Oscillating Mullard5-20

[icebear]

After getting the resistors in the PSU correct, I have trouble with the thing oscillating. I measure 60V on the output bindings, and there is a loud humming. Oscillating yes?????

I'v checked that the assemby is correct regarding the schematics.
It's in both channels (stereounit) so it has to be something fundamental.

The signal from the driver goes trough the "air" and lands on the EL34's. The output goes below the incoming signal (3-4cm). Could this be the problem? Should I rather try to get the output to the transformer along the side? Anyway it gets close to the input of the EL34. Look at the pictures:

 

[dhaen]

Bjorn,

First: disconnect the negative feedback loop (short the input, as the amp will become over-sensitive).
If the oscillation stops, it is likely that the phase of the output is 180 deg from the expected. In this case, all that is required is to reverse the phase of the output transformer secondary connections.

Hope this helps

 

[icebear]

Thanks

The oscillation stopped when removing the feedback loop. All i have to do now is to switch the output. Gets back when there is music.....

 

[Duo]

Hehe, it's a pain when that sort of thing happens.

There's one good reason for OTL if you can, easier to get the phase right the first time...