Super reverb 4 ohm speaker load.

[SoulFetish]

Maybe it should read Max Power Dissipation instead? Dissipation at full power can be lower than idle dissipation in very hot biased amps

absolutely. Good point

[Helmholtz]

How are you getting these idle figures from the tool? I set grid signal to 0, but still don't see any numbers like the 24W you mention, or the 200% ?

You need to point to the operating point in the Ip/Vp graph. A little black window pops up showing plate current at different conditions.
Use the value labelled "Load Line" for Ip and multiply by plate voltage.

Or just read the idle current from the loadline graph.

[g1]

the idle dissipation (24W in the example).

How are you getting these idle figures from the tool? I set grid signal to 0, but still don't see any numbers like the 24W you mention, or the 200% ?

[Helmholtz]

I wanted to show a 30% difference for so choose -14V vs -10V, or whatever.

Actually I'm not sure how to interpret the "Max Dissipation watts (sine wave, per tube) =16.13" value in Nick's tool.

If it actually means max dissipation it can't be lower than the idle dissipation (24W in the example).

Maybe it should read Max Power Dissipation instead? Dissipation at full power can be lower than idle dissipation in very hot biased amps.

Fixed bias Class AB amps often have a dissipation maximum at some medium output.

[SoulFetish]

What made you choose an unrealistic grid bias of -9V?

That would result in an idle current of 75mA per tube or a plate dissipation of 24W (!) or 200% at idle acc.to Nick's tool.

The example illustrates why it’s unrealistic.
I wanted to show a 30% difference for so choose -14V vs -10V, or whatever.

[Helmholtz]

EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -9V operating in Class:AB1 should produce a power out of just under 17W
Max sine wave dissipation of just over 16W (per tube)*

What made you choose an unrealistic grid bias of -9V?

That would result in an idle current of 75mA per tube or a plate dissipation of 24W (!) or 200% at idle acc.to Nick's tool.

[Helmholtz]

It does, however, have a significant influence on the operational characteristic, with regard to class of operation during signal drive, and the total tube dissipation.

Now - as the discussion was about Zaa, did you also check dissipation for a full class A impedance of around 4k?

[SoulFetish]

Just take a look at the EL84 datasheet.

Recommended Raa for class B: 8k, recommended Raa for class AB: 8k, same plate and screen voltages.
Means that the optimal plate load per tube is 2k in both cases. And that's roughly what you get when doing a loadline construction.
Exact bias point has little influence on optimal load line and doesn't matter much at full power.

correct. plus,doesn't seem to have much affect on output power either. It does, however, have a significant influence on the operational characteristic, with regard to class of operation during signal drive, and the total tube dissipation.

for instance:
EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -13V operating in Class:AB1 should produce a power out of just over 17W
Max sine wave dissipation of just over 9W (per tube)*
while;
EL84 (push pull with p-p load of 10k) with plate voltage of 321, screen voltage of 319, and a grid bias of -9V operating in Class:AB1 should produce a power out of just under 17W
Max sine wave dissipation of just over 16W (per tube)*
- of course, this is assuming a full sine wave grid drive to the 0Vg limit

how this affects the perceived sound is subjective to the player/listener, I suppose

* calculation done via nickb's interactive datasheets @ bmamps.com:
http://bmamps.com/ivds.html

[Helmholtz]

okay, yeah, I see what you mean. You are going to go for the knee of the curve. Good point. Even when I might use the class A load to set the bias and operating point, I'm assuming the class B Load to account for it.

Just take a look at the EL84 datasheet.

Recommended Raa for class B: 8k, recommended Raa for class AB: 8k, same plate and screen voltages.
Means that the optimal plate load per tube is 2k in both cases. And that's roughly what you get when doing a loadline construction.
Exact bias point has little influence on optimal load line and doesn't matter much at full power.

[SoulFetish]

All datasheet examples for class AB operation I've seen list the recommended Zaa for class B.

When you do a load line construction to find the optimal plate load Za for one tube in a class AB amp, there's some variation depending on operating conditions.
But Zaa is always found by multiplying Za by 4 and that assumes class B operation.

It's high output where correct plate load matters most.

Even the Vox AC30, which has a significant class A range, uses the class B primary impedance of 4k. The Zaa for pure class A would have to be 2k for 4xEL84.

okay, yeah, I see what you mean. You are going to go for the knee of the curve. Good point. Even when I might use the class A load to set the bias and operating point, I'm assuming the class B Load to account for it.

[glebert]

FYI. You can get Jensen MOD speakers in 10" 4 ohm and they are cheap. Some people have tried them with classic Fender designs and like them.

[Helmholtz]

I can find no example in any tube data sheet listing common operating conditions, where the load impedance is listed according to it's class B operation (regardless of class of operation - class AB, or otherwise).

All datasheet examples for class AB operation I've seen list the recommended Zaa for class B.

When you do a load line construction to find the optimal plate load Za for one tube in a class AB amp, there's some variation depending on operating conditions.
But Zaa is always found by multiplying Za by 4 and that assumes class B operation.

It's high output where correct plate load matters most.

Even the Vox AC30, which has a significant class A range, uses the class B primary impedance of 4k. The Zaa for pure class A would have to be 2k for 4xEL84.

[SoulFetish]

Not sure, I'm getting your point.

Yeah, after reading how my post is written, it sounds like my comment about screen dissipation was directed at you, or in response to something you posted. It wasn't.
I simply mentioned at the beginning of my post to reiterate that you were right about the OT and speaker load, and I wasn't sure anyone provided a solid reasoning for pushing the output stage beyond it's designed operating area by purposefully mismatching the load impedance.

A fixed bias class AB amp runs in class B most of the time. That's why OT primary impedance is always chosen for class B.
A 6L6 in a SR wants to see a plate load of 1.1k. So the OT has to provide a total primary impedance Zaa of 4.4k (factor of 4 for class B).

Obviously, most of the power in a class AB amp is generated when operating in class B. That's why it's a thing. There are several factors that go into choosing the primary load impedance. The class B load line is clearly an important one. But I think you over state it. I can find no example in any tube data sheet listing common operating conditions, where the load impedance is listed according to it's class B operation (regardless of class of operation - class AB, or otherwise).
But my point was in reference to the relationship of the screen dissipation curve to the 0Vg1 plate curves. However, it was not intended to be directed at you, if that's how it appeared.

[Helmholtz]

You’re correct about the secondary Helmholtz

as Far as screen dissapation goes, don’t forget, when assessing screen dissapation under the higher load impedance, the amp will be most likely operating in the class B/cutoff portion of the duty cycle during heavy overdrive and the loadline shifts vertically in a dramatic way to 1/4th the P-P Impedance.

Not sure, I'm getting your point.

A fixed bias class AB amp runs in class B most of the time. That's why OT primary impedance is always chosen for class B.
A 6L6 in a SR wants to see a plate load of 1.1k. So the OT has to provide a total primary impedance Zaa of 4.4k (factor of 4 for class B).

The "class A" region, where each tube sees 2.2k is very small, maybe up to a couple of watts, depending on bias.

[SoulFetish]

You’re correct about the secondary Helmholtz

as Far as screen dissapation goes, don’t forget, when assessing screen dissapation under the higher load impedance, the amp will be most likely operating in the class B/cutoff portion of the duty cycle during heavy overdrive and the loadline shifts vertically in a dramatic way to 1/4th the P-P Impedance.