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In Reply to: RE: SET Class A posted by Naz on January 04, 2015 at 03:03:03
Totally agree but that has nothing to do with class A or not. I had a friend who built a class AB2 amp on a Stereo 70 chassis using 2A3 tubes(one of his favorites, liked them better than 300Bs) and got 18 watts per side.
Follow Ups:
I'm not sure what a push-pull amp has to do with the topic of conversation?
(I'm also scratching my head about how an ST-70 driver board could cope with 2A3 grid current)
Push/pull was mentioned because of the idea of deep class A. SETs are just class A. Deep doesn't have anything to do with it. My friend's 2A3 amp was mentioned as a push/pull triode example. It ran AB2 to get 18 watts and he only used the chassis and transformers. The front end was push/pull with 6SN7 tubes and I think 6SN>s.
I used to do that because I could buy ST-70's for about $150-$200.00.I put them in small-town movie theatres where the owner was on a tight budget, but wanted better sound.
I set them up for choke-input, and ran lower voltages & currents to keep the stock power transformer cool.
I got 11-12 watts/channel, which was plenty of power. I also sometimes used the common EL-34 output tubes that were stock with the amp. I just ran them at 25-30 ma. each. That cooled the whole thing down and made the tubes last.
Sound quality-- especially dynamics, was far better than a stocker-- which was a stressed-out amplifier for sure!
---Dennis---
Edits: 01/04/15
So , is there a sound difference between class A1 & class A2 ?
Is one preferred over the other ?
Thank you
saki70
ST-70 NOT a Class-A amp!
---Dennis---
It's not so much that there is a sound difference per se, everything is different. As pointed out, some tubes need to be biased in A2 to get any power out of them.
I prefer to build a SET that is biased in A1 as normal but is capable of transiting into A2 cleanly to provide extra headroom (which can be almost 50%). This dictates that the driver must have a very low impedance and deliver linear swing simultaneously with some current among other things. It's tricky but very worthwhile IMO.
I've tried a couple of tubes that run constant A2 and didn't like the sound all that much but there are way too many variables to conclude that constant A2 cannot provide great sound. A1 is definitely easier to implement well.
Naz
The problem that needs to be solved with A2 all the time is that the driver must always deal with current, not just on spikes. Very, very hard to find a good sounding way to have the constant current handling without a big power tube and a very expensive interstage transformers or by resorting to MOSFETs. Direct coupled cathode to grid driver is hard to get to sound due to the power requirements; it takes a constant 5 to 10 watts output to drive the grid current from a tube cathode driver. You need to build a pretty powerful OTL amp to do this with tubes. If not the amp will simply sound hard and nasty.
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
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.)
If an output stage is biased in class A2, the driver stage behind it has a much harder job to do. Biasing an output stage in class A2 can be required by certain valves, and in other cases can be a consequence of wanting to run a certain valve at lower voltages than it was designed for.
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