Control grid Voltage / grid stoppers...

[jon_010101]

These are probably easy ones...

The amps in question are triode Williamson with KT66s... Radio Craftsmen 500a.

The control grid stop resistors for the KT66s measure a bit, um, out-of-spec ... one amp has 1k and 1.5k, the other is 1.7k and 3k. Since they don't carry current, does this even matter?

I measure ~2.5V and ~3.5V on the grids on one amp's KT66's and ~3.5V and ~2.5V respectively on the grids of the other. This is with the bias adjusted for equal cathode current (i.e., hopefully equal plate current considering they are triode-strapped). Is this "reasonable" (spec is 3V)? Or, do I need better-matched tubes? Or, new coupling caps (presently Jensen PIO)?

Advice/thoughts will be greatly appreciated...

All the Best,
Jon

 

[ray_moth]

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The control grid stop resistors for the KT66s measure a bit, um, out-of-spec ... one amp has 1k and 1.5k, the other is 1.7k and 3k. Since they don't carry current, does this even matter?

I don't think it really matters if your grid stoppers are not well matched. As you say, there is no appreciable current involved. Their purpose is to combat parasitic oscillations, Sometimes, they serve to limit current through the decoupling caps, in the event of the grid going positive for a short time, so as to speed up recovery from that condition. For output tubes, the value I've seen is between 1k and 4.7k. Others with more experience in this subject can probably advise on how to select the best value for stoppers.

I measure ~2.5V and ~3.5V on the grids on one amp's KT66's and ~3.5V and ~2.5V respectively on the grids of the other. This is with the bias adjusted for equal cathode current (i.e., hopefully equal plate current considering they are triode-strapped). Is this "reasonable" (spec is 3V)? Or, do I need better-matched tubes? Or, new coupling caps (presently Jensen PIO)?

It's quite common to find that the negative voltage on the grids (w.r.t. cathodes) are unequal, even though the cathode currents are the same. This is just the result of unevenly matched tubes and is the reason why it's advisable for fixed bias circuits to be individually adjustable. Self (cathode) bias does this balancing act automatically, within reason.

It seems, if I understand correctly what you are saying, that you might be able to achieve better matching by swapping the tubes around, so that the 2.5v tubes are paired in one amp and the 3.5v tubes are paired in the other.

 

[jon_010101]

It seems, if I understand correctly what you are saying, that you might be able to achieve better matching by swapping the tubes around, so that the 2.5v tubes are paired in one amp and the 3.5v tubes are paired in the other.

Ray, thanks for the reply... this is a pretty good idea which I will have to try. I guess it is still within +/- 20% so it's not too bad and doesn't seem to be affecting the sound quality.