Volts are not amps...
Step 1: Take the damn tubes out.
2: Get the bias supply working so the bias voltage is adjustable from, let's say -10V down to -45V or so. That's enouge range to cover any tube type.
3: Put the tubes back in and bias them.
4: Stop pratting about with the sodding bias supply from then onwards.
This is all you need to do and then forget about it forever
I have installed 150k into diode (1n4007) into 15k then 56k (voltage divider) then into 50k trimmer to gnd
56k is not on the schemo. You must mean the 47k, which is part of the second leg of the divider. It keeps you from shorting to ground through by setting the trimmer to 0v. 150k + 15k is your one leg of the divider, 56k + 50k trimmer is the other. Think of the 150k and 15k as one resistor, and the 50k + 56k as a second resistor.
ve lowered the 150k to 133k but i need to go a little lower so ill try a 100k in there and it should be about right hopefully!!!
It doesn't matter what value resistors you put in the bias supply. The goal is that without the tubes installed, you can adjust the trim pot to give you something like -10v when it's one direction, and -45v when it's all the way the other direction. It doesn't matter exactly, as long as you have a range to work with.
The idea is that every individual tube needs a different voltage on the grid to flow the correct current at idle, in this case 41ma. An EL34 might need -39v on the grid, but a KT66 might need -45v grid to get the 41mA at any given B+. If you crank the B+ up, you will automatically flow current through the tube, so you lower it back down with more (bigger number) negative voltage on the grid. That's why you set the bias range like merlin said and the forget about it except when you bias tubes. You only use the trimmer because different tubes only need a slight difference in voltage.
2: Get the bias supply working so the bias voltage is adjustable from, let's say -10V down to -45V or so. That's enouge range to cover any tube type.
This is the way to do that:
Think of the 150k and the 15k as adding up to one big 165k resistor since they're all both in series. Then there is the separate 50k + 56k to ground. The series resistor and the trimmer make up the two legs of your voltage divider; look up videos or articles on the web, however you learn the best.
Your voltage divider is the 165k resistor and the trimmer resistors. Diode doesn't matter for what you're doing here. Ignore the trimmer for a second and just focus on what the 165k is doing. It sets the maximum voltage you can put on the grids of your tubes. If you put 150v- into the 165k from your transformer, you might have something like 120v- coming out on the other leg. If you make this SERIES resistor BIGGER, it lets LESS voltage (LOWER NUMBER) through to the divider. A 400k instead of 165k might give you something like 90v- instead of 120v-.
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Your SERIES resistor set the MAXIMUM voltage (BIGGEST NUMBER) that can be applied to the grid. and your TRIMMER resistor LOWERS the voltage down to 0v- from the maximum.
So, what you do is shrink the 165k to a certain value so you get around 50v- on the "output" leg. This is your maximum negative voltage from the bias supply.
Then you TURN DOWN this voltage by making the trimmer resistance SMALLER. When your trimmer is all the way up you have 50k ohms. As you turn the trimmer DOWN you get a SMALLER resistance. This lets more of your maximum voltage go to ground. This way you have less voltage going to the grids of your tubes.
No tube in this amp is going to need more than -45v or -50v on the grids so that's a good number to shoot for.
