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In Reply to: Re: Thank you, but your amplifier is out of my range... posted by Guy on May 9, 2005 at 09:59:30:
and lovely VAC pa80/80 for sale in the Asylum's classifieds. Very tempting. There may go my prejudices!Thanks, all.
Yours,
Guy
Follow Ups:
NT
Nope. OTLs can operate in Class A, AB, or B, just like any transformer-coupled tube amplifier. Some do operate in Class A up to a certain power output and then flop over into AB mode or some variant thereof. Futterman-type amplifiers (e.g., Transcendent, NYAL, etc) operate in AB or B mode mostly.
Circlotron type OTL's operate in Class AB as well. Mostly B.
Using a circlotron with 8 6as7g's with 550ma. idle current each bank. When one bank rises to 1.1amp the other bank will be at or approching zero. That would give you 1.1amps across 8 ohms. That's about 5watts rms. From there on up, it's Class B.Tre'
Have Fun and Enjoy the Music
"Still working the problem"
Paul Speltz wrote a very interesting white paper on the operation of the circlotron that was available on the defunct ASOG website. According to his argument (based on actual experimental measurements), the Atma-sphere circlotron can go more into Class A than one would calculate by the standard method you have applied above. I am struggling to reconstruct his findings in my head, but I can't recall the gist of it at this moment. If the ASOG Tweaks section is available anywhere, you might be interested. Paul is a very smart, thoughtful guy and has no ax to grind one way or the other. But you are correct in general that the circlotron will eventually end up in Class B at the top of the power curve.
Hi Tre'I thought we had this discussion. FWIW Fisher made an amplifier back in the 50s that was class A2, like our stuff is. We can drive our tubes into cutoff, but at that amplitude the amplfier is clipping.
Now, some say that A2 is not 'true' class A, that's OK- its America. But A1 and A2 are designated that way by the engineering community for a reason...
As for me, if your post was meant to speak to my statement about the circlotron ending in Class AB or B operation at peak power, I am not qualified to argue, nor would I wish to. I will have to go back to my books to find out the differences between AB and A2 classes of operation. (I think in Class A2 the grids of the output tubes are driven positive.) I am thoroughly convinced that the Circlotron has most advantages vs the Futterman-type output stage in OTL designs, regardless of the semantics.
It's a matter of whether or not the current is moving equally up and down. Output tube bank #1 vs. output tube bank #2.
If the current goes up in one bank 600ma. from the starting point of 550ma., which it must for the amplifier to make________(you tell me) watts, the other bank can not go down 600ma. if it started at only 550ma.
In a true Class A amplifier the output stage current draw from the power supply does not change from no signal to full power. (Oh, it changes a little due to non-linearities in the tube, but only for that reason. Not because one bank has stopped going down because it has no where to go.)
If one output tube bank cannot reduce it's current at the same rate as the other output tube bank is increasing it's current, then the current draw on the supply will go up. And this is no longer the type of Class A as seen in a single ended amp or True Class A push pull amplifiers like the designs of Lynn Olsen or the Sun Audio stuff or any number of Class A Push Pull designs out there.
Have Fun and Enjoy the Music
"Still working the problem"
We just go by the measurements...If the tube does not cut off, its class A. If we drive the amp hard enough to make the tubes cut off, the amp is clipping a few watts earlier. IOW the tubes enter cutoff after the amplifier goes non-linear.
If the amp were to go into cutoff inside of its linear range, it would be a class AB2 amplifier. But it doesn't, instead that makes it class A2 according to the books.
The tube type is also very important. Its very difficult to ignore tube characteristics when designing any amp! If we used a pentode, this type of operation would not be so easy, not the least of which would be the grid current issue which is dramatically different then it is for a 300B or 6AS7G. 6C33s have a grid current character that is much closer to that of a pentode- these things can't be ignored even in theory.
Actually, the M60 is technically Class A all the way to 60watts. But that is only by definition and only because the 6as7g refuses to cutoff.In a true Class A amplifier there is equal current above and below the idle point. Zero total current change. One bank going up by X amount while the other bank goes down by THAT SAME AMOUNT. This would be with linear tubes like the DHT's. Biasing is done at the center point of operation. If the tube is biased at 60ma. it will move up to 120ma and down to near zero with very little non-linearity in that movement. The current draw on the power supply will remain almost constant. What is described in this paragraph in not what is happening in the M-60 above 5watts.
On the Atkins site, while describing the Vox AC-30 guitar amp and whether or not it's Class A,
"There is an equal amount of current swing above and below the quiescent point, which indicates an average change in plate current of zero, which is another characteristic of a true class A amplifie" He shows some nice graphs that explains what I'm saying.Tre'
Have Fun and Enjoy the Music
"Still working the problem"
nt
Have Fun and Enjoy the Music
"Still working the problem"
NT
nt
Have Fun and Enjoy the Music
"Still working the problem"
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