Martin,
yes, large signal waveforms get rather strange looking with no bypass cap, the peaks getting rounded off; it's like there's a compressor whose ratio increases with signal level, and it takes a very large input signal to get it to clip hard.
I ended up using individual resistors for each cathode due to the signal cancellation effect of a shared cathode confuses things too much.
With a shared unbypassed cathode, at low signal levels the opposite polarity signals at the control grids appear at the cathode and are cancelled out. However, as signal level is increased and the tubes enter cut off for part of the cycle, when in cut off the control grid signal no longer appears at the cathode, and so the cancellation effect is lost (until the tube conducts again).
Independent cathode resistors have the benefit of being consistent in regard of bypassing over the full cycle and also allows unmatched tubes to settle at closely similar static currents.
Anyway, those was my findings, I'll look forward to your investigation into this topic.
Pete.
cathode bias confusion
Moderators: pompeiisneaks, Colossal
Re: cathode bias confusion
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martin manning
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Re: cathode bias confusion
pdf, what I'm seeing is consistent with your observations regarding the shared cathode resistor and presence of a bypass capacitor.
I think the answer to the question of how hot to bias is nicely summed up by Aiken (taken from your post above):
"If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation."
With this I'm back to "hot enough to get good tone, but not so hot as to exceed limits and redplate," which depends upon B+ and load impedance. Assuming that the B+ and load impedance are reasonable, "getting good tone" is likely to mean biasing hot enough to stay away from crossover distortion at high signal levels. If crossover distortion still appears when the bias is at 100%, then the only way out would be to reduce the drive level.
So, Andrew, you might want to go back up in idle current if you can hear an improvement, and you don't get into redplating with sustained high volume playing.
I think the answer to the question of how hot to bias is nicely summed up by Aiken (taken from your post above):
"If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation."
With this I'm back to "hot enough to get good tone, but not so hot as to exceed limits and redplate," which depends upon B+ and load impedance. Assuming that the B+ and load impedance are reasonable, "getting good tone" is likely to mean biasing hot enough to stay away from crossover distortion at high signal levels. If crossover distortion still appears when the bias is at 100%, then the only way out would be to reduce the drive level.
So, Andrew, you might want to go back up in idle current if you can hear an improvement, and you don't get into redplating with sustained high volume playing.