[John L Stewart]

That circuit I put on Audiobanter is something I looked at about 10 years ago. I had seen in the old tube manuals back more than 60 years ago some rather odd power tubes. Odd to me, anyway.

These would include 6AC5 & 25AC5, a pair of very high mu power triodes. The others are 6B5, 6N6G & 25B5, 25N6G.

All of these are now quite expensive, I guess the collectors need them to keep the old stuff running. I wondered about simulating them with easy to get tubes. If one thinks of it, most power pentodes when run as triodes with G1 & G2 strapped together have quite a high mu.

I checked some 6F6s in that mode & found mu to be about 50, so OK for some kind of trial. If we check the old tube manuals we find that the recommended driver for the 6AC5 was the 76, a medium mu triode. The 76 is the same as the 6P5G but on the older base.

The trial circuit is quite simple, a 6F6 DC driven by the cathodes of parallel sections of a 6SN7GT. The anodes of the 6SN7GT could be either connected to the B+ supply, the OPT CT or the plate connexion of the 6F6. That varies the amount of NFB applied to the anodes of the 6SN7GT. The resulting reach thru to the 6SN7GT cathode is (1/mu). I’ve put the test circuit & results into attachments.

I never did build the complete PP UL circuit. I realized that the driver plate(s) direct to B+ version was used commercially in some radios in the 30s. If you look at the base connexions on the octal 6N6G & 25N6G they are identical to many common power tubes such as the 6F6GT, 6K6GT, 6V6GT & so on. In the circuit they are self biasing, no cathode resistor required. But they did not catch on. Probably more expensive to build these tubes than the corresponding pentodes with there two cathodes, etc.

Anyway, an interesting exercise.

Here are links to a family of tubes suitable for an all triode PP UL amp.
http://frank.pocnet.net/sheets/049/6/6AC5GT.pdf
http://frank.pocnet.net/sheets/127/2/25AC5GT.pdf
http://frank.pocnet.net/sheets/127/6/6B5.pdf
http://frank.pocnet.net/sheets/084/6/6N6G.pdf
http://frank.pocnet.net/sheets/127/2/25B5.pdf
http://frank.pocnet.net/sheets/049/2/25N6G.pdf

Trying to add some schema's now, see what happens!

Snow & Sun today, so I guess it will be the ski's. Looks like the bike is parked till Spring.

Cheers to all, John L Stewart