Metropoulos Forum

Got a problem. Fired up this new build 2203, going through testing. Got to the point of all tubes installed flipped the standby and it seems OK until I took a plate voltage reading, or at least I think that's what triggered it but then I get this high pitch whale out the speakers.

I've given it a once over but can't see anything obvious, could it be the output transformer primarys are reversed inside the transformer? If so is there any way I can test for this without having to pull it all apart?

If not this, any ideas? So close.....

Right.... reading some more, when I built it I chainged V1 pin 2 and 7 from the instructions so the 68k resistor is now on pin 7 going to the Jack socket and pin 2 is going to the pot. I chainged this around because I noticed it on a different lay out and was told this was the way Marshall did it originally.

Could this be the problem? If so what else needs to be swapped around from the instructions to make it the way Marshall did it originally and get rid of the noise sent to test me from Beeelzeebub and his minions?

Sounds like phase reversal essentially turning your negative feedback loop into a positive feedback loop. Remove the wire from the 100k(?) negative feedback resistor to the speaker jacks. If the squeal goes away, you have the OT primary wires reversed (alternatively reverse the PI output wires to pins 5 on output tubes).

Swamp resistors

Guys thanks for the speedy replies.

Axeman, yes there are swamp resistors fitted. Is there something spscifically I should be addressing? Can post a pic tomorrow if that would help.

My suspicion as you said Shkti is some sort of Phase reversal. That's why I'm wondering because I came accross a thread that spoke about grid wire reversal having the same effect as OT primary reversal. So knowing that I've reversed the two wires (pin 2 and 7)on V1 and that's my only deviation from the instructions, well other than snubbers I can't help thinking that has something to do with it.

Shakit, I will take your advice and give it a try.

Missus is back home tomorrow and requiring attention so it may be a day or so before I can get back in to it.

Thanks guys

Sorry axeman, if I understand correctly your saying the same thing.

So if I reverse the orange and green wires at the turret board that go to the swamp resistors, essentially feeding bias voltage to pins 5 of the out put valves this would have the same effect as reversing the out put primary's?

I think that would be easier to do than flipping the output primary' s.

Thanks guys, that's got to be it. I'll give it a go.

Could be a phase reversal like metion above but V1 high input is crazy picky about input wire placement.
I recommend a shielded input and a 68k on the grid pin.

Yep, totally my fault.... I wired the swamp resistors in reverse.
Got it swapped over and biased up, everything seems O.K.

Thanks for everyone's help.

Cool