Individual Variable Bias on all Four Tubes

[talbany]

The 100k bias feed resistors...
Grid bias feed resistors from 220K to 100K, decreases the time constant of the AC coupling to the output tube grids.. helping to prevent a transient distortion known as blocking distortion..also changing the high pass shelf..... Gotta run the #'s
It might be a good idea to use the 100k's for the bias feeds to speed up the time constant of the individual bias supply's as well..Especially w/ .1 couplers.....Just a thought

I have used split bias adjustments on all of my amps. One thing that you have to make sure is that the 220K grid resistors (bias feedpoint) are at AC ground. If you dont do this then you create a variable high pass filter for each output tube (due to the action of the bias set resistors). Adding a small e-cap at the bias feed to the 220K's to ground will correct the issue. The only downside is that you need to make small adjustments as the caps take some time to charge.
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Early on tried it with caps in and without and couldn't really hear any audible difference... Gary have you run the numbers on that.. Any idea as to what frequency range were talking here...I know it varies depending on pot setting..The caps are a good idea...

Tony

[Bob Simpson]

I'm not sure I follow what Gary is saying.

Where do you put these capacitors and why are they necessary?

I'd like to know about this as well...

Do you have a picture/drawing?

Bob Simpson

[glasman]

Here is an example.

Note that is very similar to what I do in my own amps. The actual bias feed is pretty high impedance in this example but illustrates the point.

C8 and C9 are the caps I am talking about. If they are not in the circuit, the grid to ground AC resistance can range from 253K to 353K ohms. With C8 and C9 in circuit, the grids always see 220K to AC ground.

The 100K -33K allow for a WIDE range of tubes to be used. About -25v to -65 volts. 25 turn trimmers.

Gary