My Marshall is now a Theremin..parascitic oscillation

Hi guys,
I've been working on a super bass that I converted ti '69 SLP specs with a Metro amps board. This amp had an oscillation problem before the conversion and unfortunately, still has it.

With the presence beyond 12:00 and the treble channel up past 10:00 I get a squeal. I can control the squeal by turning the presence knob up and down and/or by putting my hands near the tubes (ala theremin)

I have so far replaced the circuit board, fitler caps and rewired a good portion of the preamp. I also added 5.6K swamp resistors.

The problem still exists.

I checked the layout to the bes of my abilities following the Ken Fisher tips in Gerald Webers book.

Any ideas.

I'm running out of places to look. I'm trying to see if I can figure it out before I spend money sending it to a pro.

Thanks,
Paul

 

Hi Bruce. BTW cool modular preamp!

Let's see if I can answer your questions.

 

I do get it into both channels but the treble channel is a lot worse.

 

Hi John,
I can stand anywhere from 1-20ft away and it will oscillate, squeal, and do the buzzing thing under the notes.

I'm not sure If I understand the "coupling when my hand is near the Power tubes" part. What seems to happen is that I can quiet down the oscillation or even control the pitch when I place my hands close to the tubes. It will still do it whether my hands are there or not. Does that make sense?

Thanks,
Paul

 

I believe it does do it with the volume on the guitar down. I will have to verify tonight.

 

I just verified. It does squeal with the guitar volume off. It's a lot worse with the volume up however.

Does that mean something?

 

The higher is set the presence or the volume the"deeper" the sound gets. It sounds like the amp is bearing down if that makes any sense.

I didn't think to check the front panel.

 

Hi Bruce.
I did try different cables and guitars. It will do it with any cable and a guitar doesn't even have to be plugged in.

It seems to always be there, just really pronounced as the volume and or presence goes up.

If this is important, I can research it more.

 

Thanks John,
Heres what I have two grounds going from each input to the buss bar on the back of the pots and one ground wire coming from the the negative terminal of the filtercap under the board. Using the method below, all the grounds have very low ohmage readings.

should I still add a ground from the bussbar back to t filter cap in the power section?


I was reading in Gerald Webers book about checking the grounds in the amp in general he suggests that I use an ohm meter, connect one end to the center tap ground (PT) and then check every ground in the amp, if we have any unusually high readings, we may have an issue.

Any thoughts ?

 

BTW, on a 100W Marshall the PT center tap isn't connected to ground, it's connected to the mid-point of the first filter cap stack; there's a bridge rectifier arrangement, so you will definitely find a lot of resistance between the center tap and ground . [/B][/QUOTE]

Good that will save me from having to find the PT color codes.

BTW,
does it matther where I attach my grounds on the buss wire on the back of the pots? (between treble and volume 1 as opposed to bass and mid pot for example)

Thanks again,
Paul

 

O.K. This is really interesting

I'm not sure how to explain this. It seems like the inputs are acting like antennae. The shorter a cable I plug in, the worse the sound. I can plug un a 3' cabble and move areound the excess and hear difference things depending on where I put it.

I did ground the pots. It seems to be making a small difference but not enough.

*****I did notice something else*******
I was looking at the OT. The marks on the outside I were told was just some mold or something is more likely score marks on the bell cover from arching. Could the OT be damaged and causing the oscillation as well as the amp to seem underpowered?