5g9 Tremolux, Need Bias mod Advice to bias each 6V6GT Separately

[nickb]

I you have such a wide spread in your spare tubes, then it's quite likely that the transconductance has a large variation too, probably due to wear. So, you can fix the the bias but they are still unbalanced because of the gain difference. This is a guitar amp so I suggest you chose a pair that are not so far apart and just enjoy the result.

I promise the world will not come to an end.

[peeler]

I have tested all the tubes I have on a Mighty Might and yes, it does not test for transconductance by giving values and is not providing much in the way of deeper tube health information but they all test well in that device. I also understand you can have perfectly fine NOS tubes that are very different in bias reading. I am shooting for 3ma-8ma range for a tube set as I've read it gives great results for guitar amplifier sound. And, I don't have enough of each tube type by the same manufacturer, materials and construction, to match them up as close enough. I do hear a difference in tubes of different types, enough so that I'd like to use two of the same tube.

[peeler]

Schematic found online for an adjustable bias pot mod for the 5G9 tweed Tremolux:

Bias Mod | Telecaster Guitar Forum

[Enzo]

Making it adjustable is simple, as we have already discussed. The problem is when you want to make each tube adjustable on its own.

[octal]

I think that grossly mismatched tubes is bad news with a bias vary trem; you're going to get thumping/subsonics. Having said that, if you really want to do this, you could break the cathode to ground connection of the hotter tube of the pair and stick a low voltage zener diode between the cathode and ground. You'll have to experiment with the voltage of the diode till you get the bias of the hotter tube equal to the colder tube. Remember, zeners are used backward compared to regular diodes, so in this case you'll connect the cathode of the output tube to the cathode of the zener and the anode of the zener to ground. Once you have the current matched, you can then use the regular bias adjustment tricks to get the actual current you want. (adjusting the negative bias voltage.)

[tubeswell]

What Bloomfield and nickb said

[Chuck H]

What Bloomfield and nickb said

Second that^^^^^^^^^^^^^^