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In Reply to: RE: Behaviour in overload of PP EL84M cathode bias amp? How to examine for that? posted by Timbo in Oz on September 04, 2014 at 16:27:15
When you say overload, are you talking about departing from class A?
Overload can be tough to test for, if you put a load on your amp and attempt to measure power/distortion, it will fall apart as it edges into class B, which is why cathode bias is a poor choice for such a design.
The best shot that you have is to design for the power you want in class A. You may potentially be able to put a voltage regulator under the cathodes of the output tubes to keep the bias from wandering all over the place, but that would be a topic of recommended research, not a solid suggestion.
Follow Ups:
yes, or no?!
I can't tell.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Your description was about the driver stage and the power supply.
The driver stage is not going to be the dominant problem with a cathode bias amp going from A to B, the cathode resistor/capacitor will be.
For the power supply, you really have two options.
1. A real choke input filter, which would provide adequate regulation for the wide range of currents demanded.
2. Tie the B+ feed to the output transformers from the first node of the power supply, which would minimize voltage changes from filtering impedance, but still isn't as good as above.
Do note that these power supply mods assume that you know to run the amp in fixed bias in the first place. With cathode bias, you need to stick to class A.
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