Push-pull

[Dave H]

I use the NH12AX7 model from here - PSpice

I've tried the model you posted and it has the same problem. In the cathodyne circuit it has the plate to cathode voltage as low as 20V when the tube is overdriven. I measure 100V for a real 12AX7.

[J M Fahey]

As a general rule, Wikipedia is a very POOR source of information.
Simple as that.

1) it's end user written. *Anything* can (and does) slip in unmolested.

2) only way something is revised if it's *way* too outrageous, but not otherwise .
So very poorly written articles stay there.

3) it's only "advantage" is that it's widely available, nothing else.

4) if anything, if an article lists sources, go get and read them instead, because they are presumed right but are often mis quoted or taken out of context.

5) even if an article is well written, sometimes it's somebody's "pet theory", only shared by few, if any.
As an example, you can find two completely opposite articles on capacitors, one "faith based" explaining the differences on "sound stage" or whatever based on the cap colour or price, while a "boring Engineer" writes "caps is caps" in his .... and both are "truth".

In a nutshell, Wiki is a very shallow and unreliable pool of knowledge.

6) this is the main point: in the waveforme you showed, *of course* both will be exactly the same, no matter whether the tubes are biased pure Class A or any degree of Class AB, from "almost crossover distorting" to "almost Class A", because no matter what each tube does or contributes, the transformer will work like a signal adder and "stitch" both signal halves together, no matter what.
What you *should* have plotted is current through each tube.
There you will find the "truth".

[Steve Conner]

The little circuit shown in Wikipedia is definitely a push-pull amplifier. Push-pull just means that the amplifier circuit is in two sections, one for each direction of current flow to the speaker.

You don't have to use a transformer to combine the outputs of the two sections. It just happens to be the most cost-effective way to do it in a tube amp that has to drive an 8 ohm load. But even with tube circuitry you'll find circuits like the Futterman OTL, White cathode follower and SRPP, that provide push-pull drive to a load without using a transformer.

Single-ended audio amps have to run in Class A for an undistorted output, but push-pull ones can be A or AB. Push-pull transistor amps can run in AB biased so cold that for all intents and purposes it's B. But if you tried this with a tube amp it would sound horrible.

The little Wikipedia schematic doesn't show any bias circuitry. This would make it Class-C with too much distortion for audio use.

[überfuzz]

Dave H - This is an explanation I found when I sniffed around the net. Maybe it could be improved according to you suggestions.