Just commenting here to appreciate your work
@santiall .
I have started my Tube guitar amp journey with a Bugera 6262, and recently closed off (for now, haha!) with a JVM410H. Even though you weren't fully involved into the development of the 6262, it's a cool thing to come "full circle" in that regard. At least for now, new tube amps are certainly on the horizon ;-)
Just a question on the bias-ing of the JVM410H - I read all these current measures, like 30mA being cold, 40~mA being more preferrable, but all of this is of course in reference to the plate voltage. My question is: does the plate voltage fluctuates heavily from amp to amp, or is it pretty much 480V (not talking about tolerances of a Volt or 2), or is it another value? Reason I am asking is that although I am technically capable of reading out the plate voltage with my 600V multimeter, if I can spare that step I am working safer.
Also, posing this question allows me to thank you again for taking the time to engage in this forum. I have seen the Leon Todd videos, Tone Talk and read through the entire thread, and I find it absolutely remarkable, that you take the time to answer every single question with unparalleled patience - legend material there!
Hi, it is me who appreciates your appreciation!
The bias/plate voltage... ok, for any amp, not only the JVM, when we measure the current what wewant to do, albeit indirectly, is to calculate the plate (anode) dissipation. In a very simplified way, what we have is that the dissipated power is the product of the anode voltage by the anode current, so 480V multiplied by 35mA (480x0.035) equals 16.8W.
Now, a EL34 maximum plate dissipation power is 25W, 16.8W represents approx the 70% of that and 70% is the "adopted default internet standard" so there you go.. here we got a ballpark for any amp using EL34s and a HT voltage of around 500V. You bias to that and you are safe.
There are a couple of caveats though, what we measure is the cathode current, which is the anode current PLUS the screen current in a pentode or tetrode (EL34, 6L6, etc). That screen current is approx 10% of the anode current, so by calculating the power in excess, we are safe.
Why all this, because at the end if the plate voltage fluctuates, so will the current and hence the dissipated power. Do we care?, hmm depends on you to be honest. As per the calculations above, even if the current and plate voltage go up by 10%, the power will go up by 12% or so, instead of 16.8W, around 20W, which is 80% of the total anode power, still safe (rememeber that this numbers still include the screen power).
Does the plate voltage fluctuate... mostly depends on your mains voltage and your bias current. The higher the bias current, the higher the losses in the transformer so the plate voltage will go down. That's why biasing is always a bit of a back and forth process between tubes till you get to the value you want.
We don't have much control of the mains voltage unless you use an AC source or a voltage stabilizer. In most countries, the tolerance of the mains is "guaranteed" at +/-10%, in some Australian places with 240V, the max will be capped to +6% (which coincidentally is the same as 230V+10%). When designing an amp, we have to make sure that the amp operates safely at those raised mains voltages but for you, once again, depends on how picky or how tolerant (pun intended) you are about the exact bias values.
As for the multimeter, if you still feel wary of the voltages, I'd suggest you to get a couple of cable hook probe tips. I use them all the time and are safer than normal probes. The tip isn't as exposed until you press the button on the top and you can clip them before powering the amp on and/or to the bias resistors while adjusting the bias. These ones:
hope it helps!
Santiago
