5f6a, Adding adjustable, balanceable, bias?

[Realist]

Greetings,
I'm building an 5f6a, and will need to redesign the fixed bias to adjustable/fixed. In addition, I was interested in being able to balance the bias between the O/P tubes. I had this the O/P tranny in an amp at one time and averaged 6 ma difference between tubes. I would have to switch them to achieve the lowest imbalance. (unmatched tubes)
Would the bias in question work to alleviate this problem? Thanks

[Chuck H]

Would the bias in question work to alleviate this problem? Thanks

Sure... At idle... When the amps not doing anything

The best matched tubes will operate the same, not just idle the same. Tubes that match at idle could be drawing very different current when pushed. I would think that two tubes of the same make and construction are more likely to operate similar when amplifying if they have the same bias settings even if they idle a bit differently. Change the bias so they idle the same and now if they were operating similar, they won't. You could test them with an input signal to see how much current each tube draws in operation. If they're off you could then choose to build an assymetrically adjustable bias circuit. Matched tubes are hit or miss for guitar amps anyway. Some amps sound much better with mismatched tubes. True. I wish I could remember where I read it but even Ken Fischer was an advocate for mismatched tubes. I think it's part of the formula that makes crossover distortion sound more like "swirl" than a buzzy rattle.

6mA is pretty far off but if your amp sounds good I wouldn't worry about it. As long as the tube that draws more current isn't drawing too much you won't hurt anything.

[Realist]

My concern is more with the O/T than the tubes. If the tubes were causing the mismatch the higher current would stay with the same tube. For example I could have 7ma mismatch, then reverse the tubes and have a 5ma mismatch. I presume a matched set would give 6 and 6.

[Chuck H]

Got it. Well... Have you tested all the components in the bias and PI circuit? If, for example, you have a cap leaking a little voltage onto a power tube grid that would skew the bias. If the voltage is correct I would think the current should be too. Otherise any imbalance in the OT would cause a dissimilar voltage drop across the higher current side.

[Realist]

That tranny was in an amp that I sold. I had replaced it because it had the incorrect impedance for that amp. The new tranny didn't exhibit the same issue, in the that amp. I am quite a ways away from having this new amp ready. Perhaps I should wait and see how it works out in the new amp. In the meantime I am reading 50.2 and 46.4 ohms to the center tap.

[Chuck H]

I am reading 50.2 and 46.4 ohms to the center tap.

That's normal. The two sides of the wind are usually different lengths because the wrap is smaller on the first half turns than it is on the second. The DCR is small enough to be negligable compared to the actual impedance and it's the number of wraps that make for accurate transformation. The imbalanced DCR reading should not cause a notable difference in current balance between the tubes.

[Realist]

Thanks chuck,