1965 Princeton AA964 Bias Problem

[plexi50]

I have a 1965 Princeton and put a set of new TAD 6V6GT power tubes in it. I can not get the bias to settle at -34vdc. At turn on the bias voltage is -35vdc and slowly drifts down to -34vdc. But then it keeps going south and lower in voltage. I have made adjustment values to the bias resistor many times and can't get these new tubes to settle at -34vdc. Plates are equal 418vdc. Screens are 416vdc. I don't have this problem with the original RCA 6V6GT tubes that i still have here for comparison testing.  A party just wanted new tubes.This is a first.

[Leevi]

Have you checked the cap (electrolytic) of the bias circuit?
/Leevi

[plexi50]

Have you checked the cap (electrolytic) of the bias circuit?
/Leevi

Yes all caps are new. One moment. I just see something. I see a 10K resistor where an 18K resistor should be. Back momentarily.

[plexi50]

I put the original RCA 6V6 power tubes back in and i have a bias of -34.2vdc. Is there a problem with the tubes being sold today or what? I understand there are slight differences in power tube build. But i don't expect to buy new tubes and have them not bias or settle eventually what bias is called for. What is up?

[pdf64]

It’s good to check bias voltage, but it’s far more important to check what its effect on anode or cathode current is.
-34V only applies when everything is bang on bogey / nominal.

[HotBluePlates]

... put a set of new TAD 6V6GT power tubes in ... At turn on the bias voltage is -35vdc and slowly drifts down to -34vdc. But then it keeps going south and lower in voltage. ...

I put the original RCA 6V6 power tubes back in and i have a bias of -34.2vdc. ... i don't expect to buy new tubes and have them not bias or settle eventually what bias is called for. What is up?

If you're monitoring voltage on Pin 5, then it sounds like the new tubes have some grid current that is making the grid voltage less-negative.

Give it some time & see if it settles down.  It only takes ~4.6µA to develop 1v across 220kΩ.

My tube tester lists ≤2µA as the spec for 6V6-style tubes, but not many testers directly measure that current.

[plexi50]

... put a set of new TAD 6V6GT power tubes in ... At turn on the bias voltage is -35vdc and slowly drifts down to -34vdc. But then it keeps going south and lower in voltage. ...

I put the original RCA 6V6 power tubes back in and i have a bias of -34.2vdc. ... i don't expect to buy new tubes and have them not bias or settle eventually what bias is called for. What is up?

If you're monitoring voltage on Pin 5, then it sounds like the new tubes have some grid current that is making the grid voltage less-negative.

Give it some time & see if it settles down.  It only takes ~4.6µA to develop 1v across 220kΩ.

My tube tester lists ≤2µA as the spec for 6V6-style tubes, but not many testers directly measure that current.

I expected that the new tubes would eventually settle. But they didn't. After many attempts at fiddling with 27K - 28K and in between bias resistor values i put the old RCA tubes back in and both settled at -34.2 vdc. I had the new tubes sent back.

[pdf64]

… i put the old RCA tubes back in and both settled at -34.2 vdc…

It’s certainly good for the bias voltage to be stable.

What was the resulting anode or cathode current?
With real world valves, for a given set of anode current, anode voltage, and screen grid voltage, we expect control grid voltage to be up to +/-20% of the bogey nominal level.

[Latole]

I agree with PDF

It is the cathode current that must be measured to confirm that the tubes have the right bias.
The negative grid voltage is only a means to achieve it but not an end in itself.

The "original" tubes may have had the right bias with -34 vdc but it doesn't mean that the new tubes will have the same bias

6V6 = let say 12 watts
Plate volt ; 418
12/418 = 28 ma X 50 % = 14 ma
At 50% it the best bias to me , some like 60%.

What do you read with you grid negative voltage ?

 Re-check plate voltage and adjust bias ief needed.

[Latole]

AA964 Princeton

To have more negative voltage

Replace 100K with smaller value
And/ or
Replace 27K with higher value
Check diode