The one that got put away time and again.

[boater sam]

Very much a shot in the dark to me but sounds like too much negative feedback. Something like C49 short?

By the way. thank you for the circuit Andy.

Tracy

 No, C49 is OK

Clue? feedback circuit.

[boater sam]

Not switches or R29.

Clue?  Consider the cathode and feedback arrangement on the EL84. remember that it is drawing current OK.

Alistair is comfortably warm, the weather has been kind today.

[boater sam]

OK, the snooker has finished, smashing match, shame there can't be a draw.

In fairness to Alistair, he was on the ball. Not disconnected but..................

The cathode circuit with the negative feedback is unusual, the cathode current is via the secondary of the output transformer. So the output valve is drawing about the correct current but it is all down to the screen, the section b of the output transformer is completely open, the anode has zero volts.
What amazed me is the lack of distortion, if you could accept the low, but sufficient to listen to, volume you could accept that it was just a low emission valve.
The EL84 showed no distress, and with a new transformer without the hum cancelling winding, the valve is fine, the volume is huge, the sound is clear.
I would not have thought that so much current could pass by the screen without it getting red hot.

Hope you enjoyed the brain exercise. Thanks for taking part.

Sam.

[boater sam]

OK, the snooker has finished, smashing match, shame there can't be a draw. All set for another.
I will have to put the mod capacitor from the anode to ground as the gain is now such that there is out of phase instability .
Thanks for taking part.

Sam.

[BusyBee]

Thank you for putting me out of my misery lol. So the output stage was perhaps working more like a cathode follower for audio, essentially a triode output? That would explain the non-distorted output.

Well done Alistair!

Tracy

[Murphyv310]

R 23 low value.
Oops didn't fully read the thread

[boater sam]

R23 is fine, why did you think that Trevor?

Sam.

Gotcha, OK. Sam

[Murphyv310]

If R23 was low in value output will drop and of course distortion in the output stage will not occur.
I've done one of these with an open B winding but had no output at all. Very odd you got anything at all as the G2 is decoupled by C42.

[Murphyv310]

Typo.
C46 is the G2 decoupler
Makes me think C46 may be O/C as well, C28 will do a good job of filtering hum even if C46 was open.

[boater sam]

C46 was leaky and replaced, no change.

I think the reason there was any output at all is due to the cathode current being drawn through the feedback circuit and the output transformer secondary and to HT via the screen grid.

The cathode resistor and decoupler do not go directly to ground, its an unusual circuit.