Fixed bias for double single ended?

[H Chandler]

Does it make sense to used a fixed bias scheme for a double single ended amp, or for a big output tube that draws a lot of current? Since I'm using a PT with a bias tap anyway, wouldn't it make sense to fix bias, say a pair of 5881's, rather than dealing with a large wattage cathode resistor, wasting I^2R energy, and heating up the chassis? Is there anything I'm failing to consider?

On a single ended, would this affect the drive, and clean overhead like in a P-P amp?

Thanks,

Howard

[Don Symes]

While I don't have an answer (surprise), I do have a couple of suggestions.

If there's room in the chassis, give each output tube its own bias pot (and grid coupling cap to the driver) so you can try mixing radically different tubes - a 6V6 and a 6550, say.

Use Big, Chunky turrets to connect the bias feed resistors, and run your outputs' cathode connections through enough other Big Chunky turrets that you can fiddle with mixing fixed- and cathode-biased tubes in an output.

I think the Hi-Fi guys call that setup Parallel-Single-Ended, if you're looking for an adjective.

Hope this helps!

[MWJB]

Yes, there's no reason not to fixed bias a SE amp. It will tighten up response and the front of the note won't compress off (if that's what you're after).

Heating the chassis via the cathode resitor isn't really a consideration, you're likely to be running those 5881's around 50-ish mA, which is still going to generate lots of heat if they hang under the chassis.

You should still have plenty of drive (crunchy distortion) being SE, P-P cancels a lot of these crunchy harmonics in the output stage.

[H Chandler]

Assuming a champ circuit then, would I simply build the little bias board like found in the BF Pro and apply the bias voltage to the grids of the output tubes?

[MWJB]

You also need to ground the power tube's cathodes, do this via 1ohm 2W resistors (one per tube) for easy bias reading. Why not fit a fixed/cathode bias switch...Kevin O'Connor has schematic on his website?

I would use the 5F6A style bias circuit, make it adjustable by using a 50K multiturn cermet pot in place of the 56K load resistor, add a fixed resistor in series to ground (33K-39K?). Then, if necessary, fine tune the value of the 15K dropping resistor to give you the sweep you need (bigger = more current, smaller = less current).

When setting up for the first time, power up without the power tubes, then set the negative voltage at pin 5 to -50v to -55v before installing power tubes and fine tuning the plate current.

[Bruce / Mission Amps]

One of the most usefull SE amps I ever built was a tweed Princeton 5F2A type amp in a universal chassis with adjustable fixed bias, a 100ma PT and a 10 watt 5K OT.
Why?
I used that amp to match all kinds of power tubes that used the same pin out as a 6V6-6L6-KT66-EL34 etc., ... using that adjustable fixed bias, I was able to come up with very usable data to match power tubes.
You should do it without hesitation.