Why was the 6AS7 dropped as a TV damper?

[maxhifi]

Originally Posted by benman94

Here's the article I was looking for from Williamson:

http://www.keith-snook.info/wireless...perlatives.pdf

Cathode feedback windings are great, IF you can drive the output stage. In the case of the typical triode or triode strapped pentode/beam power tube, cathode feedback windings just won't work.

In the case of the pentode/beam power tube, this is certainly an approach that deserves more exploration. But again, it comes at a cost, higher driving voltage at the grids and a much more expensive and unusual OPT.

That's a classic article. Given the number of McIntosh amps sold over the years, and the enduring popularity of the QUAD II, I wouldn't totally write off cathode feedback - but for someone building at home with off the shelf parts, it's not really a realistic option. I'm sure that Sowter could do a one-off, but get our your wallet!

Back to the original topic, I think the 6AS7G was used most widely by the military, in all sorts of regulated power supplies. In my years of collecting I have come across multiples of military surplus 6AS7G and 6080, but not even one in a consumer targeted box or package. I even built a regulated power supply using one, and it worked really well for what it was.

One strike against the 6AS7 for audio is it's not that linear - sure, all the second harmonic is cancelled by push-pull, so it still does decent with distortion measurements this way, but have a look at the characteristic curves, they are not evenly spaced at all, and lots of "knee" near cutoff. In comparison to a linear triode such as a 6SN7 or a 2A3, they are very compromised.