Biasing and Tube Selection

[lowell]

If I have 4 power tubes: 2 cathode biased via 1 resistor, and other 2 fixed biased, is the quiescent bias point of pair A affected when the cathodes of pair B are lifted, and vice-versa? I lent my amp to someone for a month so I cannot just throw it on my bench..

[Bruce / Mission Amps]

If I have 4 power tubes: 2 cathode biased via 1 resistor, and other 2 fixed biased, is the quiescent bias point of pair A affected when the cathodes of pair B are lifted, and vice-versa? I lent my amp to someone for a month so I cannot just throw it on my bench..

Hmmm ... I would say yes but only so much as the B+ rail is unloaded with only two tubes and the plate voltage rises, which will result in the remaining two tubes operating slightly under biased and hotter.
Of course the other minor issue is the the speaker load now needs to be connected to the next lowest impedance tap.

[sean k]

I have no idea except to say that if one set is cathde biased then the voltage in the tubes will be B+ minus the voltage on the cathode whereas the other set will be B+ plus the fixed bias voltage. so if B+ is 350V ones at 330, both with bias at 20V, and the others at 370... inside the tubes.

Would the voltage change in one set of tubes affect the other set of tubes?

Good question! I'd like to know if it would and why and even why this is done.

The first amp I designed from scratch, the only one so far actually had a set of 6AK6 tubes in class AB1 P/P and I did both cathode and fixed bias... so whatever the cathode was fixed at I could add a little negative voltage to that.

So what am I saying. No Idea except I'd like to follow this thread. Sorry.

[lowell]

Ok, so it seems it most likely not an issue, and I'll check for sure when I get my amp back. If it's too much of a bias change I'll just back off the quiescent current a bit in full power mode.

Sean K this is in regards a power tube selection switch, via lifting the cathodes of either pair... I failed to mention that I have 6V6 cathode biased, and 6L6 fixed bias. It is nice to be able to select them.

Right now the amp is wired so the grid signals cancel via a switch, but the switch is $10 and takes a good while to wire it up. I want to save a bit of $ and time since I'm making a few of these things.