Voltage needed to drive power tubes

[harry]

...if you have an idea of how much power (Po) you expect from the tubes (and there's sufficient plate voltage and plate current 'swings' to develop that power), and know the OT Zpp and the tubes' rated average transconductance (gm) value, here's the equation that can be backsolved to find the PI output signal (Vg.rms = half of Vgg) needed to produce that power:

Po = 0.9*(Zpp/4)*(gm*Vg.rms)^2

...so:

Vg.rms ~ (1/gm)*SQRT[(4*Po)/(0.9*Zpp)]

...it'll get you within ±10% of the actual design value.

Going off:

Zpp = 4500 ohms
Po = 25W
gm = 9.5mA/V for 6L6s (what happens if I'm using two?)

gives 16.53V rms = 23.37V peak or about 46.75V peak to peak!


So the problem lies in the power stage?

[harry]

The impedance selector switch was wired incorrectly. I worked this out by measuring the RMS voltage on the OT primaries, and looking at the RMS voltage on the secondaries with a second meter.

As it is, 250V across primary gives 10.2V across secondary on 8 ohm tap (although my load is actually 7.5 ohms)

Assuming a primary of 4.5k as per the manufacturer's specs, this is correct.

This means the output is 13.9 watts

Still not there...

[Old Tele man]

...14W for 2 x 6L6GC is indeed low...Zpp = 4.5K should get you around 40-45W, Zpp = 5.6K is needed for 50-55W.

...what Vgg (volts peak-to-peak) do you measure at the PI plates? That's the drive signal going into each respective output tube, so should be around 47Vg.pk...what's the bias voltage at?

[R.G.]

...not a problem for me, because ALL pepper tends to look like fly-shit to me (ha,ha)!

Me too. I just hate it when I can dimly see the pepper crawling...

[harry]

Sorry - should have mentioned - it's cathode biased.

[harry]

25.5 Vrms at power tube grid...

so about 36V peak / 72V peak to peak


Actually the PI is unbalanced, so one side is only putting out 28.3V peak


Heater voltage was low - upping that gives me 16.7W at clipping (estimate - my preamp has too much gain for me to gauge properly) or 30W at full tilt...


I'll try balancing the preamp and see what happens...

[Old Tele man]

Sorry - should have mentioned - it's cathode biased.

...did you remember to subtract the cathode bias voltage from the plate voltage to get the actual "usable" plate voltage?

...that's one reason why a cathode-biased amp ALWAYS produces less output power than an otherwise identical fixed-bias version will--there's less available "usable" plate voltage.

[6267]

The front end has so much gain I'd make sure there's no weird clipping or oscillation coming in and disrupting output.

For kicks, try 16 and 4 ohms and see if that brings up the power.

I can get at least 16-17 volts across 8 ohms peak (not rms!!) with 6v6 in class B, so your 6L6's with that much drive should be able to do at least a 20v burst! Something is fishy. Check all solders and speaker connections. What's your heater voltage (i.e. is power transformer & PSU so overloaded it sags a ton??)

[harry]

What is the B+ on a 5D8?

By calculations, I'm dissipating 19W per tube.. which is about 63% of the maximum 30W.

Some amps bias EL84s at 100% when cathode-biased, right?

I heard it's not a good idea to do this with 6L6s.. but would upping it to 22W per tube dissipation be okay?

[Old Tele man]

...a real 6L6GC has an average plate power dissipation (Ppd) wattage rating of 30W, so 22W is 73% of it's working level...slightly high.

...with Class-AB1 operation it's advisable to keep the idle wattage at or below 70% of the tubes Ppd wattage, regardless of bias method.

...how did you calculate the idle wattage? did you remember to first subtract the cathode bias voltage from the measured plate voltage, ie: (Vp-Vk)?

[harry]

I cheated and used Weber's bias calculator

Weber Bias Calculator

But yes, I have been doing calculations using 363-25 = 338V from plate to cathode.

It sounds good as-is, so I might just keep it slightly cold.