Marshall clone oscillating via presence pot

[Doug]

I've got a 1987 circuit marshall clone that's high freq oscillating when the presence control is turned down past 6. It's stable with presence at 10. When I increased the feedback resistor from 47k gradually up to 220k it became more and more stable. Strangely, 47k feedback resistor and 4k7 presence pot was entirely unstable, but I changed the feedback resistor to 220k and the pot to 22k and it was much more stable. Isn't that the same 10:1 ratio? Might I need a cap on a plate resistor somewhere? Any help much appreciated.
Thanks
Doug

[ampcession]

I would try redressing the leads to the early gain stages.

[Doug]

Should these be shielded?

It's a kit and came with a prewired board and unshielded leads. Moving the wires around seems to have no effect, is shielding my next step?
Thanks
Doug

[Mars Amp Repair]

Also,
Be certain you haven't swapped the plate connections from the OT. THis will indeed cause massive oscillation as the feed back (controlled by the presence control) becomes positive feedback instead of negative feedback.

glen

[Doug]

Don't think I have them swapped

I've got a white lead and a red lead, white goes to v4, red to v5. Also v4 is fed by the 82k half of phase inverter and v5 the 100k half.
Should I try the other way?

[Mars Amp Repair]

Sorry,
I don't have a schematic & the one I've found on Schemo heaven has different designations for tubes. I think you might be speaking of the secondary connections of the OT.

To clarify, I was referring to the primary connections of the OT to the power tube plates...not the secondary of the OT where the feedback is connected.

If you hook it up wrong (swapping the OT primary wires) it will probably make lots of noise, but if you only flip the standby switch on long enough to know it's wrong, you shouldn't have any issues.
I have seen some amps that behave just like you're describing with the OT primaries reversed & I have also had one old Mashall Major that howled so loud I almost fell of my seat! Still didn't hurt the amp for that short period.

I forgot to ask if this ever worked? I'm assuming you just got this or just built this. glen

[ampcession]

You should have a white lead and a red lead for the plates on pins 3 on each output tube. The brown lead is connected from the center tap of the OT to the first Pi filter section on one side of the choke. Mars is correct about the need to verify the correct orientation of these two leads. Reverse either the red and white leads, or keep them the same and reverse the grid leads on pin 5. IOW tube number 4 will now be fed by the 100k side of the PI. Try that....be prepared to shut it off very quickly because it can be very loud. That's why my first intinct was that your ot wires are oriented correctly, but since you have been increasing resitor values in the FB circuit, that may be allwoing the feedback network to be putting positive feedback into the loop at more tolerable volumes. Put the stock resistor values back in and try swapping the leads. If the amp goes balls to the wall oscillating then you had them right the first time and you will need to trouble shoot the whole amp. Try this first. Good luck

[Doug]

Right... ot primaries

OK, Guess who had the primaries reversed... I thought it was right according to schematic, but the schreiking howling said otherwise, how bad was this on my OT? I raised the feedback to compensate but played it this way several hours. Once the primaries were right, the amp was much quieter and all oscillations gone.
Thanks

[Mars Amp Repair]

Doug,
IT wouldn't hurt anything...the big issue would be if things actually started to get hot & even then you would have smelled the OT lamination getting hot & the OT itself would have gotten hot.
Glad the issue is solved...g

[isaac]

Although, I know this is an old post, and I have a habit of posting to these 2007 posts... but yea i had the ot leads switched on a tweed bandmaster clone I built, and it had bad oscillations also, I reversed them, and it was better, although, I think I actually just swapped the power tube inputs instead of primaries, but that helped, maybe if someone actually replies to this, would it help to switch the primaries still?

[Joe L]

Although, I know this is an old post, and I have a habit of posting to these 2007 posts... but yea i had the ot leads switched on a tweed bandmaster clone I built, and it had bad oscillations also, I reversed them, and it was better, although, I think I actually just swapped the power tube inputs instead of primaries, but that helped, maybe if someone actually replies to this, would it help to switch the primaries still?

There isn't any harm of just swapping the inputs, at least not technically. But I think it might cause some confusion down the road if you forget, or sell the amp to someone else.

So after I determine which is which as far as the transformer leads, I make sure the inputs match the layout and swap the leads if necessary.

[isaac]

I think the problem I had, was just the power tube input wires were travelling a pretty good length and they weren't shielded or anything, so they picked up noise, but my bandmaster sounds awesome.