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In Reply to: RE: Very good point. posted by sser2 on March 03, 2017 at 14:00:55
"In a typical AB2 tube stage, drivers output impedance should not exceed 500-600 Ohm. This can only be achieved by a combination of low Rp PP driver stage AND a step-down interstage transformer."
It can also be accomplished using cathode followers reactively coupled to the output stage with chokes (or a center-tapped choke). Altec did this in the 1570B, and I used it for my A2 211s.
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Thanks for posting this. A very specialized case.
The output stage works in pure class B2. 811 tubes (not very linear, to put it mildly) are biased at about zero volt. Although cathode follower has low output impedance, 6W6 triode connected, at 25 mA cathode current, will have difficult time delivering the calculated 42 mA peak grid driving signal current and 1.5 W driving power required for full output power of this amp. Tons of power, but a lot of distortion. Probably was intended as PA amplifier for sporting events, like Daytona 500 races.
Not to get into a long-winded discussion about this, but ... :)
I have to disagree regarding the 811A. I used them for years as linear amplifiers in audio-modulated RF service (single sideband). Their IM and THD are not significantly different than other tubes. As for the 6W6, I haven't used them myself as power drivers, but they do have huge peak current capability due to the robust heater design. That's the same reason I used a parallel-connected 6BL7 follower to drive each of my 211s. Can't speak for the Altec, but in my case, distortion measurements indicate no perceptible change as the output tubes enter A2. Distortion goes up gradually with increasing power, but there's no "step" at A2. That tells me the low output Z and power delivery of the followers is at least the equal of an IT.
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