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01-18-2024, 08:33 PM
Hi Guys
Why waste valuable plate voltage as heat?
I would eliminate the dropping resistors and go with the circa 400V plate voltage.
1k-5W per tube screen is good in a guitar amp, as TUTs state, because guitar amps get clipped a lot.
Is the goal actually to have maximum output? If so, install a fixed-bias supply and go from there. A pair of EL-84 / 6BQ5A can produce 30W easily given the right voltage environment. There is absolutely no advantage in running the tubes at their maximum heat - just because you can and they will do it, does not mean you should or that they like it. Set the idle current to a proper value.
If this needs some warming up tonally, add a Body control so warmth can be dialed in at will. And use a concertina splitter instead of the tone-sucking Schmitt.
The numbers you have are about right for a pair of 12W tubes, ie no more than 12W output class-A. Cathode-bias without a bypass cap is as close to class-A as most hobbyists get. There is often an economic reasoning proffered for cathode-bias , but to me it is an admission of fear of dealing with negative voltages - who can understand them? Personally I find cathode biased amps to be a waste of an almost good circuit. Is that too harsh?
If you think that Va is limited to 300V for EL-84 / 6BQ5A then you are mistaken. TUTs explain why. The data sheet may say so, but it is wrong. Do not confuse applications with tube limits.
Should we assume the Bogen 20 would normally produce 20W? Do you have any idea of the PT specs? The label on the rear panel should state the power inut as VA - the fuse rating tells you a limit of protection, leaving you to guess how much lower the true maximum is.
Have fun
Why waste valuable plate voltage as heat?
I would eliminate the dropping resistors and go with the circa 400V plate voltage.
1k-5W per tube screen is good in a guitar amp, as TUTs state, because guitar amps get clipped a lot.
Is the goal actually to have maximum output? If so, install a fixed-bias supply and go from there. A pair of EL-84 / 6BQ5A can produce 30W easily given the right voltage environment. There is absolutely no advantage in running the tubes at their maximum heat - just because you can and they will do it, does not mean you should or that they like it. Set the idle current to a proper value.
If this needs some warming up tonally, add a Body control so warmth can be dialed in at will. And use a concertina splitter instead of the tone-sucking Schmitt.
The numbers you have are about right for a pair of 12W tubes, ie no more than 12W output class-A. Cathode-bias without a bypass cap is as close to class-A as most hobbyists get. There is often an economic reasoning proffered for cathode-bias , but to me it is an admission of fear of dealing with negative voltages - who can understand them? Personally I find cathode biased amps to be a waste of an almost good circuit. Is that too harsh?
If you think that Va is limited to 300V for EL-84 / 6BQ5A then you are mistaken. TUTs explain why. The data sheet may say so, but it is wrong. Do not confuse applications with tube limits.
Should we assume the Bogen 20 would normally produce 20W? Do you have any idea of the PT specs? The label on the rear panel should state the power inut as VA - the fuse rating tells you a limit of protection, leaving you to guess how much lower the true maximum is.
Have fun
