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Class A Push Pull - how is this ?

kappclark

kappclark

AK Subscriber
Subscriber
Must be missing something here but I am wondering how a class A amp can be push pull ? I thought push-pull indicated class B -- Is this just way to say driver stage is class A and output/power is class B ?

I get this 'confusion' from looking at this schematic at oddwatt:
5751-SRPP-KT88-Push-Pull-Tube-Amp-Schematic.png
 
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Class a simply means that neither output device ever goes to 0 current.
Most push pull tube amplifiers are running class a most of the time if I had to guess.
Not that unusual at all.
 
Class of operation and push pull versus single ended are not related, other than the fact that single ended amplifiers can only be Class A.

CLASS A - Both tubes are conducting all of the time. Advantage is low distortion, disadvantage is low efficiency. Seen only in low powered specialist amps.

CLASS AB - Both tubes are conducting at small signal levels but one cuts off with larger level signals. This avoids crossover distortion, but also retains decent efficiency. This is the most common.

CLASS B - tubes biased to full cutoff, only the "top" or "bottom" tube is active at a given time. Suffers from severe crossover distortion and is unsuitable for hifi amps. Used in public address amplifiers and radio receivers historically.
 
Exactly what they said. AB is typically used just because you can squeeze more power out of the same tubes while still getting good overall distortion performance.

These are a bit of a strange design. The output stage is actually a differential amp, or a long-tail pair setup. Usually you see this as the phase inverter and the output from that stage feeds into the output tubes or whatever happens to live between the inverter and output tubes. The lower tube is running in grounded grid configuration, and input is via the cathode. I can see how this works with triodes, or even with pentodes, but with UL I honestly don't know. Seems to me you'd have two signal inputs, one at the cathode and one at the screen, and I don't honestly know what that would look like at the output. I'd have to wonder exactly how equal but opposite those are working. Likely would depend significantly on how accurate the taps are on the transformer.

but anyway, this sort of output stage has to work in class A otherwise there would be a portion of the signal that is not reproduced at the cathode and you'd end up with this ugly output that is clipped both top and bottom.
 
I think that many Class "A" push-pull amps may be Class AB. Anyhow, there are some well-known Class "A" push-pull guitar amps. My Matchless is advertised Class "A," EL84 push-pull (with cathode bias and no feedback). For various factors, it sounds and responds much differently than most Class AB1 push-pulls (with fixed bias and feedback).

The last part of Randall Aiken's "The Last Word on Class A" explains it much better. :)

https://www.aikenamps.com/index.php/the-last-word-on-class-a
 
Thx for the link -- very helpful .. Much more reading to do, and music listening, of course..

From the Gospel according to Aiken: (this was my very question - how can a push-pull be Class A ?
----
For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.
 
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JohnT123 said:
I can't make heads or tails out of that circuit. I shall head to the corner and don the pointy hat. Seems like something done by an engineer just because they could. :confused:
Click to expand...

Almost looks like a cathode follower on top 12SL7 half, with a concertina (cathodyne) phase inverter on the bottom half. With the bottom half of the sine wave grounded, then picked up by the grid of the lower EL84. No gain stage.

emoji - thinking (small).jpg
 
its actually a mu follower. Has reasonable gain actually, but basically its like one triode with a cathode follower.

nothing picked up by the lower EL84 grid. Its held at AC ground through the cap, the cathode swings relative to the grid instead of the grid swinging relative to cathode like a typical tube input.
 
gadget73 said:
Exactly what they said. AB is typically used just because you can squeeze more power out of the same tubes while still getting good overall distortion performance.

These are a bit of a strange design. The output stage is actually a differential amp, or a long-tail pair setup. Usually you see this as the phase inverter and the output from that stage feeds into the output tubes or whatever happens to live between the inverter and output tubes. The lower tube is running in grounded grid configuration, and input is via the cathode. I can see how this works with triodes, or even with pentodes, but with UL I honestly don't know. Seems to me you'd have two signal inputs, one at the cathode and one at the screen, and I don't honestly know what that would look like at the output. I'd have to wonder exactly how equal but opposite those are working. Likely would depend significantly on how accurate the taps are on the transformer.

but anyway, this sort of output stage has to work in class A otherwise there would be a portion of the signal that is not reproduced at the cathode and you'd end up with this ugly output that is clipped both top and bottom.
Click to expand...

Long tail circuit symmetry depends on how high is transconductance of tube used there. Power tubes usually do not have it high enough. Looks like this circuit needs a lot of global feedback to make distortion level acceptable.
 
for_p1 said:
Long tail circuit symmetry depends on how high is transconductance of tube used there. Power tubes usually do not have it high enough. Looks like this circuit needs a lot of global feedback to make distortion level acceptable.
Click to expand...


possibly the UL tap is creating some local feedback to make it more linear? Never experimented with one of these, just guessing.
 
gadget73 said:
its actually a mu follower. Has reasonable gain actually, but basically its like one triode with a cathode follower.

nothing picked up by the lower EL84 grid. Its held at AC ground through the cap, the cathode swings relative to the grid instead of the grid swinging relative to cathode like a typical tube input.
Click to expand...

Interesting design. Any advantages over "swinging" AC grid?

Thanks!
 
depends what you're doing with it. You get fairly low input impedance so its a bit less prone to noise pickup. On the down side you need more oomph to drive it.

mostly its grounded out of necessity on this one though. This is the same thing you see on a long tail pair inverter. The second grid goes to ground through a big cap for the same reason. Grid is held steady, cathode swings, and you end up with output at the plate. The cathode of the tube that gets a conventional grid input is a low impedance source, so its got enough snot to drive it.
 
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