Fixed Bias Issue

[passfan]

I have a new build of a Naylor SD60 circuit and I have a bias issue. When I check the fixed bias on standby it's the same -56 volts all the way out through the grid resistors to each output tube. When I flip off standby I get 33ma's at one tube and 99ma's at the other with no more than -3 volts difference between the two on the bias voltage. I can also hear the amp loading down coming off standby in the form of a hum with a slight change in pitch as the amp settles in.

) I have swapped to a different set of output tubes, no change
)swapped between the pairs , no change.
)re wet all the solder connections on the socket in question.
)verified the grid and screen resistors.
)pulled the power tubes completely and checked pre amp plate voltages and bias voltages. No load down when the output tubes are out.

It seems to stay with the socket and only when a tube is present. At this point my thoughts are to change the socket out. I'm wondering if anyone has experienced this before.

[ampgeek]

A bit of a head scratcher for sure!

Are your current sensing resistors fairly well matched?

Likewise with your power tube grid leaks.

Anything odd or unusual around the PI coupling caps?

I don't think that I have ever seen the bias voltage differ by more than a fraction of a volt so 3'ish sounds a little high to me. Of course, it is hard to point at that as the root cause with a 3X current flow imbalance!

Good luck,
Dave O.

[Firestorm]

Screens? Cathode connections?

[passfan]

Took the one ohms out and grid leaks are 4.7 and 4.8 k respectively. We'll see how it reacts when I change the socket. I used older cinch sockets out of hammond organ chassis's. I'm wondering if it has some carbon tracking somehow.

[Stevem]

If the amp has a feedback circuit than unhook it and than swap the OT primary leads on the tubes, does the problem stay with the primary wire?

[passfan]

Going to check this afternoon. First is swapping the PI tube out with known good. Then jumper the plates together at PI and lift feedback to check for oscillations. Then I will swap primary wires to check OT. Will post results this evening.

[passfan]

Removal of feedback killed an oscillation in the presence circuit. Going to try Martins presonance circuit.

[TUBEDUDE]

So the socket swap solved the current draw issue?

[passfan]

Presence was oscillating through one side of the PI and driving that one output tube to load down at idle. As the output tubes load and unload the bias changes. Didn't need a new socket. I simply lifted the feedback off the impedance switch and it all went away

[TUBEDUDE]

Congrats.

[passfan]

Now I'm trying to figure the source of the oscillation and I discover the plate wires are crossed and need to be switched. Still have the stock presence circuit in and it is still trying to oscillate so I went to the plexi presence circuit. All is well now. I guess the Naylors presence circuit is so complex it drove the original plate mixup to a frequency that couldn't be heard. I like things like this. They make me learn