Power tube grid conduction and PI caps

[EFK]

Please bear with me - this may seem like a silly question but I can't stop wondering about one aspect here, theoretically. I have been troubleshooting an amp design which I suspect may be suffering grid conduction; there is a nasty 'extra' distortion riding on the initial breakup sound when hitting the amp hard. Here is the theoretical part; the PI output caps are some old CDE black cats I had around (never used) which I tested at nearly 400V for leakage and had none. They are 1000V rated, so they are big. However, they are only .015 (actually measure .017) so one would *think* they would charge/discharge quickly enough to avoid grid conduction. What I'm wondering is, are old caps of such large size actually slower to charge/discharge despite the fact that they are not leaky and despite the assumption that a .015 cap is a .015 cap. So the question here is - in this particular location and function, can a cap of large physical size (I assume there is a lot of mylar in there) and older design be detrimental despite the fact that it serves its purpose (blocks DC, no leak) and measures appropriate value?

[Jazz P Bass]

.015uf is .015uf.
The voltage rating is how much V the cap can take before it goes poof.

[Enzo]

I am much more inclined to think you are seeing parasitic oscillations from lead dress issues, instead of some esoteric thing like grid current. Move your power tube grid wires as far from the plate wires as possible. Try moving the wiring from the PI tube around.

[EFK]

If this seems like it would be better suited to the troubleshooting forum please feel free to move it! I wanted to investigate a theoretical aspect so posted it there, but maybe it should be in troubleshooting. Sorry! I have chopsticked/rewired this to death, I don;t think it's a lead dress issue although it certainly could still be a parasitic somehow. If was wondering about the physical size of the caps - I realize it probably sounds stupid but couldn;t help but wonder if the physical size in some way affects the charge/discharge time.

About the first minute of this video, while he's still relatively clean, is EXACTLY my issue.

http://www.youtube.com/watch?v=KMoRs9L07s8

Hear that nasty, buzzy crap riding on the notes that somewhat fades in/out as the note is struck hard, then decays? Like a buzzy speaker voice coil. In my situation, it seems tied to the PI as if I change the bias of the PI (shared cathode paraphase) - and I assume therefore how hard it is hitting the power tubes - the buzzy noise changes. Big stoppers (gradually increased, now up to 47K) have helped, but not eliminated it completely. It's there at lower edge of breakup volumes, not so much if I back off guitar volume or pick gently, but I can hear it when I hit the strings hard or turn guitar volume to '10.'

I've also considered crossover distortion, although I've never heard it to my knowledge. This is cathode bias, 6L6, I can;t run it any hotter as it's idling at 26W and rises to a little over 30W at full volume.

[Enzo]

Size? Electricity can go all the way around this earth seven and a half times in one second. A footlong piece of wire takes about a nanosecond to traverse. A few millimeters in size shouldn;t matter. MAybe at microwave freqs, but not audio.

I'd be surprised if you had ever heard a guitar amp that lacked crossover distortion to some degree. Crossover distortion takes place where the signal crosses over from negative to positive, so it is always present in the signal if you have it at all. That is the polar opposite from occuring on peaks.

[MWJB]

Sounds like oscillation to me.

Any pics of the amp & circuit in question.

[Chuck H]

If you have two 30V zener diodes and two 1n4007's you can do the Paul Ruby mod. One zener (cathode toward ground) in series with one 1n4007 (cathode toward tube) parallel with each grid load resistor. If it's grid conduction you should hear a marked improvement.

How are you calculating bias?

The vid does sound like crossover distortion to me. But parasitic oscillation and crossover distortion can sound similar. I'd like to see a pic of the innards also.

[EFK]

I don;t have pictures. How did you determine 30V for the zener? I just ordered a pile of them in a lot of different values from mouser as I've been reading up on that, also want to try the "Chuck H" zeners across the cathode R approach to try fixing the bias.

I measure plate to ground, cathode to ground, subtract cathode from plate then divide cathode V by value and mutliply that by the plate - cathode V, then divide answer by 2 for a bias per tube. To check it at full volume, I'm using my LCR meter to inject a 1 khz signal into the input, crank to 10 into a dummy load and so the same measurements. Currently I'm at 26W and 33W per tube but then I have 7581A in there so it shouldn't be a problem. I had them in a fixed bias amp and they are matched within about 2 mA.

I've looked more into blocking and I keep seeing references to "look at video of Jimi at Monterey" and it doesn't sound like that big, farty, exploding, phasey mess. Once I wind up the volume, the annoying noise disappears and it actually sounds quite good and remarkably tight for a "tweed-type" amp. So, maybe I should be looking more at crossover or parasitic issues.

[Chuck H]

The 30V was a guess and probably close enough for testing purposes. But for the actual value of the zener for the PR mod you would measure the cathode voltage at full output and choose a zV a volt or two higher OR if you use the zener across the cathode R you would use a PR mod zV a volt or two higher than the cathode zV.