(08-07-2017, 10:28 PM)Hangman Wrote: Rickfox2,
The bias adjust is trimpot AP1. I don't know the location, but I don't believe there are any other trim pots in the R50H.
Bias voltage doesn't really tell the whole story. what you need is the Bias current. a good way to estimate the idle power dissipation is to measure the cathode current (which is approximately equal to the plate current at idle) then calculate the plate power dissipation using ohms law. IxV = P
ideally you want somewhere between 60-70% maximum plate dissipation at idle. So for an EL34 the max is 25W. so 25*.65 =16.25 W
we know V (its fixed) so we adjust the bias voltage to give us 16.25/390 = 41.66ma
unfortunately the R50H doesn't have an easy way to measure cathode current. so you can either make a bias probe or buy one. you don't have to buy one of those goofy "bias kings" or anything like that. If you already have a digital multi-meter, use something like this:
http://www.ebay.com/itm/8-Pin-Tube-Plate...Sw-0xYfSSm
they are pretty easy to make, but this is a pretty reasonable price. using the above method will allow you to set the bias for any fixed bias amplifier. although the bias probe will only work for the tubes listed in the ebay auction. thats an issue of pinout.
As I recall, with the Marshall I measured current at one of the pins of the EL34. Then did the math and adjusted accordingly. If I understand you correctly, this can't be done with a R50H?