Grid resistor balancing

[TonyM]

I have a 100w marshall clone I built with the help of amp tech 30 years ago.
There is a trimpot between each pair of output tubes connected to the grid via each 5.6k grid resistor. See attached photo.
I'm trying to understand the trimpot function, what its supposed to do and the procedure for adjustment.

[tubeswell]

Pull the output tubes out, set up a pair of V-meters with test clips (one each going to ground) from the output tube grid pins, switch everything on, and twiddle the trimpot to see how the bias voltage behaves (simultaneously on each grid pin). This will give you a better understanding of how the pot functions.

[stratomaster]

It looks like a way to bias balance between pairs on a single side of a push pull.

The ideal way to adjust it would be to put a meter across the cathodes of the two tubes set to mV.  Then turn the pot until you get to 0.

You'll have to measure the voltage across the 1ohm bias resistor for each tube to check absolute bias setting.  In theory this should allow you to use unmatched sets and match them per side to each other at least at idle.  Given that unmatched tubes will perform differently at different setpoints you'll likely have to bounce back and forth between measurements to ensure you're at 0mV difference at the cathodes after adjusting gross bias.

Pretty clever idea.  I don't trust tiny trim pots to serve as bias leaks however. Especially not with a cantilevered configuration like this. 

[TonyM]

Thank for your responses.

[pdf64]

As far as I can see, it seems ill conceived.
There's no VDC dropped across grid leaks / grid stoppers.

Unless using a vacuum tube volt meter, it's misconceived to measure V DC from grid to ground.
As a regular meter will add a significant path to ground that doesn't exist in normal circuit operation.

[tubeswell]

As far as I can see, it seems ill conceived.
There's no VDC dropped across grid leaks / grid stoppers.

Unless using a vacuum tube volt meter, it's misconceived to measure V DC from grid to ground.
As a regular meter will add a significant path to ground that doesn't exist in normal circuit operation.

Yep. Which is why I suggested measuring the effect of twiddling the pot on grid voltage. It’s too difficult to tell what the builder was doing just from looking at those photos without a schematic