Biasing Twin Reverb with 2 pairs of tubes

[rockman627]

I got a Twin Reverb , 135 watts with master volume that had a loud hum. The output balance and heater balance could not adjust enough to quiet it. It had 3 sovtek 5881wxt and 1 6L6gc. I had 6 5881wxt spares. Plates on v10,v9,v8 and v7 measured 435dcv. I put my dual bias into v8 and v7 and saw how the 6 spares measured and how the original tubes measured. I managed to find two that measured around 34mv and two around 27mv. I put a 34 and 27 in v10 and v9 and a 27 and 34 in v8 and v7. I was able to adjust the output and heater balance pots to nearly dead quiet.
It seems v10 and v9 are one pair and v8 and v7 are another. Is my tube placement correct? It seems 27mv is a little cold but I'm thinking 27 and 34 average out to about 30 of per pair. Does it really work that way?

[pdf64]

Does it really work that way?

Pretty much, yes.

Plates on v10,v9,v8 and v7 measured 435dcv.

That seems a lot lower than expected, normally would be 500V.
Can you recheck?
What voltage do you measure at the OT CT node, eg at the standby switch?
https://el34world.com/charts/Schemat..._135_schem.pdf

[g1]

Yes, that is how I would do it as well. The 'push' side ends up balanced with the 'pull' side. I guess 2 of the tubes end up working harder, but not much you can do about that without using a matched quad.

[Helmholtz]

OTOH, if you put the two 27mV tubes on one side of the OT primary and the two 34mV tubes on the other, it might be possible to adjust for equal currents on each side using the balance/matching control.
That way all tubes would run essentially at the same idle currents.

[rockman627]

That Twin won't be back until Tuesday. I'll have to check it then. I'm also going to see if Hemholtz' idea will work. Thanks guys.

[The Dude]

OTOH, if you put the two 27mV tubes on one side of the OT primary and the two 34mV tubes on the other, it might be possible to adjust for equal currents on each side using the balance/matching control.
That way all tubes would run essentially at the same idle currents.

^^^^^^Yes, that. That is precisely what the bias balance control is for.

[rockman627]

Today the measurements at the standby switch is 507dcv and all output tube plates are 505dcv. I think I got my notes mixed up. Sorry for the mixup.

[rockman627]

Yes , I got the two 34mv's in v10 and v9 and two 27mv's in v8 and v9 and was able to adjust for pretty low hum. Thanks.