Push-pull

[überfuzz]

I'm simulation a power stage, push-pull class AB, in LTspice and have some questions regarding push-pull, class AB. Here's a plot of the signals going into the out transformer.


And here is wikipedias view on a push pull amplifier.

I don't get it, the power stage I simulate is supposed to be a push-pull class AB power stage, but it's clearly different. What am I missing?

[Enzo]

Please describe what you are asking. WHAT is different from WHAT?

I see your plot of two waveforms at opposite polarity, I assume representing the signals at either end of the output primary winding of a tube amp. Then you link us to a very simplistic digram of the basic push pull transistor output stage.

I will accept your claim the tube amp is operating in class AB. The transistor amp is clearly operating in class B. The tube amp is driving a center tapped output transformer and has a phase inverter to oppose the polarity at the two ends. The transistor drawing has no transformer and does not use phase inversion.

[überfuzz]

So wikipedia has got a illustration of a class B amplifier listed as a class AB?

[Enzo]

I may be missing the obvious, but I don;t see "class AB" on the wiki page. In fact there is precious little on that page. As far as I can see it only says "push pull."

The wiki circuit as drawn would work poorly, it is just a simplified drawing to demonstrate the push pull concept.

[überfuzz]

The illustration is listed as "Class-AB push–pull amplifier" in the main article.

[Enzo]

OK, I admit dfefeat - where is the main article?

[Dave H]

If you are driving it with a sine wave it should be out of phase sine waves at the ends of the output transformer because that is the signal out of the PI to the power tube grids. LTSpice generated the plot below when I tried it.

Push Pull.pdf

[potatofarmer]

OK, I admit dfefeat - where is the main article?

It appears to be this: Amplifier - Wikipedia, the free encyclopedia

[überfuzz]

Sigh!! I am an engineer and tend to use Wikipedia as a quick source at the job. No double check has ever revealed errors. Now, for fun, I try to get a more fine grained idea of how tube amplifiers work. -There is so much BS on the net it's staggering! It seems even my trusted friend Wikipedia. :-/

[überfuzz]

If you are driving it with a sine wave it should be out of phase sine waves at the ends of the output transformer because that is the signal out of the PI to the power tube grids. LTSpice generated the plot below when I tried it.

Yeah... I kind of got that part covered. Wikipedia only nudged my perception of what a push-pull set up does. :-) Do you tinker a lot with LTspice?

[Enzo]

Thanks PaulP, I did not see a link to that article.

Uber, that isn't a schematic, it is a skeletal representation of signal relationships. Look at the Class A example a couple images higher. There is no indication that any biasing would be needed or that DC would need to be blocked from the output. The drawing only is there to show that the full wavefor going in is also what comes out - though at higher amplitude and reversed polarity.

Now drop down the the class B example. There they drew nothing but a transistor. The output at the emitter shows that the signal has been rectified, but there is no clue as to how it went about that, unless they just used half the transistor as a diode.

And again down to the Class C example. Note that the "schematic" for the Class C is identical to the one in Class A. A bunch of other stuff needs to be there to have a real circuit. The difference between the class A and C situations would involved differeing stage bias etc.

These are not actual circuits. Now look at the very first image - it even calls it a practical circuit. THAT circuit would work if you built it. It is an actual circuit. But the others are more loike block diagrams, only intended to show the relationship between input and output waveforms.

[Dave H]

...Do you tinker a lot with LTspice?

I don’t use it all that much. I used it years ago to check out their switch mode power supply ICs and I have my three tube guitar amplifiers drawn up which is how I could quickly generate the Push Pull plot. I found out recently that some of the 12AX7 models on the net are not accurate but it is fun to play with. The plot below is of the speaker output and current in one half of the OT of an AC15/30 type amplifier showing it’s not class A as the tube is cut off for part of the cycle.

AC15.pdf

[überfuzz]

Enzo - I wasn't really looking for missing parts, more understand the wave forms. Seems more class B to me, than AB. I found one of my old school books, it had a more detailed explanation. Thanks!

Dave H - Well the tubes are approximations, for obvious reasons. Anything in particular you think should be adjusted or altered? Apart from the cut-off being ill approximated I think their quite all-right.

[Dave H]

Dave H - Well the tubes are approximations, for obvious reasons. Anything in particular you think should be adjusted or altered? Apart from the cut-off being ill approximated I think their quite all-right.

It was only the minimum plate to cathode voltage I've found so far, otherwise the simulation looks the same as the scope.

[überfuzz]

Dave H - What model do you use, I mean 12AX7 model? Lately I've used this out of convenience, I just copy paste it.

.subckt 6AV6 Ax Gx Kx
Ga 0 a Ax Kx 1
Ca 0 a 1n Rpar=1
Gg 0 g Gx Kx 1
Cg 0 g 1n Rpar=1
Bak Ax Kx I= 340n*log(1+uramp(V(a)))*
+ ((75+V(a)+97*V(g))/(1+uramp(V(g)/-8)))**1.4
Bgk Gx Kx I= 5u*uramp(V(g)+0.2)**1.5
Cak Ax Kx 0p5
Cag Ax Gx 1p7
Cgk Gx Kx 1p6
.ends 6AV6

Well, now I guess I have to look it through.