Individual Bias Pots

[Enzo]

Tom I wouldn't assume that. In 100 watt amps with two bias controls - like those Marshalls - each control handles two tubes. It would be the same with a balance control.

You know, it i snot strictly speaking a balance control, but in many amps with dual bias controls, The procedure is not usually to adjust each side. They usually have you probe one test point and adjut the bias there, then you put the meter between the two test points and adjust the second control of zero. zero meaning the two currents were equal. SO you are balancing it in a way.

[TomCarlos]

Got it and thank you !!!

[patlaw]

Thanks for the discussion! Sorry to be late posting back here, but I didn't receive notifications of these replies.

[Delta362]

I just had one of these amps come thru this past month. I replaced the 15k resistor that attached between one out leg and the base of the balance pot with a 10k resistor and a 22k pot, leaving the balance pot in the circuit. It's a very simple mod.

[pdf64]

Patlaw....

Here is what Hiwatt has published. It shows the two control. I guess you can make a four control but I have never tried it.
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Just to note that another pair of coupling caps and grid leaks will be required, separate for each output valve control grid, so as to achieve DC isolation between them.
So the load on the phase splitter will increase, probably reducing its gain and max signal swing.

[TomCarlos]

Just to note that another pair of coupling caps and grid leaks will be required, separate for each output valve control grid, so as to achieve DC isolation between them.
So the load on the phase splitter will increase, probably reducing its gain and max signal swing.

PDF... Just to be clear... referring to the diagram, the options on the bottom for the "Improved DR 504" .... If an amp has four output tubes, would your preference be to take the single option (on the left) and repeat it four times or would you be ok with the double option (on the right) and build a second circuit?

[Helmholtz]

Result would be the same.
Note that more control pots require lowering the series dropper R (which sets the range).
In the example: 22k with single control, 10k with 2 controls, 5k with 4 controls. A different amp is likely to require different values.

But pay attention to what pdf 64 wrote:
With 4 controls/tubes you need to separate the paralleled grids on each side, requiring a separate PI coupling cap for each tube and 4 individual bias feed resistors.

The different resistors from lower pot end to ground (22k vs. 33k) with 2 controls must be an error.

[tedmich]

RV6 pots, 25k linear, locking! Thank you Apex Jr!

I am most assuredly ...biased.