JVM Forum

Why is it not made so important that we check Plate Voltage before we bias the JVM?
With other Marshalls it was always stressed that the Plate Voltage has to be taken and then with some calculations, we can bias the amp.

No difference from any other amp. It should be checked to be sure that the valves are in the safe area but in any case biasing at certain percentage of anode dissipation won't make it a better bias. The recommended bias values are given at nominal mains voltage.

I see. So if I bought new Output Tubes for my JVM or JCM800 and then only adjusted the bias without bothering to know the plate voltage, i'ts still ok to run the amp without concern?
Or am I misunderstanding something?

correct, don't worry at all.

Imagine you choose to bias at 72% bias dissipation, 18W, which is already a random value, in the ballpark but random as it could have been 68% or 74%, you don't know where the tone is "better" unless you test for a particular pair of valves and your personal taste. So you go, measure the HT, and bias it to exactly 72% plate dissipation which is for example 37.5mA per valve at your plate voltage (480V). Please note that we already made a mistake as we didn't take into account the screen current, we are measuring cathode currents, so our actual plate disipation is slightly lower, around 65% most likely.

Now imagine you have a second amp, you change the valves and bias it to 37.5mA without measuring the plate voltage. Let's go to an extreme case and imagine your plate voltage is very high, 500V, so you are having a plate dissipation of 18.75W and instead of 72% you have 75%, again without considering the screen currents... still only a 3% error which is making errors on the safe side. If we consider the screen current the real plate dissipation is about 67.5%.

In short, if you bias within reasonable and normal current values you don't need to worry of variations in the HT. Whether the tone changes is a different problem but unfortunately there is no perfect "70% dissipation" value to guarantee that.

I had that as an the original idea from this Forum, but that idea kept getting shaken from other things that I have read elsewhere on the internet.
But now I understand. Thank you Santiall.

Great explanation, Santiago. Thanks!!