Ceramic capacitor causing instability?

[DumbGuitarPlayer]

I recently acquired a Marshall DSL100, and like many, I found it to be very fizzy in the high end. So I set about implementing a slight mod in the preamp to filter some of this high end out. In doing this, I put a 390pF Silver Mica cap across the load resistor on the preamp tube, and a 47pF ceramic across the volume pot (one leg to ground). It did a great job of removing the fizz.

However, lately I've been noticing some volume issues with that channel. Not huge, but enough to be noticeable. I'll be playing, and suddenly the volume will increase slightly and often-times go back down to where it was.

The silver mica is rated at 500V, and the Ceramic is one at 1KV, so I don't see where voltage would be breaking either of them down, especially the ceramic. Could it be the ceramic capacitor is causing this, and I should replace it with a more stable silver mica?

Or could it be the silver mica cap across the plate resistor on the gain stage? The leads on that capacitor are suspiciously thin for 500V rating, which is what the bulk box was marked at the electronics surplus store.

[Pedro Vecino]

A 500V mica capacitor is perfectly stable in that role, but their installation may have caused a loss of quality in the original resistor welds. It is a possibility if you've done the welding directly above the resistor..
DSL potentiometers pins are small and close together. It requires much attention to weld something on these tracks to do well and safely.
Itīs more likely a problem of this type that a component failure

[Enzo]

It probably has nothing to do with the caps. Mich more likely just a problem on its own. Possible when you were in there adding caps, you may have knocked some other connection loose. I think that is what Pedro was saying to you too.

[DumbGuitarPlayer]

Thanks for the response, Enzo! You're both right. It has nothing to do with the caps. I simply wanted to eliminate them before considering the issue as separate. Unfortunately it leaves me with a problem that seems dynamic and doesn't present itself unless I'm playing in the lower registers (say, heavier riffs and notes). It seems almost like a midrange spike or volume spike that happens. Interestingly, it doesn't occur as readily when I let chords or notes ring. It seems to respond to the increase in input signal from my pick attack, and it doesn't happen every time I strike notes or chords. I know it sounds retarded, as in a "try controlling your picking, har har" solution but rest assured it's not.. It seems to happen the most when the deep switch is engaged and there is an abundance of bass. It's almost as if my signal or volume is being choked at a certain frequency, but cuts through intermittently. Im going to go check the usual stuff, plate voltages, grids, etc and see if anything seems strange. I'm gonna put my detective hat on and try and figure this thing out, but any comments would be appreciated. Obviously I've swapped tubes, tried different guitars, eliminated effects, etc.

I'm not sure if I mentioned this before but it seems to only occur on the ultra channel so it leads me to believe it could be a capacitor around V3. I thought briefly it could be heat damage to the 100K load resistor (R1) since I've soldered a cap to it, but It read 100.7K even after I heated it up briefly with a soldering iron. All the solder joints appear solid on the main board and EQ board.