Anybody ever tried zero bias with the screen and control grid tied together?

[Austin]

There was a hi-fi amp that used about a dozen 12AX7s as output devices. I think it was one of the E.A.R series designed by Tim de Paravicini. In an interview he said that the 12AX7 grids had to be driven quite far positive to get enough output power.

So, I wouldn't be surprised if the grids were positive even at idle, which would make it "zero bias", and imply that he got the idea from the old ARRL handbook.

The amp got some pretty good reviews at the time.

All of these zero bias schemes have the disadvantage that the grid drive circuit needs to source considerable current. It is more like driving the base of a BJT than the grid of a tube. The signal from a guitar would be nowhere near strong enough, so I don't see how a "one tube amp" could be possible. If you want a one tube amp, use something like an ECL86, or 6BM8 or whatever.

In zero bias push-pull, driven by an interstage transformer, the driver circuit is always loaded by one grid or the other, so the distortion issue is not so bad.

I got the single tube amplifier idea from here, A 6C45 based Single Tube Single Ended Amplifier
It seems interesting that 7 watts output power is possible from a single 12ax7 dual triode in pp class b.. Another option might be a 12bz7 amp, similar to a 12ax7 but half the plate resistance. So with both sides in parallel the plate resistance is 15k. Would be cool to try the 6c45 in something, I been looking for an excuse to buy a couple as there are still being made I think.

[Austin]

Here might be a possible way to try it, cathode follower and positive grid bias, all grids tied together.