Bieworm wrote: ↑Mon 01/04/21 2:38 am
I only have a 1000uf and a 4700uf 63V electrolytic cap .. should I try the latter?
Those are BIG caps for this purpose. What you gain with larger caps is a longer time constant. If this is a bias problem, that means a sustained overdrive will take longer to create the fizz. However, as the signal falls away, the unwanted distortion will stay with you longer because bias will take longer to normalize.
About Jack's suggestion on lowering the 6V6 cathode resistor and fire up the bias to 100% ...makes sense in a way to avoid blocking distortion or is it just going to make things worse? This amp is so damn loud already. I even added a 1/4 power switch to be able to play it at home
Just so we're on the same page regarding terminology, blocking distortion is what happens when a too-large signal draws grid current on positive peaks, causing the grid of the power tubes to move negative. That can have a similar effect, but it's a totally different cause, and the smallish coupling caps that are typical in guitar amps tend to minimize it as a problem.
What I was referring to earlier is cathode bias shift, and it's specific to self-biased Class AB1 amplifiers. At idle, the DC voltage on the cathodes is determined by the current through the tubes and the value of the cathode resistor. However, when the tubes are driven with a signal, they draw more current, because this is AB1 operation. That additional current creates more positive DC voltage at the cathodes, pushing the tubes in the direction of cutoff (the grids become more negative, relative to the cathodes). Depending on the specific combination of resistor value, signal level, RC time constant and signal duration, the result can be significant crossover distortion. The tubes will operate closer to Class B than AB1, and they will continue to do so until average current falls and the cathode capacitor discharges.
I want to stress that this might or might not have anything to do with the problem you're having. It's just one of the things to nail down, given the nature of what you're experiencing. This is why I suggested monitoring the DC voltage at the cathodes while you listen to the problem. If you have a DVM, clip lead it to the cathodes and ground, and watch what happens when you smack the amp. You might find that certain cathode voltage levels correspond to what you're hearing.
Not sure how practical this might be for you, but another approach would be to temporarily ground the cathodes and apply fixed grid bias. In a pinch, you could use three 9V batteries in series and a 100K pot. The batteries will last many hours in that configuration, much longer than you would need to do a listening test.
Jack
Seriously, it's the issue of bias changing with power output in self-biased AB1 stages. The more current flows through the tubes, the higher the voltage across the cathode RC becomes. I'm surprised this isn't better known in the guitar amp world, but maybe the audible effect is mostly hidden because the tubes are already in overdrive when it happens. Anyway, this is why I suggested that he measure the cathode voltage when he's playing through the amp. If there's a significant increase, he might need to lower the R (hotter bias) or substantially increase the C, as you suggested.
