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Old 2nd March 2012, 08:12 PM   #1
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Default Cathode Bypass Capacitor concern

I have a class A/B push pull cathode bias amplifier (6V6 tubes) that is bypassed with a 25v/25uf sprague atom electrolytic capacitor (not an old one but a new black case with green writing). My concern is that at idle or even under low signal scenarios the voltage across the cathode resistor is 21vdc which is fine but when the amp is pushed to max signal I have seen the voltage jump to 29vdc but obviously for a fraction of a second. I had my meter hooked up and on min/max setting I got for a low 19vdc up to 29vdc for max.
Does anyone know if these caps can handle that peak voltage surge or should I replace with a 50v/25uf cap?
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Old 2nd March 2012, 08:43 PM   #2
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Well you could probally leave it like that forever, but to understand all that is happening at the cathode you really need to observe the AC. That means a scope. What is the value of the cathode resistor?
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Old 2nd March 2012, 09:32 PM   #3
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Well you could probally leave it like that forever, but to understand all that is happening at the cathode you really need to observe the AC. That means a scope. What is the value of the cathode resistor?
The cathode resistor is an 8 watt 250 (I measured 255 with my meter) wirewound Brown Ohmite. And I don't own a scope but one is on the list of things to get in the future to help in my hobby-tube addiction. I guess I could bias colder (bigger resistor) to drop voltage but I like to run a little hot because of the auto bias nature of the circuit.

Last edited by famousmockingbird; 2nd March 2012 at 09:35 PM.
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Old 2nd March 2012, 10:06 PM   #4
SY is offline SY  United States
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Why not spend $1.50 to get a 50V cap and not worry about it any more?
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Old 2nd March 2012, 10:17 PM   #5
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Why not spend $1.50 to get a 50V cap and not worry about it any more?
That would be too easy

My first reaction was to instantly order a new cap and replace. Then I thought to myself well am I being paranoid and overthinking this because I checked the schematic and the original spec is a 25v voltage rating. I know it won't hurt to replace the cap to be safe but I thought it might be funner to post and let people chime in their .02
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Old 2nd March 2012, 10:20 PM   #6
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The voltage increase is due to second-order distortion. Perfectly normal. It is why cathode biased output valves need to be biased quite 'hot' with no signal, as the bias gets 'cooler' with signal. The bypass cap should be OK as most electrolytics can cope with short-term surges up to about 20% above rating.

I'm not sure why you sometimes see a reduction to 19V. Could be grid current charging up the coupling cap from the previous stage, so you get blocking on really big signals.
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Old 2nd March 2012, 10:45 PM   #7
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The voltage increase is due to second-order distortion. Perfectly normal. It is why cathode biased output valves need to be biased quite 'hot' with no signal, as the bias gets 'cooler' with signal. The bypass cap should be OK as most electrolytics can cope with short-term surges up to about 20% above rating.

I'm not sure why you sometimes see a reduction to 19V. Could be grid current charging up the coupling cap from the previous stage, so you get blocking on really big signals.
The amp sounds good to my ears, so I guess I will leave the 25v cap in. If I find myself losing sleep over it I will replace to a 50v rating. I couldn't find info on surge voltage for the sprague atoms so if a 20% surge is the norm than I think I will be ok.
As far as push-pull circuits go I thought that second order harmonics created in the output stage will be cancelled out in the OT. Are you talking about second order harmonics from previous gain stages that are in the output? Please correct me if I am misunderstanding the concept.
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Old 2nd March 2012, 10:58 PM   #8
DF96 is offline DF96  England
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Second-order harmonics and DC shift generated in the output stage are cancelled in the push-pull OPT so don't appear in the output. They don't cancel at the cathode. If cathodes are joined together they add there. A PP output might be generating 10-20% second-order internally at full output, with equal amounts of harmonic and DC hence the large bias shift.
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Old 2nd March 2012, 11:32 PM   #9
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Second-order harmonics and DC shift generated in the output stage are cancelled in the push-pull OPT so don't appear in the output. They don't cancel at the cathode. If cathodes are joined together they add there. A PP output might be generating 10-20% second-order internally at full output, with equal amounts of harmonic and DC hence the large bias shift.

thanks for clarifying that for me
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