Well-kept secret?

[benman94]

I hate to drag up an ancient thread, but I've had the opportunity to restore the Stromberg-Carlson AR-410 amplifier (albeit for someone else) that Tom Bavis mentioned. Mein Gott! Talk about a very nice sounding amplifier. And it measures much better than the typical 6V6 design from the same era. I was getting a solid, actual 10 watts out of it, cathode bias of course, with only 0.5% THD and negligible IM once recapped and cleaned up. All this with a rather shoddily designed 12AT7 input and phase splitter stage to boot! It does bust the plate and screen voltage specs, but not terribly. I would still expect acceptable life from the 6F6 outputs.

Most cathode bias 6V6 push pull amps produce 8 to 10 actual watts of audio power on the secondary, at 1% or more THD with ear-ripping IM.

I am seriously considering doing a partial clone of the Stromberg 6F6 amplifier. The input stage is less than ideal; a 12AT7 makes a pretty terrible voltage amplifier and a marginal cathodyne if you're after low THD. A much better front end would consist of say a 6SF5 or 6F5 DC coupled to a 6J5 or 6C5 running as the phase splitter. The 6SF5 is much happier with ~90 volts on the plate than a 12AT7 would be. You could probably get down around 0.1% THD to 0.2% THD at clipping with a similar, moderate amount of feedback and the front end I described. A 12DW7 would also work admirably in such an arrangement and would save a hole in the chassis and 0.3 amps of heater current.

The 6F6 should have lived on longer as it is an exceptionally linear pentode... once you bust the plate and screen voltage specs a bit.