Getting only 25% of power from 5f6a Bassman.

[Anthon]

Another troubleshooting concerning my DIY Bassman  :

I measured the output with 1kHz tone, and it seems it delivers about 25% of the supposed power. (special thanks to PRR and CJ for suggesting how to do it without a scope). I tried both 6v6 and 5881 tubes - they give about 6W and 12W resp.
It makes sense, because it sounds hardly louder than my DIY Princeton while running 6v6.

I guess only 1 power tube is doing its job.

Some clues:
- all voltages are within tolerance, except on the cathode of the phase inverter (after the 470ohm resistor). I get 44V instead of 34V.
- I tried swapping all the tubes, no improvement.
- The problem persists on both 6v6 and 5881 tubes (25% of the theoretical power output).
- The bias is okay - 40mA at 440V of plate voltage with 5881 tubes.

I bet something is wrong the phase inverter, but can't see the problem.

 

[CJ]

look at the AC grid drive voltage and tell us what it is at full volume,

how many milli volts are we injecting, try 25 mv at 1 K Hz

measure plate and cathode DC voltages for all preamp tubes which should not take long

the health of an amplifier is determined by it's bias voltage

 

[Anthon]

I went full retard on this one: forgot to install the coupling cap between the phase inverter and one of the power tubes.  :
So indeed, only 1 power tube was doing its job.

I installed the missing cap,  measured the output, and it's about 45W with 5881 tubes now.

 

[CJ]

put a switch on the cap for power soak/SE operation, or not, probably will sound like crap,

45 w is good!

 

[PRR]

> about 45W with 5881

Which is what a 5F6a should do.

I thought you were building a half-5F6a??