2550x Troubleshooting

[joe14vf]

Hello everybody!
I post in this old thread if it's ok with you, because my problem is indeed troubleshooting a 2550x build..
First of all please forgive me if I sometimes write somewhat funny or utterly wrong, English is not my first language.

This is my first experience with a guitar amp build, scheme and layout are SDM's.

Here it goes:
If I put the it in "triode mode", everything works fine, no undesired noises, loud humms or problems at high volumes.

As you might have guessed my problem is with the switch in pentode.
With a test speaker connected (it's a 35w 4Ω car speaker) when I slightly turn up the presence knob (all others are to zero) I get a low frequency hum that increases it's frequency as I turn the knob up.

If instead I connect a 8Ω 100W speaker (from a  yamaha T100C, that I know is fine and works fine when the selector is in triode) as soon as I turn standby on I get a very loud squeak/whistle. I don't know if turning up any knob has any effect, because I turn the amp off immediately for fear of blowing the speaker or something else up.

The output selector was in the correct position.

I tried going trough all the solders, taking all the wires connected at the bottom side of the board to the top side, bypassing the switch and connecting the screen grid resistors directly to the choke and capacitor but the problem remains..

To give a complete picture I add that my output primary is 3200k instead of the 3400k indicated in the schematics.

 

[sluckey]

I started a new topic for you. Please post a schematic or link to your schematic.

[joe14vf]

Thanks,
here they are.

[shooter]

a very loud squeak/whistle

typical problem is phasing, to test, swap the wires feeding the power tube grids and re-test.

[Pietro]

check if you aren.t applying positive feedback instead of negative feedback.

[joe14vf]

Thank you both for the quick responses, turned out it was the grids that needed to be swapped..
I could have sworn I already tried that.. 
Evidently I did not..

Anyway, now it seems to be ok.
Time to bias it properly and to try it out.

Thanks again for your help