two bias pots vs one

[BobW]

This question probably has been discussed in detail in the past, and in the event it has, I apologie, but here goes:

Is anyone able to hear any significant difference (besides Scott Lerner of course ) when balancing the bias on both tubes to the same idle current draw vs. a few mAs of imbalance?

Two pots give you the capability to balance the current draw at idle, but am wondering if it really makes any real difference in say less than a 5 mA delta? I usually just swap tubes positions to compensate for circuit differences, live with the least imbalance, and go from there.

Since not all tubes have the same gain and OTs are not balanced, I'm not sure adjusting both sides exactly at idle accomplishes much if the delta using only one pots is initially small (< 5mA).

[Bob-I]

I've been back and forth on this one. As far as I can tell, if you have a well balanced OT, equal DC resistance the 2 bias pots won't make a difference. I have a Weber OT in my Twin clone and the difference in tone when the 2 bias points match is huge. I don't see any really difference in the AC output, just DC bias.

[drz400]

Current changes as signal is applied
so it only matters to get a perfect idle balance if you dont play the amp
Balancing the side via the phase inverter adjuster might make more difference until you hear some people like it better mismatched

[908ssp]

I like two bias pots because you can run unmatched tubes. With the cost of NOS tubes it would be nice to be able to bias tube individually. I could do that with my Ganesha and it worked great. My JTM 45 Bluesbreaker clone will have two bias pots GEC KT66 aren't easy to come by

[BobW]

Current changes as signal is applied

Agree, and it's non-linear on both tubes as well. They are matched only at idle. So, adjusting on the phase inverter side does make more sense, once the dc bias is close enough. What determines close enough? I don't know, but using a less than 5mA delta rule of thumb, as least for me, is close enough. At my age, I can't hear any difference or see any scope differences.

I like two bias pots because you can run unmatched tubes. With the cost of NOS tubes it would be nice to be able to bias tube individually

Well you can run unmatched tubes with only one bias pot too. If the bias delta is small enough it may not make a difference. However I can certainly agree with wanting to be extra safe when using expensive NOS tubes, but am not convinced setting both DC bias' equal implies the gains (current draw)will be equal.

[dogears]

LOL!

I like my seperate bias pots. I feel it is safer on my tubes, especially if I run them hot. So, I guess the answer is yes you can hear it, since you may be able to run bias levels that would be possibly unsafe if you had a single pot and not perfectly matched tubes. You'd be hearing the bias levels and not the dual pots fwiw.

Also, KT77 tubes have a much smaller sweet window. A delta of 5ma is noticeable. In 6L6 it is not, usually.

This question probably has been discussed in detail in the past, and in the event it has, I apologie, but here goes:

Is anyone able to hear any significant difference (besides Scott Lerner of course ) when balancing the bias on both tubes to the same idle current draw vs. a few mAs of imbalance?

Two pots give you the capability to balance the current draw at idle, but am wondering if it really makes any real difference in say less than a 5 mA delta? I usually just swap tubes positions to compensate for circuit differences, live with the least imbalance, and go from there.

Since not all tubes have the same gain and OTs are not balanced, I'm not sure adjusting both sides exactly at idle accomplishes much if the delta using only one pots is initially small (< 5mA).

[sdp1234]

Can someone point me to a schematic using 2 bias pots?

[BobW]

A delta of 5ma is noticeable. In 6L6 it is not, usually.

Scott, I also agree with that caveat. I should have indicated 6L6s in the mix and not all tubes. btw, I do have two bias pots in my latest build, was just looking for what is considered close enough when using one pot.
have a great 4th o July

[drz400]

Just want to make a note that a Matched tube at idle bias does not = matched tubes. The Gm (transconductance) should be very close from tube to tube, an individual bias control doesnt help you there.

[Johnhenry]

Beside's all the above, it'll let you run a mix of different tube's, on a 100 watter with dual bais pot's you can run 2 X KT66's and 2 EL34's with the bais set right on both set of tube's, and i have also did this on some 50 watt amp's, 1 X 6L6 & 1 X EL34, might not be your cup of tea, but like Dogear's Said, it make's it possible to run mismatched tube's.
Johnhenry

[David Root]

drz400's note about Gm is worth remembering. I use a digital power tube tester which reads Gm and current draw at 325 and 400V. Gm varies too and not necessarily in step with the current, although I have not seen the level of variance in Gm you can get in current draw in the same tube type.

[BobW]

Well it seems this thread took off in different directions, and considered all the possibilities why one pot isn't good.. I should have stated from the beginning, I wasn't considering one bias pot with a 4 tube output or a mix of different tubes, it was for a two 6L6 application only.

As drz400 and David root stated, the Gm doesn't always follow the dc bias setting, so setting the dc bias exactly the same on both tubes does not insure current draw from both tubes anywhere between 10% - 100% will be the same, anyway. Even if the both sides of the OT and long tailed pairs were exactly the same. That's why I initially suggested if one bias pot is able to put you in the delta of less than 5mA, then two bias pots doesn't buy you anything for this particular application.

enough said, the can o worms is open....