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Vintage threads from the first ten years

Re: Blackface Bassman filter caps

5/29/2000 7:39 PM
Peter S	Re: Blackface Bassman filter caps I think what Ray is saying is for a brief moment(1 or 2 seconds) no harm will result in shorting the cathode to ground, and in effect you take the cathode resistor and bypass cap out of the circuit.....if the noise is being generated by the cathode circuit of the tube, it will temporarily go away by doing this, which can lead on to coclude that that's where the noise was being generated. Personally I'd prefer to unsolder the cap and/or resistor and substitute known good ones. PS

5/29/2000 7:39 PM

Peter S

Re: Blackface Bassman filter caps

I think what Ray is saying is for a brief moment(1 or 2 seconds) no harm will result in shorting the cathode to ground, and in effect you take the cathode resistor and bypass cap out of the circuit.....if the noise is being generated by the cathode circuit of the tube, it will temporarily go away by doing this, which can lead on to coclude that that's where the noise was being generated. Personally I'd prefer to unsolder the cap and/or resistor and substitute known good ones.
PS

5/29/2000 8:27 PM
Ray Ivers	Bruce, I'll try to respond to the comments from your last post one at a time (I hope they show up italicized like when everyone else does it...). I guess I need a better understanding of what you're doing!! If you jump across the bypass cap, you've effectivly grounded the cathode... ZERO bias. Is that what you are trying to do in this test Ray? Not at all - that's just an unfortunate side effect of this particular 'find-the-bad-cap-quickly' technique. It's been my experience, in a testing situation, many people can't hear the difference between a 25uF cap and a 5uF cap when used as a cathode bypass cap in a preamp section. Most of the old caps I test are way over their stated value anyhow so paralleling one only works if the cap has gone VERY low in value or has come unsoldered. I absolutely agree. I almost never parallel caps. Shorting the cap out with a wire would bring the cathode to ground and adversely change the way the stage works!! Yes, it will - but it will also tell me if the cap is making noise or not. The stage will continue to function in 'contact-bias' mode. If the cap is already shorted, there will be no difference in the operation of that stage with the wire across the cap... Yes - and this condition would be indicated to me by the lack of a 'thump' when I bypass the cap. An open large-value cap would be indicated by an increase in the amp's hiss level with the jumper connected. actually I'd be surprised if the tube was still alive and working close to spec after any real operating time with a shorted bypass cap. *I don't know, Bruce - it seems to me that the little tubes especially will continue to put out some kind of sound even after tremendous long-term abuse. Of course, if the tube is dead, my technique won't be too effective... Peter's right, too - it IS best to unsolder the caps to check them - but I can test an entire Fender chassis before the soldering iron is hot, without affecting the vintage resale value at all for those who care about such things. I know I particularly prize the amps I own that are completely untouched - so much for being 'enlightened'! Ray

5/29/2000 8:27 PM

Ray Ivers

Bruce,

I'll try to respond to the comments from your last post one at a time (I hope they show up italicized like when everyone else does it...).

I guess I need a better understanding of what you're doing!!
If you jump across the bypass cap, you've effectivly grounded the cathode... ZERO bias.
Is that what you are trying to do in this test Ray?

*Not at all - that's just an unfortunate side effect of this particular 'find-the-bad-cap-quickly' technique.

It's been my experience, in a testing situation, many people can't hear the difference between a 25uF cap and a 5uF cap when used as a cathode bypass cap in a preamp section.
Most of the old caps I test are way over their stated value anyhow so paralleling one only works if the cap has gone VERY low in value or has come unsoldered.

*I absolutely agree. I almost never parallel caps.

Shorting the cap out with a wire would bring the cathode to ground and adversely change the way the stage works!!

*Yes, it will - but it will also tell me if the cap is making noise or not. The stage will continue to function in 'contact-bias' mode.

If the cap is already shorted, there will be no difference in the operation of that stage with the wire across the cap...

*Yes - and this condition would be indicated to me by the lack of a 'thump' when I bypass the cap. An open large-value cap would be indicated by an increase in the amp's hiss level with the jumper connected.

actually I'd be surprised if the tube was still alive and working close to spec after any real operating time with a shorted bypass cap.

*I don't know, Bruce - it seems to me that the little tubes especially will continue to put out some kind of sound even after tremendous long-term abuse. Of course, if the tube is dead, my technique won't be too effective...

Peter's right, too - it IS best to unsolder the caps to check them - but I can test an entire Fender chassis before the soldering iron is hot, without affecting the vintage resale value at all for those who care about such things. I know I particularly prize the amps I own that are completely untouched - so much for being 'enlightened'!

Ray

5/29/2000 11:44 PM
Peter S	Ray, You're absolutley correct, and I didnt mean to imply that there was anything wrong with your diagnostic method....if fact it's pretty clever, and I remember an old timer showing me a similar method years ago in a high production shop where time was very critical. I'm all for any method that improves efficiency, without compromising safety. Your point about leaving solder joints untouched is a good one, although I must say that it's a rare old amp that hasnt had a few joints resoldered, or at least needed them. PS

5/29/2000 11:44 PM

Peter S

Ray,
You're absolutley correct, and I didnt mean to imply that there was anything wrong with your diagnostic method....if fact it's pretty clever, and I remember an old timer showing me a similar method years ago in a high production shop where time was very critical. I'm all for any method that improves efficiency, without compromising safety. Your point about leaving solder joints untouched is a good one, although I must say that it's a rare old amp that hasnt had a few joints resoldered, or at least needed them.

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