Loss of power in my AA764 new build...

[chikov]

Hello friends. About 9 months ago I moved from Houston, TX back to my homeland Moscow, Russia and recently decided to build AA764 amp head from scratch. I had someone here to wind for me a round power transformer (toroidal?). I used a normal output transformer from StewMac. After I completed the build it started working rightaway but it is not as loud as it should be. It is only at 2.7watts and I think it should be at 5 Watts or so. Plus, it does have some buzz when I play low E string on the lowest notes. Does anyone know what might be the problem?
The voltages are pretty hi. It is around 400V at node 1 and 365 at node 3. I did add another filter capacitor Before node 1 (33uf). Also, to reach and maintain around 35 mAmps Bias on my 6V6 I had to Use 1.5 k cathode resistor.
When I inject the Sine wave at 500 hz my output transformer begins to make a faint hi pitch sound ( I usually hear it from my Load resistor). Is it normal?
Finally I do see assymetricaL clipping on my scope when I take a measurement from my output jack.. is it normal for type A Output amps?

[pdf64]

BF Champ https://el34world.com/charts/Schematics ... ematic.pdf
A 1k5 cathode resistor on the 6V6 seems like it will be the cause of low power output and asymmetric waveform.
A tiny bit of audio from an OT is normal

[Stevem]

What does your drive wave form look like going into pin 5 of the output tube and what is its amplitude?
You need like 8 volts peak to peak of drive audio if a recall right.

With a class A output stage the drive voltage should not exceed by much the D.C. Voltage on the cathode.

[pdf64]

To fully drive the power tube. the pk-pk signal at the power tube control grid should be about twice the Vdc at the cathode.

[chikov]

BF Champ https://el34world.com/charts/Schematics ... ematic.pdf
A 1k5 cathode resistor on the 6V6 seems like it will be the cause of low power output and asymmetric waveform.
A tiny bit of audio from an OT is normal

I dropped the wall voltage with my variac down to needed 360V on the node 1 but the bias amp flow has not changed much... hmm...

[chikov]

What does your drive wave form look like going into pin 5 of the output tube and what is its amplitude?
You need like 8 volts peak to peak of drive audio if a recall right.

With a class A output stage the drive voltage should not exceed by much the D.C. Voltage on the cathode.

I will check on it tomorrow ( i left my shop already), but it is something I didn’t know about. I did add 1.5K grid resistor on my output tube to reduce a chance of oscillations. May be it causes all the problems?... Thank you for the input

[chikov]

I’m having strange readings. When I inject a strong sine wave signal I am getting on my grid of the power tube 2.3 V DC, but on the cathode 41vdc. Does it make sense?
However i do get 52VAC on the grid...
I have 388V on the plate of 6V6, but 396V on the pin 4 (screen). Can it be that my electrones flow to the screen instead of a plate? And that is why it is not as powerful? What would be a good way to fix it?

[pdf64]

52Vac at the power tube grid will be overdriving it.
When the Vg-k reaches / exceeds 0V, the g-k path becomes positively biased and forms a tube diode.
That rectifies the ac at the grid, resulting in the Vdc you're measuring.
The 1k5 grid stopper is not a problem.
The 1k5 cathode bias resistor is though, for it to rise up the 41V indicates how far off centre bias the tube's operating point is.
Consider that a cathode bias resistor both restricts plate current and reduces the plate-to-cathode voltage available for signal swing. Hence as it is increased in value above the point of centre bias, output power becomes increasingly restricted.
I suggest to use the proper 470 ohm cathode cathode resistor, tweak the variac to get a 360V HT, and re-assess.

[chikov]

Thank you for your help pdf64. I studied the voltages one more time but I kept coming back to the fact that I had higher voltage on pin 4 ( screen) than on pin 3 ( plate). I increased the value of the resistor between the node 1 and 2 from 1.5 to 3.7k or so and the voltages changed so I have 387 on the plate and 386 on the screen. It helped a lot. The volume went up to 6.5 Watts max. I will keep playing with it until I am semi satisfied but I thank a lot all of you who helped me. Sometimes a few helpful words can calm you down and figure things out eventually

[Stevem]

Depending on your needs you might want to make the second node of the power supply in parallel so the preamp section does not see any voltage drop, it will call for a another filter section to be used , but it's worth it.