HMMM, no takers on this one? g
this may be a really common issue with these amps, so bear with me.
I have noticed on quite a few of these Marshalls that the 22pf anti oscillation cap that goes between the screen grid & plate of the power tubes burns up.
I have always thought that this was a result of some catastrophic issue with the power tubes & a resultant oscillation that this cap could not handle.
I just repaired one of these issues that also took out the OT.
The amp came back in a few days & the scenario was similar, but this time the guy fortunately turned it off before it went into meltdown.
I replaced the 22pf with a 1kv cap & also one screen resistor that had obviously gotten hot (the one on the tube with the cap).
I fired the unit up & verified the bias. All seemed ok. As soon as I plugged in the guitar, wammo, there was the oscillation just screaming. very high, but audible.
I determined that tapping the input jack had the most effect on the oscillation. The connections to the input jack were not bad but it was obvious that the contacts were not making well.
Then of course it went away not to rear its ugly head. I replaced the input jack just because it didn't make sense to just clean the contacts & take the chance of this occurring again, as well the originals have very thin metal contacts.
Has anyone (everyone) experienced this type of issue?
As an aside, I have also noticed that some amps are very intollerant to connnecting my audio gen to the guitar input & immediately causes this type of oscillation...I think that issue is the ground loop created when doing that.
Well I did have a Marshall that was oscillating on me only when I hooked the speakers up in 16 ohms. At 4 & 8 it was fine. It turned out that the secondary wires coming out of the Output tranny were on the right pins at the tranny but Marshall placed them in the wrong holes on the pcb board connection so I swapped them on the transformer side and the problem was gone. If your suppresion cap is blowing and the screen is getting hot I suspect a bad tube or maybe a bad tube socket as that screen shouldn't really get that hot.
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