JMP fades out when powered off

fanofjimi

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I am almost positive (as I rarely play it) that this is a new problem. What could have changed or broken?
 

marshallmellowed

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I don't think most users play while turning their amps off, but my guess is, if they did, they'd hear the same thing. As mentioned, depending on the circuit, once switched off, electrolytic caps in the PS can continue to power the circuit until their voltages bleed off.
 
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Minegoto11

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When my JMP is in standby and silent, the sound returns and quickly fades out when I power off. Does anybody know what would cause this?
I wouldn't worry about it too much, as long as its not popping and squealing.. I was lucky enough to have a few conversations with Jess Oliver after I got my first 1987x. I had an amp go up in smoke while in standby so I asked him how long I should leave it in standby before switching the power off.... He told me to turn the guitar all the way down and switch the power off first, wait a few seconds then flip the standby. Some say that's not right and will damage the amp... Well Jess Oliver was the vice president of Ampeg and designed a lot of their amps...
I've been powering down that way for years and haven't had any issues outside of normal maintenance, and I use my fav 1987x almost every day,,,
Welcome to the forum, a lot of good info here and a lot of good people willing to help out,,, good luck
 

Georgiatec

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Looking for something I don't remember what.
When my JMP is in standby and silent, the sound returns and quickly fades out when I power off. Does anybody know what would cause this?

Totally normal, it will have always done. It's just that you started noticing it. Some people recommend instead of going back to stand by, just power off and keep playing your guitar to drain the filter caps.
 

RobS

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my amps have always done that if signal from the guitar was still going into them upon powerdown - nothing to be concerned about
 

neikeel

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As above.
Totally normal- filter caps discharging. Amps with bleeders will do it less but stock JTM45, JMP50 will often do it.
 

danfrank

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Normal!
Normal!
Normal!
Lol
This is caused by the bias cap (10 uf + 10uf) draining much faster than the main B+ filter caps. This causes the power tubes to be underbiased for the 2-3 seconds that the reserve in the B+ filter caps takes to drain out through the power tubes. That's why the sound comes back up in the amp.
This doesn't harm anything in the amp, in fact it has a benefit... The B+ caps end up draining faster through the underbiased power tubes so there's less possibility of them holding lethal voltage after the amp is turned off... The 56k bleeders need to bleed less voltage when the amp is turned off.
 

NickkiC

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My ‘81 JCM 2204 started this after replacing the original electrolytic caps.
2203 don’t.
 

Dean Swindell

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When my JMP is in standby and silent, the sound returns and quickly fades out when I power off. Does anybody know what would cause this?
That's odd. I would expect that on a newer amp where relays might be involved in some way but not a classic amp unless somebody did some things in there. Incidentally, it's best to warm up an amp on standby but not to power it down; just a small thing but reduces tube wear.
 


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