Push/Pull vs two tubes parallel (Single Ended)

[54Goldtop]

Why does a Push/Pull amp (with two tubes) put out more wattage than a class "A" single-ended with the same two tubes wired parallel?

[vibratoking]

If you have to ask, you have to read. Whatever explanation you get here will most likely be incomplete. Check out the sticky:
http://ampgarage.com/forum/viewtopic.php?t=9023

[54Goldtop]

Not sure which article I'm to read? I am aware of the basic differences between class A, class AB, single ended, push/pull etc... I'm looking for something beyond "push/pull is more efficient" - Even some experienced builders can't seem to answer this question with much depth - HELP!

[matt h]

(deleted)

[54Goldtop]

I am aware of the biasing/voltage swing contribution to the wattage difference and realize this is a big piece to the puzzle - Is there anything else to be considered?

[matt h]

(deleted)

[54Goldtop]

Sounds like I need to do some more reading - Thanks for pointing me in the right direction... Could I trouble you to explain in a little more detail? What is the nature of the push/pull that you're referring to? I've read just enough to step on myself and look silly - I would appreciate any and all help.

[matt h]

(deleted)

[hans-jörg]

This is vague, oversimplified, and conceptual for the sake of brevity and my raging headache.

Imagine the voltage at your plate:

In SE, it's gonna be like a n (think of that looking like an upside down u)

in PP, it's gonna look like 0. (an n and a u mated)

From peak, to peak--voltage swing is double.

ascending and descending letters! Beautiful

[Merlinb]

Why does a Push/Pull amp (with two tubes) put out more wattage than a class "A" single-ended with the same two tubes wired parallel?

Because push-pull amplifiers can operate in class A, class AB, or class B. Class AB and B are more efficient, so the tube can safely tolerate a higher HT and lower load impedance than in class A, which means more power.

Single ended amplifiers are stuck with class A. Yes, OK, you can run a single ended stage in class B, but you would only be amplifying one half of the signal! A push pull stage amplifies both halves even in class B.

[xtian]

I was going to say, "Imaging pedaling a bicycle with both feet on one pedal." But then I was afraid the guys would tease me for making a long-reach, Gerry-style simile.

You know. Like water works.

[pdf64]

If operated in class A at the same conditions, 2 in parallel should put out the same power as 2 in push-pull.
Given the same power supplies etc, the push-pull version would have lower hum from the B+ (as B+ ripple, being common mode, would be largely cancelled out in the balanced output stage) and for the same frequency response could have a smaller cheaper lighter output transformer, as the static mag fields from either side of the primary cancel out.

Class AB allows the power tubes to be run hot for part of the cycle because they are also in cut off (ie cooling down) for part of the cycle.
So the time averaged plate dissipation can be within the limiting value.

[54Goldtop]

This last post from PDF64 is the sort of detail/explanation I was looking for and makes sense to me - But I'm new to all this so - Does everyone agree with PDXF's input?

[54Goldtop]

By the way Peter, I checked out your band and thought the guitar/amp sound was excellent - I couldn't make out what was being used for the amp - Is it something you built?

[pdf64]

I'm glad that was helpful.
Along similar lines, given a push-pull class A amp, its power output won't increase if the bias voltage is increased to put it into class AB.
Well, with a saggy B+, the instantaneous power output may increase some due to the static B+ rising, and even the continuous power output may be a little higher, as less current is being drawn.

Similarly, it's unlikely that it would be possible to turn a class AB amp into class A by reducing the bias voltage, as the limiting value of the tubes' plate dissipation would probably be reached before that happened.
Something else would have to change too, eg VB+ reduced, OT primary impedance increased.

Thanks for the positive comment, I'll have to edit the older videos to include that info.
I tend to use a few amps, a BF Fender based build, a JTM45 based build and a tweaked AC30, all generally through a FluxTone Model 10 1x12 open back.