Motorboating on Channel 3 of my amp.

[raza]

I have a problem with a three channel amplifier.

Turns out I have motorboiating noise.
It is a 3-channel amplifier with Gz34 as a rectifier.

Noise was only hear on channel 3 LEAD. Usually when going from 2 to 3.

I have done the typical thing that is done when there is motorboating. Lower capacitance of power supply capacitors. Increase the value of the first series resistance of the power supply.
And I managed .... that the noise of motorboating only appears for 10 seconds on channel 3 and then leaves not to return.

Any idea how to fix it at all?

Attached is the BOGNER scheme which I have followed but with my own food source.
The chanel 1 is clean, ch2 plex mode, ch3 blue.

[Mike_J]

What power tubes are you using and what value grid stopper resistors are they using. Should be attached to the grid socket of your power tubes. It has been my experience increasing the resistor value can stop motorboating but you don't want to over do it. I am sure there are other fixes for the same problem. Hopefully one of the more knowledgeable people on the chat will be able to respond.

[pdf64]

...I have done the typical thing that is done when there is motorboating. Lower capacitance of power supply capacitors...

That’s moving things in the direction that will make motorboating worse.

[Mike_J]

Your amp uses a tube rectifier. There are limits as to how much filtering you should have on the first stage of the power supply. Depends on the tube you are using.

[Leevi]

Check the wiring by using a chopstick methode. Check also the grounding
/Leevi