JCM800 Presence Control high pitch Squeel/Ring

[pleasantbullet]

Just completed my JCM800 build, 50watt 2204.

I used a pair of old Transformers from the mid 60's, from an old Tube PA i found a year and a half ago.

Few things.
The heater windings didn't have a center tap, so i used 100ohm virtual center tap.

The B+ windings also didn't have a center tap, so i went With the bridge rectifier.

The power transformer also has an extra winding for bias voltage, at 62vac, so i decided to use that for the bias as i was having trouble getting the negative voltage from the bridge rectifier. i know its sloppy in the pic, i'm going to neaten that up, and get rid of the extra diode.

Another mistake, i didn't mean to put the two filter caps up front, they were supposed to be behind the transformer on the back, but i got a turned around when i was laying it out, and had to roll with it.

Also, I ordered the board hastily from Watts tube audio, and i didn't realize there was a drop down menu to change it from eyelet to turret. I was expecting turret, and got eyelet... so ya.

Everything works great, awesome sounding amp so far. minus a high pitch tone on the presence knob.

This tone, goes away completely at 10, and slowly comes in around 7, and is greatest between 5 and 6. Then as you back the knob all the way down to 0, the tone is gone, but when you play the guitar the amp snaps and crackles, or distorts in a very un pleasing way.

Otherwise, the amp is great, i can only turn it up to 2 or 3 before my wife gets pissed. For some reason she says "that amp is different, it makes my organs vibrate... i dont like it".

0.1uf cap, 25KL pot, 4k7 resistor and 100k ohm NFB resistor.

Oh, and the last thing. the amp i took the OT from, said 16ohm, 8 ohm, and 6 ohm impedances on the back.

My speaker is hooked up to the 8 ohm wire, and i'm not using the other taps, only the 6 ohm (yellow) is connected to the NFB resistor.

I tried swapping the NFB connection to the 16 ohm connection to see if that made a difference, but i just got a crazy booming deep oscillation as soon as i played a bassy note. so i went back tot he 6 ohm.

I've cranked the amp, boosted it with a pedal, and played with some effects. all is well, save for the weird presence control noise.

any suggestions would be great.

I've read a post on another site, where someone had the same problem. but it acted in reverse.. they couldn't turn the presence up at all.... where as i cant turn it down. weird.

cheers.

[cbass]

Could theOT primaries be backwards?

[martin manning]

More FB (from 16-ohm tap) made it worse, so I'm with cbass. Try swapping the blue and brown wires.

[pleasantbullet]

I've been hesitant to try that as everything else sounds fine. But I will try it tonight and get back to you guys.

[pleasantbullet]

swapped the output transformer primaries.
Problem solved.

thanks guys, now on to the fun mods.

[cbass]

Cool rock on