Marshall JVM Is Frying Bacon!

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River City Studios

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Hey everybody,

Finally took some time off day job to work on a new project in my studio and I'm having amp problems. I have a 77 JMP 2203 that was modified by Rockstah. When playing certain low end notes (especially palm muted bar chords) the amp emits this crackling sound that I can best describe as the sound that bacon makes when you fry it in a pan. Here is the signal chain:

Les Paul -> Mogami guitar cable -> Marshall -> Mogami speaker cable -> Marshall 1960BX cab

I have completed the following trouble shooting:
-Tried 3 other guitars
-Tried 3 other guitar cables
-Tried 2 other speaker cables
-Tried another cab
-replaced the pre-amp and power tubes with 2 different sets that were known to be good
-Cleaned the tube sockets and pots with contact cleaner
-biased the amp

Problem still exists.

In reading other forums (Metroamp), someone mentioned that failing/failed screen grid resistors on the 12AX7 sockets could cause a crackling noise like I have described. Does this jive with anyone here? If so, is it possible to change these on my own without electrocuting myself?

Unfortunately, I live in an area that is deprived of quality amp techs. I've taken amps to the few that are available and they sent my "fixed" amps back without being fixed. I guess I could box it up and ship it to someone, but I'd rather avoid the expense, if possible. Any guidance would be appreciated.

Thanks in advance,

--Dan S.
 
Did you try another speaker jack? Did you clean all of the pots?

Otherwise I'd say take it to a tech (or ship it if needed). Where are you located?
 
I did clean the pots. Didn't help. I'm in the Central Valley, CA. 3.5 hours to SF. 4 hours to LA.
 
I have a Laney AOR 50 that did the same exact thing, and it ended up being the pre amp volume pot going bad.
 
Just a warning if u take it to a tech, be sure to tell him not to change the spec of any of the components at all. I once took my modded 77 jmp to a tech and he changed many components back to stock as "he" preferred these amps stock which was common at the time. Anyway, there are a few builders on this board that are familiar with the mod 5 that might be able to help you if needed like Steve from , Jerry or Brian from bfg mods.
 
The frying bacon description makes me think it's related to capacitors
 
My JVM always sounded very "transistor-like" with high gain and high-volume.
 
shane159":2ohel0ke said:
Title says JVM, post says JMP :confused:

Sorry. Had been reading about similar Marshall issues all morning. I guess "JVM" was on my mind. My amp is a Rockstah modded 77 JMP 2203. My apologies.
 
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