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Old 02-08-2018, 01:14 PM
benman94's Avatar
benman94 benman94 is offline
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Why was the 6AS7 dropped as a TV damper?

The 6AS7 ultimately ended up being a tube without a purpose after TV manufacturers (including RCA) stopped using it as a TV damper, after RCA stopped promoting it as an audio output valve to which it was very well suited (arguably better for high fidelity applications than the 6L6), and stopped being used as a pass element in regulated power supplies.

It was in all probability shunned by RCA for audio use because it would directly compete with the 6L6 which RCA had been promoting since 1936. Still, a push pull 6AS7G with cathode bias could produce 10 watts or so at only 2 percent total harmonic distortion, right in line with a pair of cathode biased 6L6s while remaining about as efficient as a pair of 2A3s, 6B4Gs, or 6A5Gs. Add in that the distortion characteristics of a triode are much more benign, and the plate resistance so much lower, than the 6L6 and the 6AS7G looks rather attractive by comparison. The grid swing necessary is ungodly, but it is possible to drive a 6AS7 in push pull with itself to clipping with a single 6SN7 R-C coupled to the 6AS7 grids.

Take a look at this schematic from RCA's Ham Tips, circa September-October of 1948:

http://n4trb.com/AmateurRadio/RCA_Ha...amtips0803.pdf

Also see this speech amplifier developed in the year prior:

http://n4trb.com/AmateurRadio/RCA_Ha...amtips0704.pdf

Clearly the 6AS7 works admirably in audio service; RCA didn't abandon it as an audio tube for any practical reason relating to the valve itself.

In pass element service, the 6AS7 was simply surpassed by even higher perveance tubes. Why use two or three 6AS7s when one of the newer types will suffice?

But in TV damper service, I see no logical reason to choose a 5V4G or any of the dedicated 6 volt damper diodes over the 6AS7. Any ideas?

Last edited by benman94; 02-08-2018 at 01:29 PM.
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