Individual bias pots in a Twin?

[greiswig]

Hi,

I'm interested in being able to mix and match tube types (outer pair EL-34, inner 6L6) or to accommodate 6L6's that aren't perfectly matched. Can anyone point me to a diagram that shows multiple bias pots used in a Twin-style amp?

Thanks!

-g

[Andy Le Blanc]

I'd go for two bias pots, for two pairs of tubes.

I built a bass rig some years back with individual controls for each tube.
The issue became balancing the available current for each and was a time
consuming procedure. Touch one and all the rest were changed.

[greiswig]

I'd go for two bias pots, for two pairs of tubes.

I built a bass rig some years back with individual controls for each tube.
The issue became balancing the available current for each and was a time
consuming procedure. Touch one and all the rest were changed.

Ooh...hadn't thought of that. Makes perfect sense, though. Okay...what is the trick to creating two bias controls?

[Structo]

I installed dual bias pots in my D'lite.

They work very well and I can balance disimilar tubes pretty well so far.

[ChrisM]

Good to know, thanks Tom for the diagrams. I just traded a pedal I built for a bunch of RCA Blackplate 6L6GCs and GE 6V6s, doubt they are matched though.

[tictac]

The key is to have an individual bias set resistor from each bias adjust pot to ground. I have this setup on my ODS clone and have no problems with one adjustment affecting the others.

I also have a rotary switch that allows you to plug in your DMM to a pair of test points and then select each tube you want to adjust. Make bias setting and monitoring a breeze...

TT

[greiswig]

Thanks, Tom...seeing it, it's pretty stupid simple. I should have been able to figure that out.

[Structo]

As it happens those values worked in my D'lite.
As long as you preserve the values you have.
So in the first picture you have 20K.

In the second there are two 40K but because they are in parallel it is 20K again.

You may have to adjust values for the twin but I have plenty of range on 6L6's in my D'lite.