Gain of output valves

[martin manning]

The OT model only has inductances and series resistances for the coils. B+ remains at 450V when Vg1 is driven to zero, i.e. there is virtually no sag.

I’m not sure how this happens, but I suspect it’s the primary inductance itself. The path on an Ia vs. Va-k plot is significantly affected, where the conduction loop shrinks and widens considerably at 50 Hz vs. 400 Hz with the same PI input voltage.

[Helmholtz]

The OT model only has inductances and series resistances for the coils. I’m not sure how this happens, but I suspect it’s the primary inductance itself. I can see that the path on an Ia vs. Va-k plot is significantly affected, where the conduction loop widens considerably at 50 Hz vs. 400 Hz.

That would be a "static" model with a constant primary inductance.
The lower the frequency the lower the inductive reactance of the primary which shunts the reflected secondary load.
Means that the plate load impedance lowers and increasingly becomes inductive, so the load ellipse widens while getting steeper.

There's more happening with a real transformer.

[martin manning]

There's more happening with a real transformer.

I'm aware ;^) In fact if I increase the input signal to drive Vg1 back to 0V, I can almost recover the 100W... 97 or so.

For anyone who hasn't seen it, this is what a (simulated) load "line" (green trace) actually looks like. Screen current and Vg1 and Vg2 are also plotted. [updated with revised OT model]

[Helmholtz]

In fact if I increase the input signal to drive Vg1 back to 0V, I can almost recover the 100W... 97 or so.

That makes sense as a lower magnitude plate load means lower power tube gain.

[Mark]

This is what I’m seeing on the CRO.

Nothing really unusual in your pics.
From the display it seems you're using a 50Hz signal? Would also explain the slanted tops.
If so, available output will be lower than at some mid frequency (e.g 400Hz) because of increased magnetizing current and OT losses.

Output voltage should be measured using a good RMS voltmeter connected directly to the OT secondary wires (not across the load resistor).
This avoids possible errors due to contact resistance.

It was at 50Hz and I changed the frequency to 1KHz and it didn’t make any difference. I subbed another Twin output transformer in the amp and that made no difference. I also put a new set of TAD 6L6’s into the amp and that made no difference, in fact they dragged the voltage rail down from 430VDC to 420VDC with no increase in output.

I’m thinking the only thing left is the power transformer that was rewound.

[Mark]

Do you know the condition of your output tubes? Are they new, and do they meet specification?
Bottom line, I think you will need 450V B+ to get 100W of clean output power.

H Martin, I have put new TAD 6L6’s in the amp and the output was similar to the 5881WXT’s in the amp. I subbed another Twin output transformer into the amp and it made no difference.

I took a few measurements with the original output transformer and the new valves and I found under load the plates were at 385VDC, the screen were 382 to 384VDC and the bias’s voltage was -44VDC.

It wasn’t what I was expecting to find.

[Mark]

In fact if I increase the input signal to drive Vg1 back to 0V, I can almost recover the 100W... 97 or so.

That makes sense as a lower magnitude plate load means lower power tube gain.

Funny you should say that, I rebuilt a Bassman 100 and it was putting out 98W RMS. So it’s rather frustrating that the Twin isn’t outputting something similar.

[martin manning]

I have put new TAD 6L6’s in the amp and the output was similar to the 5881WXT’s in the amp. I subbed another Twin output transformer into the amp and it made no difference.

What did you have for plate voltage with the substituted Twin OT?

[Mark]

I have put new TAD 6L6’s in the amp and the output was similar to the 5881WXT’s in the amp. I subbed another Twin output transformer into the amp and it made no difference.

What did you have for plate voltage with the substituted Twin OT?

The voltages and performance from the amp was exactly the same.

[Helmholtz]

I took a few measurements with the original output transformer and the new valves and I found under load the plates were at 385VDC, the screen were 382 to 384VDC and the bias’s voltage was -44VDC.

So that's another 15V less at the plates.
Using the power formula with a Vsat of 100V gives me 74W before the OT.
Assuming an OT power efficiency of 90% results in an output of 66.7W.

[Helmholtz]

I rebuilt a Bassman 100 and it was putting out 98W RMS. So it’s rather frustrating that the Twin isn’t outputting something similar.

Did you measure full load B+ with the Bassman?

[Helmholtz]

Does your PT have a 220/230V primary tap?
If so and measured heater voltage is on the low side, you could use that to increase B+.

[Mark]

Does your PT have a 220/230V primary tap?
If so and measured heater voltage is on the low side, you could use that to increase B+.

The heater voltage is 6.34vac. I can measure the main voltage and adjust the voltage selector IF the voltage is around 230vac. It would be interesting to see if that helps or the generator regulation of the transformer is the issue.

[Mark]

I rebuilt a Bassman 100 and it was putting out 98W RMS. So it’s rather frustrating that the Twin isn’t outputting something similar.

Did you measure full load B+ with the Bassman?

No, I didn’t take any measurements as the amp was delivering full power. I wish I had though.

[Mark]

I took a few measurements with the original output transformer and the new valves and I found under load the plates were at 385VDC, the screen were 382 to 384VDC and the bias’s voltage was -44VDC.

So that's another 15V less at the plates.
Using the power formula with a Vsat of 100V gives me 74W before the OT.
Assuming an OT power efficiency of 90% results in an output of 66.7W.

This certainly makes sense with what I’m seeing.