68 Princeton Reverb Bias Mod Question

[SBax]

Hi,

I would like to add a bias balance to this AA764 to keep the RCA power tubes as long as possible. It already has a bias level.

Existing Bias Level



I would like to add this mod. Does it look ok?



Also, should I raise the bias on the lower reading tube, or lower the bias on the higher reading tube?

Thanks.

[Jazz P Bass]

How is that supposed to make the tubes last longer?

[SBax]

How is that supposed to make the tubes last longer?

That comment didn't come out right. The power tubes have about 10 years on them and were fairly close when I installed them. Now they are about 8 mA apart. Instead of paying the current ridiculous amount for NOS RCA Blackplate, I'd rather balance them through this circuit mod.

[Jazz P Bass]

The tubes are wearing.
If you put the tubes in a test circuit, you will find that the one with the lower current draw will have a lower maximum output than the one with a higher draw. (regardless of 'bias' setting)
So in effect, the tubes as they currently sit, are not matched.
Which is o/k in a push pull setup.
By adjusting the bias you will not get the lower draw tube to output more voltage. (trying to 'match' them)
In fact, by biasing the lower draw tube hotter, you are beating it harder.
The whole concept of bias has most amp repairers totally confused.

[pdf64]

I don't think that the mod will work; the bias voltage on the modded leg will likely fall significantly as the trem intensity is turned above minimum.
Bearing in mind Jazz's point, I think that if you want to do this, the trem intensity control will need a dual track pot with a bias supply for each.
Pete

[SBax]

Ok, thanks guys. A dual track vibrato pot with a separate bias supply is too much and getting unreasonable. Theoretically, if I disconnect the vibrato pot out of the bias circuit, would that mod work? I realize only one side of the bias circuit would lower the voltage. But the other side would stay at an independant steady voltage regardless how I adjust the balance pot. Is that true?

[Enzo]

Just my opinion, but I think you are chasing a dream. These amps are not remotely precision circuits. Putting precision balanced tubes into them has no effect on all those other sloppy tolerance parts. it is even possible the tubes are aging slightly differently because one was stressed less than the other. ANother possibility is the one tube is already tired and is slowly losing emission, yes. But how long can you chase that?

[stokes]

What Enzo said,but I would add,in guitar amps you arent looking for strict,precise tolerances,might be okay for hi-fi,but in my PP amps I prefer a match no closer than 5ma's,to me it gives a better grind when you push those power tubes.