'69 JMP troubleshooting

[LPman]

I've noticed today that when I play sustained notes on the high B and E strings there is a barely audible crackling background noise behind the notes. I can't hear it when I play on the lower strings, only during sustained notes higher up the fretboard (bending and sustaining notes between the 7th and 15th positions on the high strings mainly).

No other issues, the amp is dead quiet, it sounds killer as always but I'm wondering if a cap might be on the way out or something? A preamp tube? Any ideas what should I check out first?

 

[rocker68]

Interested in the responses. Good description. Cheapest move is roll preamp tubes with known good one. One at at time starting with v1. If you pull the chassis, one thing I always do is check pi plate resistors for drift. Easy to do while I'm inside and eliminates that variable. Scope it for any oscillating effects to help determine where in the chain the root cause is . Your description does mimic a cap issue or cold solder / connection as a possibility. What model amp ?

 

[LPman]

It's a '69 metal plate 50w JMP, originally 1986 circuit with some minor mods here and there but nothing specal. The best amp I've ever played. It was serviced and restored with period-correct NOS caps and resistors a year ago.

I'll swap preamp tubes as a start, then check those components if necessary. I noticed this pehenomenon today with my one of my P90 equipped LP Specials. I'll recheck tomorrow with a standard LP too.

Meanwhile If the problem described sounds familiar to anyone here please suggest what other possibiities to look for inside the chassis if the problem persists after tube swaps. Thanks folks!

 

[Tatzmann]

Could be many things.

In what shape is your impedance selector?

Have you tried it on another outputwinding yet?

If you were using it at 16ohm, try it at 8.

Crackle still there?