6V6GTA plate dissipation in cathode bias push pull amp

[MWJB]

Measure the resistance from the 6V6 pin8 to ground, tubes removed, just to eliminate any margin for error.

[pdf64]

Surely if the cathode voltage dropped with a 300 ohm resistor, then the original resistor must have been running at more than 300 ohms?
Have you got any Vdc on the control girds of the 6V6 (I'm thinking of leaky coupling caps etc from phase splitter)?

[pchilson]

Measure the resistance from the 6V6 pin8 to ground, tubes removed, just to eliminate any margin for error.

Tubes removed, Pin 8 to ground, both tubes. 255R

[pchilson]

Surely if the cathode voltage dropped with a 300 ohm resistor, then the original resistor must have been running at more than 300 ohms?
Have you got any Vdc on the control girds of the 6V6 (I'm thinking of leaky coupling caps etc from phase splitter)?

Should this be a dead zero amount of vdc? I measured with the meter set to 200vdc and get a slight reading of .1 vdc on each tube.
Would that be considered leaky or within tolerance?

[pdf64]

0.1Vdc on the control grid won't upset anything. The trem intensity control may affect that voltage. Best to measure with the 6V6 removed.

[tubeswell]

Then try lowering the screen voltage

[pchilson]

0.1Vdc on the control grid won't upset anything. The trem intensity control may affect that voltage. Best to measure with the 6V6 removed.

Pulled the 6v6s. Measured the same .1vdc until I dimed the tremolo intensity control then it went to 34.2vdc.
Put the 6v6s back in and again the .1vdc until I dime the tremolo intensity control but with tubes in it goes to 14.2vdc.

[pchilson]

Then try lowering the screen voltage

Are you referring to increasint the 1k cement resistor as you stated earlier about following the 5E3 design?

[tubeswell]

yes. Try it to see how it affects tube current

[pchilson]

yes. Try it to see how it affects tube current

Ok, I swapped in a 5k for the 1k and my screen vdc dropped about 20 vdc but measuring and calculating I still come up with 19 on my IPD. How is lowering the screens voltage supposed to affect this?

[pchilson]

Well, I'm kinda stumped here now. Nothing I've tried has lowered the IPD any significant amount. This thing is running the tubes hot as a champ.

[Bruce / Mission Amps]

Not really likely but it is possible that you have an ultrasonic oscillation going and your tests can't tell the difference between real. "no signal" idle current and the current flowing from an oscillation added to the idle current.

I'm off to look at a schematic

OK Here's something:

Something is off there on your claim that you see +34vdc with the intensity control turned up... that circuit is a low frequency oscillator and the output AC signal is superimposed across the power tube's grid load resistors.
That does turn the power tubes on hotter and cooler to give the amp it's "tremolo".
That .1F cap from V2 should be blocking all kinds of DC from getting to the power tube grids from the intensity pot but allow the AC wobble to offset the power tube's "bias" setting.
The grounded intensity pot is what is giving your power tubes their ground reference for proper bias setting.
I wouldn't be surprised to find you have a bad intensity pot, bad pot ground or lead wire(s)..... and maybe a leaky .1uF cap too.
Take a clip lead and hard ground the common connection point of the two 270K grid load resistors on your power tubes and check your idle current again.

[tubeswell]

Ok, I swapped in a 5k for the 1k and my screen vdc dropped about 20 vdc but measuring and calculating I still come up with 19 on my IPD. How is lowering the screens voltage supposed to affect this?

what do you mean '19 on your IPD'? What is the voltage on the cathode(s) now?

[pchilson]

Not really likely but it is possible that you have an ultrasonic oscillation going and your tests can't tell the difference between real. "no signal" idle current and the current flowing from an oscillation added to the idle current.

I'm off to look at a schematic

OK Here's something:

Something is off there on your claim that you see +34vdc with the intensity control turned up... that circuit is a low frequency oscillator and the output AC signal is superimposed across the power tube's grid load resistors.
That does turn the power tubes on hotter and cooler to give the amp it's "tremolo".
That .1F cap from V2 should be blocking all kinds of DC from getting to the power tube grids from the intensity pot but allow the AC wobble to offset the power tube's "bias" setting.
The grounded intensity pot is what is giving your power tubes their ground reference for proper bias setting.
I wouldn't be surprised to find you have a bad intensity pot, bad pot ground or lead wire(s)..... and maybe a leaky .1uF cap too.
Take a clip lead and hard ground the common connection point of the two 270K grid load resistors on your power tubes and check your idle current again.

Well Bruce, you are not only a man but you are 'de Man.

New numbers after connecting the common connection point of the two 220k grid load resistors to ground with a clip lead. (yes, 220k not 270k as the schematic reads, I've found a couple variances from the schematic so far).

Plate - pin 3 = 380 vdc
Screen - pin 4 = 375 vdc

Cathode resistor = 256R
Voltage drop across the cathode resistor = 24.3 <---- Big change

24.3 / 256 = .0938 / 2 = .0469
380 - 24.3 = 355.7
355.7 * .0469 = 16.68 IPD <---- A bit high yet but a significant change

I will check and clean the ground connections and see what goes from that.

[pchilson]

what do you mean '19 on your IPD'? What is the voltage on the cathode(s) now?

Sorry, I meant 19 watts idle plate dissipation.

See my above post after Bruces post