Finished EL34 tube amp... but oscilates weird..

[TubeManiac]

Ive finished my tube amp and it plays music !!

But there occurs a weird oscillation. See my youtube vid.
YouTube - El34 Diy

The moment i touch something with the screwdriver was the cathode resistor to ground from a EL34 tube.

Any one has ideas to kill that oscilation sucker ? This evening im going to try to remove the feedback.

 

[Itsmee]

Pull the ECC82, see if it still oscillates, if it does, you know the problem is in the output stage

 

[izozaya]

Also try to disconnect the feedback loop. If oscillation stops, it is 99% chances that you swapped the two anode wires in the OPT, thus working in positive feedback mode rather than negative.

Cheers

 

[Miles Prower]

That kind of squealing on power up and power down suggests marginal stability. As for why wiggling wires might stop it, check for cold joints as well.

I would also suggest bypassing that bias resistor for your floating paraphase. That design doesn't have balanced cathode currents, and it does need bypassing.

And do tell us the schemo is wrong, and you have a DC return resistor on the grid of the upper paraphase triode.

 

[dcgillespie]

While this is not related to the stability issue, the grid return resistors for the output stage also appear to be backwards. Usually, the lower value resistor belongs with the upper tube, while the higher value resistor belongs with the lower tube. Connected as shown, the AC balance could be compromised and should be checked as well.

Dave

 

[TubeManiac]

Missing grid resistor of the upper paraphase to ground is in my amp 😉 ive used 1M.

Funny thing is when i only use one channel ( put out the other two EL34 ) it works without making weird noises.

But still its missing power. Even my Sansui EL84 from the 60's can even go much louder.

And ive seen that my B+ is to low. It only stays with 4x EL34 to 330. And with 2x El34 350.

 

[TubeManiac]

I would also suggest bypassing that bias resistor for your floating paraphase. That design doesn't have balanced cathode currents, and it does need bypassing.

And do tell us the schemo is wrong, and you have a DC return resistor on the grid of the upper paraphase triode.

Hmm i don't know what you mean ? what do you mean with bypassing the bias resistor ? remove the 150k resistor ?

 

[tinitus]

missing grid to ground resistor before ecc82 ?

 

[tinitus]

close to similar design

 

[Miles Prower]

Hmm i don't know what you mean ? what do you mean with bypassing the bias resistor ? remove the 150k resistor ?

No. I meant something like a 100uF capacitor in parallel with the 1K resistor between the cathodes of the ECC82 and ground. You can't remove the 150K resistor, otherwise it won't work at all.