Push/Pull Pair Removal Theory & Questions

[nickb]

we're discussing a P-P configuration, so where are the composite load lines?

those charts show 400V B+ in both conditions, and all of the calculations are derived from an assumption that B+ never changes. With a real world transformer B+ will increase as you move from a full-load to a half-load condition.

granted, if the transformer has a good regulation spec then the difference may not be much, but if it's got a poor regulation spec (like most MI transformers) that change could be significant. it's probably worth taking that into account and using a new tube characteristic with the proper B+ when doing the comparison. and while we're at it, we should also be looking at composite load lines.

It is a composite load line - it's a curve. I expect you are used to seeing a sharp break in the composite curve. It's just not like that in real life.

The higher B+ with just two tubes will increase the power out even further and therefore strengthens the case.

[SoulFetish]

To be even clearer, tubes have internal resistance/conductance which appear in parallel with the load. But, I though I was clear about being overly simplistic in case what LT said didn't seem to make sense. Did I explained P=R*I^2 incorrectly?

[nickb]

To be even clearer, tubes have internal resistance/conductance which appear in parallel with the load. But, I though I was clear about being overly simplistic in case what LT said didn't seem to make sense. Did I explained P=R*I^2 incorrectly?

The problem was not that the math was wrong but that it appeared to support the "removing two tube quarters the power" view. I think it's important to be clear that it's not the case.

[SoulFetish]

It is a composite load line - it's a curve.

I noticed this the interactive datasheets. It's a nice touch. I understand the transition from class operation to be smooth. How did you calculate the shape of the curve in operation for the graphic?

In the second load line you drew to illustrate your example, wouldn't this destroy the EL34s??

[nickb]

I noticed this the interactive datasheets. It's a nice touch. I understand the transition from class operation to be smooth. How did you calculate the shape of the curve in operation for the graphic?

In the second load line you drew to illustrate your example, wouldn't this destroy the EL34s??

I think that is beyond the scope of this discussion. Suffice to say it's an iterative approach.

The point about using two tubes vs four without changing the output impedance causing excessive dissipation is valid has already been made several times above. In my view, it the reason that changing the OT tap is the better option. Reliability should be the #1 consideration. No-one's gonna cheer you if your amp's dead.

[SoulFetish]

The problem was not that the math was wrong but that it appeared to support the "removing two tube quarters the power" view. I think it's important to be clear that it's not the case.

hmmm, now this seems counter intuitive to me. I'm not saying your wrong, Nick. But, if removing 2 tubes into the same external load doesn't quarter the power, there must be another significant resistance... I need to think about this.

[SoulFetish]

The point about using two tubes vs four without changing the output impedance causing excessive dissipation is valid has already been made several times above.

Fair enough.
But am I expected to actually read these threads? Have you seen the length of bob p's posts?? "brevity is the soul of wit", brother.
(just kidding, B. you're always a good read)

In my view, it the reason that changing the OT tap is the better option. Reliability should be the #1 consideration.

Word. That would be my concern as well.

*But, I would still like to know how you coded that page someday.

[nickb]

hmmm, now this seems counter intuitive to me. I'm not saying your wrong, Nick. But, if removing 2 tubes into the same external load doesn't quarter the power, there must be another significant resistance... I need to think about this.

The problem comes from the fact the the tube cannot be modeled as a current source in parallel with a resistance for anything other than small signals. Just look as the plate VI curves, they are not straight lines.

[Malcolm Irving]

...
In the second load line you drew to illustrate your example, wouldn't this destroy the EL34s??

I'm not saying they would necessarily survive in this particular case, but keep in mind that in push-pull each side is cut-off for half the time. This means that heat created during the 'on' time period can be radiated away during the cut-off period.

[nickb]

I'm not saying they would survive in this particular case, but keep in mind that in push-pull each side is cut-off for half the time. This means that heat created during the 'on' time period can be radiated away during the cut-off period.

Quite so. Also, I think the reason the "stage experienced guys" get away with it is because sound pressure for music, especially in this live case, is a normal distribution so the tubes spend most of their time at the low current end of the line.

Unless you're a shredder like Cory of course....

[SoulFetish]

Unless you're a shredder like Cory of course....

A recovering shredder.... but I've been know to fall off the wagon from time to time

[nsubulysses]

playing as fast as possible is really fun even though most of the time it doesn't sound good

[bob p]

^^ BB King rolls over in his grave.

[bob p]

It is a composite load line.

No, it's not.

We all know that single-ended load lines can't be used to model the behavior of a push-pull pair.

[Malcolm Irving]

Nick’s curves are a ‘composition’ of the class A load line and the class B load line, and the gradual transition between the two in class AB, when the 'other' side of the push-pull goes progressively into cut-off. They are an accurate representation of what either tube is doing in the push pull output stage.

Bob’s ‘composite load line’ would be the type of thing illustrated here:
Part 3 - Of Loadlines, Power Output, and Distortion
They show the operation of the push-pull pair as an overall output stage and IMHO are more useful for hi-fi analysis.