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In Reply to: RE: A2 all the time posted by Chip647 on January 05, 2015 at 12:31:12
Yes, agreed. I built a driver that would handle the requirements, which a scope and distortion measurements confirmed.
My last attempt at constant A2 was with 805s since I had everything I needed. Sound was good and I'm sure many would have thought more than acceptable, it just wasn't great like my 845s.
Many possible reasons. The curves for an 805 aren't fantastic and I'm sure that's why GNFB seems to be commonly used with this tube. I never use it but tried it for comparison purposes. The interesting thing is that although it certainly greatly assisted linearity it murdered sonics and I MUCH prefered the sound of zero NFB despite the increased distortion.
I still prefer biasing in A1 but designing with clean transition into A2 on peaks as necessary, generally brief.
Naz
Follow Ups:
Have there been any reports of success with the 811A running at low anode voltage? I don't mean whether it simply works (amplifies music), but whether this topology can compete with A1 amps like the 211/845 for measured performance and sonic qualities. I have a stash of NOS 811As I'd like to put to use in the future, but maybe they're really only useful for RF.
--------------------------
Buy Chinese. Bury freedom.
That would be the WAVAC "Music Dandy".From Nobu Shishido-- Japan.
The 811A required a power pentode to drive it-- he used a 6L6GC, or other similar, such as KT-88, EL-34, etc.
He ran 450 plate volts on the 811A plate, about 80-85 ma., and it was reverse-transformer (step-down) coupled-driven. The output transformer was conventional-- nothing fancy-- you could run about 4-to-5K.
Of course, I owned one. It was clean, fairly wide-band, lacked ultimate soundstage depth, somewhat scrambled together musical events (nowhere close-- as bad as nearly all other tube amps!), ate the tubes that drove the mid-stage power-driver for lunch every few weeks (what drove the 6L6, etc.). When healthy (new tubes!) it had 15 watts-- easily. The 811A never got stressed-- you could run it forever.
Of course, when I see something like this, I ask myself: "why not just run the 6L6 midstage right into an output transformer, and drive the speaker with that? Why add another stage"?
And there you are! One should try to be practical after all....
---Dennis---
Edits: 01/06/15 01/06/15
How does an analog amplifier, an SET at that, "scramble musical events"?
What type of memory circuit is being used to store the out of time events?
I did a bench build with the Eimac 100TH biased to +20v. It was very cool looking. It sounded really clean but I was limited by the OPT. What the world needs is a 15k, 40 watt single ended OPT. :-0 (It would probably weigh 40 pounds.)
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