50% tolerance.

Quote:

Also, most of the caps in the circuit (the coupling caps in the preamp and input sections) are electrolytics. It's recommended that I go ahead and replace those with a good polypropylene cap like an orange drop, correct?
As djgibson51 has already mentioned, the only electrolytics in the preamp are the two cathode decouplers (30uF) - again, 47uF would be perfectly OK. All the others are non-electrolytic, and not worth changing unless faulty.
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Old 4th April 2014, 03:37 PM   #5
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Wow thanks so much for the info. This totally clears up everything. I'll replace the filter caps and the cathode bypass electrolytics and then check for DC after that on signal bypass caps. When I check for DC, I don't need to apply signal, do I?

Thanks,

Will
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Old 4th April 2014, 05:04 PM   #6
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All voltages taken are without an input signal, this is called quiescent or idle voltages. You can check rated output of the amplifier with a scope and an input signal on a dummy load once you have gotten all the quiescent voltages to spec.
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