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Interesting symmetrical amp from 1955 magazine

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Wavebourn

Wavebourn

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Joined 2006
wavebourn.com
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
 

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Miles Prower

Miles Prower

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Joined 2005
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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.
 
Miles Prower

Miles Prower

Member
Joined 2005
dolphin-hsl.com
Wavebourn said:
It is AB2. Why they sound well, because of low relatively stiff screen grid supply. And it uses already -70V.
Click to expand...

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.
 
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