Class A Push Pull - output transformer specs

[merlin]

I just want to see what PP Class A sounds like in person.

FWIW, at low volume a Class-AB amp is running in Class-A...

 

[Matador]

Here is a set of load lines around your pentode operating point (300V supply voltage, 275 screen voltage, and -12V bias point, idling at 25mA). You can see that so long as the input signal is fairly small, it follows the class A load line (the lower line). Not until the plate voltage of the tube that is rising with input signal and swinging below 200V does that side transition into class B (the kink formed by the intersection of the two load lines).



Now keeping everything else the same, you can extend the lower portion of the load line by biasing the tube hotter. Raising it up all the way to 70mA results in this composite load line (changing the bias to about -8V):


This tube would be screaming in pain: it's biased at nearly double the recommended dissipation (over 20W) and would not survive long (the dashed line is the 12W maximum plate dissipation curve). However the class A operating portion of this tube extended all the way to the knee of the curves.

You can drop the B+ down to around 250V (like Merlin recommended) and reduce the idling current to around 50mA to restore operation under the dissipation limits. Note output power is down about 60%:

 

[abbey road d enfer]

A good example of amps adevertized as class A is the VOX AC series.
Actually they are their own type of class AB. For a good part of their operation they are genuinely class A, with almost constant power draw, but above a certain level, the total current increases significantly, resulting in sliding bias.
As drive increases, teh cathode voltage also increases, which tends to reduce the idle current, so at high power, the amp is running nearly class B, which explains why the tubes survive.
Some attribute the "chiming" tone to class A.

 

[emrr]

A thing you don't see any more but was most common in class A PP broadcast monitor amps before say, 1955, is screen voltage higher than plate voltage. I recall higher screen helps long term stability, attracts more stray electrons out of the vacuum. I may remember wrongly, the old books describe it.

Fully PP amps like Langevin 117 or Collins 6N, 6X, etc are cancelling most of the 2nd harmonic, so they are the sound of 3rd harmonic dominance. Transformers make 3rd. 6V6 is strongly predominately 2nd harmonic, goes nicely with PP. See RDH3 pages 14-15 for comments on distortion.

 

[SoundBlaster]

I would like to point out that the requirements for class A are LESS stringent re: the transformer. As both valves are always conducting, there is no handover from one to the other when they transition to class B. As a result, the glitch that happens at that transition doesn't "trigger" the leakage inductance between the primary and secondary. If you have a transformer with poor coupling, Class A can "fix" it.. of course there'll be less power. And a lot of the glitch can be solved by feedback.. AC30 is an example.. while not totally class A, It carries class A pretty deep into that 30 watts. When it does crossover into "B" mode, it's so loud, you're probably looking for the distortion....

 

[SoundBlaster]

McIntosh amps are notorious for their tight coupling. Necessary for their class B operation.