KT88 amp

[Helmholtz]

So this suggests the bottleneck is either OT losses, or plate supply current?

As your B+ sags by only around 25V at full power, I don't see a supply current problem. Supply current is determined by the tubes, not the PT.

Another question (sorry to ask): Are you sure your screen resistors are actually 47R and not 47k?

[hylaphone]

As your B+ sags by only around 20V at full power, I don't see a supply current problem. Supply current is determined by the tubes, not the PT.

Another question (sorry to ask): Are you sure your screen resistors are actually 47R and not 47k?

Yes they are 47 ohms.
So perhaps an OT with lower primary Z would yield more power?

[Helmholtz]

So perhaps an OT with lower primary Z would yield more power?

Use Nick's calculator to find out. I wouldn't go lower than 4k though to avoid plate overdissipation.

[hylaphone]

Use Nick's calculator to find out. I wouldn't go lower than 4k though to avoid plate overdissipation.

4.2K yields about 10W more

[g1]

Supply current is determined by the tubes, not the PT.

Maybe I'm missing the context, but I don't understand this statement. Maximum supply current is determined by the PT, is it not?

[Helmholtz]

Maybe I'm missing the context, but I don't understand this statement. Maximum supply current is determined by the PT, is it not?

A PT has a thermal current limit. Doesn't mean it can't supply more current, but will overheat.

For a given B+ the current is solely determined by the load, i.e. the tubes and the PT has no influence. All the PT can do is sag the voltage.

[pdf64]

…As mentioned earlier, you might have somewhat inefficient tubes.
E.g. Prof. Zollner found that Chinese KT66s can produce less output than original GEC types because of higher "saturation" voltage.
Don't know about KT88s, though.

Yes, modern power valves just don’t seem to produce as high an output as old vintage production ones that are still serviceable. Beam types, at least. The only apparent explanation seems to be that the anode voltage on their Vg1 = 0 curve is a bit higher, thereby reducing the max p-p voltage swing.

[Helmholtz]

The only apparent explanation seems to be that the anode voltage on their Vg1 = 0 curve is a bit higher, thereby reducing the max p-p voltage swing.

Yes, that's what I called "saturation voltage" (had seen this English term used somewhere). Zollner calls it "residual plate voltage" (my translation).
It is the voltage drop across the tube at max. signal current.

A tube with an increased saturation voltage has a more rounded plate characteristic and increased plate dissipation at full power.

[The Dude]

I get that this might be an exercise in theory and a quest for knowledge, but I'd just like to point out that "squeezing out a few more watts" isn't something to be concerned about. A few more watts will be hardly noticeable with respect to SPL. You'd need to double the output power to get even a 3dB increase.

[Helmholtz]

A few more watts will be hardly noticeable with respect to SPL.

See post #56.

[pdf64]

Yes, that's what I called "saturation voltage" (had seen this English term used somewhere). …

Yes, I’ve read that terminology somewhere, but wasn’t keen on it, as it might imply some relationship to operation in saturation mode, which is something else again.

[Helmholtz]

Fully agree, that's why I typically use it between double quotes.

Now we're in need of a better term. Any suggestion?

[g1]

A PT has a recommended thermal current limit. Doesn't mean it can't supply more current, but will overheat.

Ok. So the measured sag seems to tell us it's not a PT issue.
But I've re-read the entire thread and one data point that is missing is the make & model of PT. It would be good to know.

[hylaphone]

I get that this might be an exercise in theory and a quest for knowledge, but I'd just like to point out that "squeezing out a few more watts" isn't something to be concerned about. A few more watts will be hardly noticeable with respect to SPL. You'd need to double the output power to get even a 3dB increase.

Yes, appreciated. I'm trying to have a more "holistic" understanding of all the factors discussed here so that I may design better output stages. This thread has been extremely helpful.

[hylaphone]

Ok. So the measured sag seems to tell us it's not a PT issue.
But I've re-read the entire thread and one data point that is missing is the make & model of PT. It would be good to know.

It's custom from Hammond, with a separate winding for the screen supply. I couldn't find an off-the-shelf transformer that fit the bill. But now I have a bunch of these to try out in different designs..