Interesting symmetrical amp from 1955 magazine

[Wavebourn]

Here is a picture from Russian magazine, 1955
50W from pair of 6L6, with 6SN7 and 5SL7 tubes.
Symmetrical preamplifier/phase splitter, cathode followers to drive output tubes with grid currents, no GNFB

[Miles Prower]

The design could be improved by replacing that floating paraphase with an LTP. Although a CCS tail load and a negative rail would have been much harder to implement back in 1955 than it is today. Should have also done something about ditching one of the RC couplings, and adding bias adjust for the finals. Otherwise, pretty much how I designed an 807 amp.

Still, 6L6s without any NFB sound pretty nasty in pure pentode mode (been there; done that). Perhaps this thingy was an AM modulator?

Given the Q-Point, he wasn't gettin' no 50W from those 6L6s. The spec sheet says 26.5W, 30 if you allow for a bit of Class AB2 operation.

[Wavebourn]

It is AB2. Why they sound well, because of low relatively stiff screen grid supply. And it uses already -70V.

[Miles Prower]

Originally posted by Wavebourn
It is AB2. Why they sound well, because of low relatively stiff screen grid supply. And it uses already -70V.

Take another look at that schemo. It gives Vpp= 360Vdc; Vsgsg= 270Vdc; and Vgk= -25.0Vdc. According to the spec sheet, this is the operating point for Po= 26.5W; THD= 1.8%. It's not a 50W amp. If he's pushing it that far, then it SUX at high outputs. To go Class AB2 with this type requires: Vpp= 600Vdc; Vsgsg= 300Vdc.

Sure, using a regulated, Lo-Z screen source will help considerably the linearity of the finals. However, that does nothing for the tendency of the 6L6 to produce high order harmonics. These sound really nasty, like fingernails on a blackboard nasty. That's why I included both local NFB (10% of plate signal fed back as per O. Schade's recommendation) as well as gNFB when designing for this type.

I still suspect that this is the schemo for an AM plate modulator where linearity wasn't a big concern. Might also make a decent gee-tah amp.

[Wavebourn]

Yeah, for a gee-tah it would be nice, like a Quad-II that is very similar except feedbacks added.

For much better sound 6L6 want to see about 8K load. I use 2 pairs of them for 50W, with 375V plate, 4.4K load, 305V screen grid bias. However, with GNFB. They sound nice.

[Zen Mod]

Nestorovic Mioljub

Belgrade 1961.

[Wavebourn]

Originally posted by Zen Mod
Nestorovic Mioljub

Belgrade 1961.

This one looks better.

[Zen Mod]

Originally posted by Wavebourn


This one looks better.

certainly ;

Mioljub Nestorovic was a man responsible for Nestorovic Lab .

interesting - BudP (as known here at DiyA ) was his co-worker .

he certainly can say few more things about tricks and tips in Nestorovic Lab .....

[Wavebourn]

Here are more of ancient Russian pictures...

Guess, what is it?

[athos56]

4 broken links?