KT88 amp

[hylaphone]

6sn7 have far little gain to make any relevant global nfb usable in you circuit. You may apply it in previous stage with all consequences regard the phasing issue. If you still open to an experiment use 6sl7 as paraphase as you did but insert a 6sn7 driver stage in between

Yes, I also tried Mosfet buffers using a bench power supply for +/-150V.. but at this point in the build there's no elegant way to add a tube socket, or a dedicated bipolar supply. Next time I use this topology, will definitely plan on it.

Not quite sure what exactly you did, but it seems you're driving the KT88s into class AB2 (some grid current).
I wouldn't want to exceed the plate voltage limit (450V) of the 6SN7, though.

The plates are at 250V with 2.5mA idle current. Still tinkering with it, but so far a big improvement over the 6SL7.

[Helmholtz]

The plates are at 250V with 2.5mA idle current.

Sure, but with signal the plate voltage swings up to full supply voltage.

[hylaphone]

Sure, but with signal the plate voltage swings up to full supply voltage.

Ah didn't catch that. I'll set it up with a lower supply.

[catalin gramada]

...and 12bh7 driving 4xkt88 onset clipping at 40 cps. same circuit. Get a little bit unbalance , but hey..that.s a guitar amp...You 6sn7 should be close, 380v B+ was used.

[hylaphone]

The 6SN7 PI is now working great, much better balance and still plenty of gain to drive the grids.

However, I made another mistake in these frantic measurements - I was scoping the KT88 grids while watching output power with an AC voltmeter across the dummy load.
What I didn't catch is that the output is distorting far sooner than the grids start conducting. In fact, the best I can get is 60W clean with the tubes biased 70%

Helmholtz mentioned transformer losses but this iron is rated for 100W, that's a pretty big discrepancy.
I must be missing part of the picture here, short of replacing the OT is there a way to rule that out?
I can't scope the primaries safely, my scope is only rated for 500VDC.

I did try running the screens at higher (500V) voltage with larger resistors, didn't yield any more power.
Would appreciate any more ideas, probably splitting hairs at this point but it's a learning experience.
Thanks --

[Helmholtz]

Helmholtz mentioned transformer losses but this iron is rated for 100W, that's a pretty big discrepancy.

Transformer losses are comprised of copper (DCR) and iron (core) losses.

From the the DCR values in the OT datasheet I estimate the copper losses to be around 7%.
No info on core losses available, but I'd say total OT losses of 10% might be realistic.


Does your load resistor measure exactly 8 Ohm?

Also try to measure output voltage directly at the OT secondary wires and not at the load.

[catalin gramada]

Said you PT is 0.3A rated. What is the plate voltage at max power ? How much current you kt88 draw at max power? With 450 V into 5k 60w seems pretty reasonable..

[hylaphone]

Transformer losses are comprised of copper (DCR) and iron (core) losses.

From the the DCR values in the OT datasheet I estimate the copper losses to be around 7%.
No info on core losses available, but I'd say total OT losses of 10% might be realistic.


Does your load resistor measure exactly 8 Ohm?

Also try to measure output voltage directly at the OT secondary wires and not at the load.

Load resistor is 8.2 Ohm.
Measured right at the speaker jack, same 60W before clipping.

So 10% loss of the 70W target is 63W, more or less where I'm at. Does that jive with your estimates?

Do I have any options left to gain 10 clean watts?

[hylaphone]

Said you PT is 0.3A rated. What is the plate voltage at max power ? How much current you kt88 draw at max power? With 450 V into 5k 60w seems pretty reasonable..

Voltage at the plates is 515V idle, 490V max clean power drawing 125mA/tube. Screens are 360V idle, 340V max power.

[Helmholtz]

So 10% loss of the 70W target is 63W, more or less where I'm at. Does that jive with your estimates?

Yes.


What power do get if you use your scope to measure output voltage, using the conversion Vrms = Vpp/2.83 ?

[hylaphone]

Yes.


What power do get if you use your scope to measure output voltage, using the conversion Vrms = Vpp/2.83 ?

Approximately 60Vpp, or just over 21VRMS

[Helmholtz]

Approximately 60Vpp, or just over 21VRMS

Calculating to 55W.
Using your RMS meter probably is more accurate.

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.

[uneumann]

Load resistor is 8.2 Ohm.
Measured right at the speaker jack, same 60W before clipping.

So 10% loss of the 70W target is 63W, more or less where I'm at. Does that jive with your estimates?

Do I have any options left to gain 10 clean watts?

You should be able to make up a few watts by increasing the bias voltage (making it more negative).
That allows for more grid swing before grid current clamps the signal.

Edit: never mind - see below - greater grid swing does not lead to greater output power.

[Helmholtz]

You should be able to make up a few watts by increasing the bias voltage (making it more negative).
That allows for more grid swing before grid current clamps the signal.

I don't see how that could increase max. plate current swing before clipping.
Nick's calculator doesn't show a difference.

[hylaphone]

Calculating to 55W.
Using your RMS meter probably is more accurate.

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.

Today I received a pair of KT90, which are rated for 50W. Tried a few different bias points and still can't get beyond that 22VRMS output.
So this suggests the bottleneck is either OT losses, or plate supply current? I'm just trying to figure out what options are left to reach target power, or even to just squeeze out a few more watts.