imbalance in output tube current - fender deluxe reverb

[amplifryer]

Thanks for the ideas and suggestions. I didn't find any broken wires or connections (yet) but will double-check again today. I made some DCV measurements and it does look (to me) like the PI grids are the cause of the problem. I haven't quite wrapped my mind around how the LTPI works but from what I have absorbed from reading, 61V on V6B grid seems too much. I'm assuming the increased current through V6B is causing that plate voltage to sag to a level below V6A when it should be higher.

Not much published info on the PI grid voltages, but the little I did find indicates both grids should be close to the same voltage. I see that measurement error can be expected due to meter loading, but that would occur equally on each grid.

Does this sound like I am going down the right road?

[sluckey]

The grid voltages should be about the same. The actual voltage should be the same as you measured at the common node of the grid leaks, cathode bias, and tail resistors, ie, 67V. I suggest temporarily removing the .001 and .1 grid caps and see if both grids now read the same. I suspect the .001 may be bad and upsetting the bias on the V6A.

[bepone]

imo the problem is with 90% probability, like i mentioned before , in second grid 0.1-200V cap, or connections there

[amplifryer]

PI grid voltages are unchanged with both .1 shunt and .001 coupling caps removed. Additionally removed 820 ohm neg FB resistor with no change. Very strange....

[Stevem]

If you disconnect everything from pin 5 on each output and remove both output tubes do you see any residual dc or ac vintage there ?

How about with the tubes in the socket’s and just direct bias voltage fed to pins 5?

On the PI tube side of things is there any Stray Dc voltage on the grid with the tube removed .

If so then it could be the socket or the board leaking voltage.

[amplifryer]

It's fixed! There was a broken wire at the connection to pin 7 of the PI tube. The wire insulation was covering the break inside which was touching intermittently, especially with probe pressure.

Most of my measurements/experiments were conducted on the circuit board which likely did not disturb this faulty connection. Once I began to move wires in preparation of removing the tube socket for inspection, the break was discovered. I also had the circuit board "sandwich" separated to look for leakage paths.

imo the problem is with 90% probability, like i mentioned before , in second grid 0.1-200V cap, or connections there

Apologies and congratulations to bepone. Also thanks to all who provided guidance through this maze. I learned a lot (the hard way).

The power tube current is well balanced and the amplifier output looks good with symmetical clipping when driven hard. I have a question or two on biasing the outputs which I may ask in another thread here if I cannot find the answer(s) by searching on my own.

Many thanks! I cannot wait to enjoy this amp again.

[bepone]

imo the problem is with 90% probability, like i mentioned before , in second grid 0.1-200V cap, or connections there

Apologies and congratulations to bepone. Also thanks to all who provided guidance through this maze. I learned a lot (the hard way).

very goood, enjoy!