1980 JMP 50 Lead

[Adoom]

Hi Folks, this is my first post here, you seem like a good bunch.

I have an older JMP 50w head, model 2204. It's been a great head, I've owned it for 20 years, the owner before me was the original owner.

Unfortunately I put it through hell. I play live, in a very loud band, usually 6 times a month. It gets moved a lot, it gets played a lot, and so far it's been a champ. That said, the sound seems to be starting to suffer. Recently it began to sound like there was a treble spike, like the treble was on 10, not 4 like it was. The crunch also seems to be starting to degrade. It was re-tubed many years ago, so I'm wondering if it's time for a re-tube or if the cause is deeper than that.

I'm a player, not a tech. I can replace tubes, and fuses, but I've never gone deeper than that with this amp.

What can I do to start trouble shooting this guy.

It's still strong, it's still loud, but with my gigging schedule, I can't afford a catastrophic failure. Unfortunately I'm pretty broke, so any repairs need to be done, by me, on the cheap, and quickly between gigs.

Any help would be appreciated.

[pdf64]

So if the master vol is turned up full, it can still put out a big solid clean sound, with the right gain/tone settings?

'The crunch also seems to be starting to degrade'

Can you give more detail here?

Have you tried a different guitar lead? Speaker lead? Is the impedance set correctly on the back panel (someone may have screwed with it)?
Have you got any spare tubes, even just borrow a good ECC83 / 12AX7 from a friend?
Pete.

[olddawg]

At 33 years old it most likely needs a recap, tubes, and bias. No big deal, while inside the amp they should clean the pots, blow out the dust, and inspect for any other things on the horizon. It will not be that horribly expensive. Bite the bullet and have a pro do it.

[Adoom]

Okay, so a little update. The sound was getting thin at practice again. Between sessions I pulled the back off and did the old "tap on the tubes" scenario. On the E34L's 1 tube was fine and solid, the other let out this insane CRAAANG!!!!. To further troubleshoot (as this tube was new) I switched locations, and tapped again, the CRAAANG followed the tube, so I pulled the tube, and ran the rest of practice with just the one E34L.

Now, as I scratched my mind I went back in time in my brain. The last time this amp was re-tubed it was done by a friend of mine (at the time) who was using it for a few gigs and recording. I remember when I got it back it didn't sound the same as before. This head used to have a real crunch and a lot of balls. At the time I chalked it up to "whatever I guess" and I used a DS-1 to compensate for the lack of crunch.

Fast forward to now, I'm looking at the tubes, here's what I have.
3x ECC83's. These appear to be no-name. There's no brand stamping on these tubes.
2x JJ E34L. At least these are stamped.

It appears, to me, that my friend had it re-tubed, yes, but with what appears to be budget tubes.

Now I don't have a lot of money at all, but if I'm looking to re-tube this amp, I'd like to get it right...or close to right the first time.
What would be a good tube combination for this amp? I *think* the original would have been ECC83's and 6550's.
I am considering 3x JJ-ECC83-S with 2x JJ 6CA7. OR going with 6550's.

There's so many tube options, I'm starting to get lost in the choice out there. Suggestions?

[g1]

You are in Pickering. Does the amp have a CSA or Ontario Hydro sticker on it? Only the US amps had the 6550's, and I doubt you have a US amp. So I would suggest EL34's, 6CA7's or E34L's. Whichever you choose, you should have the bias adjusted for the power tubes, it is possible this was not done last time it was retubed.
Other than that, brand and variety are pretty much a matter of personal taste, especially the preamp tubes.

[Adoom]

You are correct, it does have a CSA sticker on it. Thanks!!

I'm looking for a high gain arrangement with crunch. I would like to ditch my DS-1 and get all of my distortion from the amp itself, what would be a good tube combo for that?

[Gtr_tech]

Don't run the amp with only 1 output tube. That can possibly damage teh output transformer since you're forcing it to try to run single ended and the output transformer won't like that for long.

Anyway....its old. It needs tubes and caps. Then it'll scream like it should.