'79 JMP crackling, and question about plate voltage

napalmdeath

napalmdeath

Well-known member
My brother has a 79 JMP 50 thats crackling, static, etc. My first thought was preamp tube, and I also did a full cleaning of jacks, pots, etc, no change. I did a full retube and bias. The PV was 391, which seems kinda low??

I think its just in need of caps, some soldering/clean-up, but that plate voltage still seems low? Is this normal? Of course the crackling isn't, but should this thing be pushing 450+ on the PV?
 
jeff bakos just fixed one of my 50 watters / crackling etc, It was some component dont remember under the board.
 
panhead said:
jeff bakos just fixed one of my 50 watters / crackling etc, It was some component dont remember under the board.
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I'm sure its something under the board, also, the preamp tube sockets are loose and wobbly AF, but all wiring seems intact. I'm not digging any further, "I'll let a tech take the zap for me", LOL.
 
I had a similar problem with my Boogie and it turned out to be a dirty power tube socket. Cleaned it with some contact cleaner and it's all good now.
 
Early 70s JMPs had 500+ plate V, mid-late 70s it varied but could be anywhere from 350-490 in my experience. Early 800s from 82-83 jumped back up to 500+. Any crackling I've ever had was solved with a good cleaning or different tubes. Bad Filter caps would lead to a more 'thin' tone with less body, less low end.
 
scottosan said:
I love low plate voltage for Jose mods. They get nice and chewy, not as stiff if you like that sort of thing. My Roccaforte was that low
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Absolutely.

I actually lowered the B+ to the preamp even further on my 83 JCM800 2204. Has amazing sag but still tight and focused.
 
Racerxrated said:
Early 70s JMPs had 500+ plate V, mid-late 70s it varied but could be anywhere from 350-490 in my experience. Early 800s from 82-83 jumped back up to 500+. Any crackling I've ever had was solved with a good cleaning or different tubes. Bad Filter caps would lead to a more 'thin' tone with less body, less low end.
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I think that's what it needs. It's old, and its probably due for filter caps. I gave it a thorough cleaning.

And, yes, it lacks some low end, and somewhat thin sounding.

I put in new JJ KT77's, a Tungsol in V1, a Ruby in V2, and Mullard CV4004 in V3. Biased it at 41mA.
 
napalmdeath said:
I think that's what it needs. It's old, and its probably due for filter caps. I gave it a thorough cleaning.

And, yes, it lacks some low end, and somewhat thin sounding.

I put in new JJ KT77's, a Tungsol in V1, a Ruby in V2, and Mullard CV4004 in V3. Biased it at 41mA.
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Buy some ARS caps, not the F&T or JJ. I've had a few with the F&T and they just sound a little 'off' compared to when I've used ARS, which leave the original tone intact. I know Filter caps aren't supposed to make tonal changes but me and a few others here will disagree. ARS are made in the same way that the LCR were back in the day I believe.
 
napalmdeath said:
I think that's what it needs. It's old, and its probably due for filter caps. I gave it a thorough cleaning.

And, yes, it lacks some low end, and somewhat thin sounding.

I put in new JJ KT77's, a Tungsol in V1, a Ruby in V2, and Mullard CV4004 in V3. Biased it at 41mA.
Click to expand...
If you’re you’re not used to vintage Marshalls it will seem thin. The very reason a resonance mod is pretty common.
 
This is one reason I don't like the 50W 2204. Low PV. Several I've had were below 400 mV. And the tone suffers for it.

And as RacerX the FT caps SUCK ARSE! Find some ARS or similar caps.
 
Racerxrated said:
Buy some ARS caps, not the F&T or JJ. I've had a few with the F&T and they just sound a little 'off' compared to when I've used ARS, which leave the original tone intact. I know Filter caps aren't supposed to make tonal changes but me and a few others here will disagree. ARS are made in the same way that the LCR were back in the day I believe.
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Agree on ARS. Love those caps. And they’re the same size as stock. The F&T look weird they’re so short. I do have a couple amps with each type.
 
I like F&T caps for preamp and ARS and LCR for mains and screen. If it’s high gain then I like the clarity that F&T brings to the table but again only in the preamp. If it’s not high gain I do all ARS or LCR. I once recapped an entire 2204 with F&T and it made the amp sound brittle and super bright - it lost the warmth it once had. I do still prefer LCR over ARS, but ARS are easier to get in the states.

Clean the power tube sockets and if that doesn’t resolve the issue, check your plate resistors for heat damage and fading. That will cause the issue as well.
 
Take a chopstick, and with the amp on, press on every component( caps, resistors, even switches, jacks, sockets, and pots), and solder joint. I usually find the crackling with this method.
 
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