question about a 74-ish marshall JMP.

[jmpizzoferrato]

i have a JMP. i think it's probably about a 74. it has the small square inicator light (but not the big one like the 800s, etc), 2 channel/4 input, 50 watt, no master.

it sounds great. i had to get both transformers replaced when i first aquired this amp. my tech did a grounding mod (star grounding i believe?) and now, about a year later, im starting to notice some things:

1. the grounding seems to be fucked, as it buzzes a lot with single coils and still buzzes considerably with humbuckers. i also get the ocasional pop when i flip the standby switch.

2. it feels like its heating up in the back more than it used to. i never kept a close look at this but i know that when i first got it, before the transformers blew, it got REALLY hot until it just stopped working. the heat seems to be comming from the read right (if you're standing behind the amp) so my assumption is that it's the power transformer.

the question: are these two problems related? if not, anyone had a similar experience with a JMP? i really want to use it in my session this weekend but im afraid that keeping that amp on all weekend would just fuck with it more and inevitably brake it.

ok
-justin

[Silverjet89]

It could be a lot of things.
The fact that it seems to be running hotter than normal would suggest a tube problem.Take a look at your power tubes while you're playing to see if any of them are glowing purple or more red than usual. You may have a bad power tube or they may need biasing or replacement. Buzzing alone could be a wiring issue, bad filter caps, bad guitar cord, etc.
There's some good info here: http://www.geofex.com/ampdbug/ampdebug.htm

But keep your hands out of the amp if you don't know what you're doing!!!

Good luck

[hammertime]

I had a 77 Marshall with one of those metal mods that I had a tech take out. Same problems -- buzzing, overheating, blowing up. I loved the sound of the amp, but it was just too unreliable. I finally almost gave it away, and I was pretty happy to get rid of it. It would have been a great amp if noone had screwed around with it.

i have a JMP. i think it's probably about a 74. it has the small square inicator light (but not the big one like the 800s, etc), 2 channel/4 input, 50 watt, no master.

it sounds great. i had to get both transformers replaced when i first aquired this amp. my tech did a grounding mod (star grounding i believe?) and now, about a year later, im starting to notice some things:

1. the grounding seems to be fucked, as it buzzes a lot with single coils and still buzzes considerably with humbuckers. i also get the ocasional pop when i flip the standby switch.

2. it feels like its heating up in the back more than it used to. i never kept a close look at this but i know that when i first got it, before the transformers blew, it got REALLY hot until it just stopped working. the heat seems to be comming from the read right (if you're standing behind the amp) so my assumption is that it's the power transformer.

the question: are these two problems related? if not, anyone had a similar experience with a JMP? i really want to use it in my session this weekend but im afraid that keeping that amp on all weekend would just fuck with it more and inevitably brake it.

ok
-justin

[linus]

In the early 80's there was a JCM800 model that was a 100 watt Non-master. It wasn't point to point but was still well made. Mine was modded with channel switching, master volume, and ran on a pair of 6L6s and a pair of EL34s at the same time. It sounded like God but after 2 years of touring one of the power tubes gave up the ghost.

I took it into three different amp guys and all three of them swore that it never should have run with that mix of tubes and that they couldn't/wouldn't duplicate it. I had it switched back to stock and eventually sold it. It just never had the same sound again.

Have someone that knows what they are doing check it out. Tube gear is not something you want to mess with on your own.

[llamaj]

there is a saying that amps sound there best when they are on the verge of blowing up. so yours sounds really good? If the transformers were both blown when you got the amp. that is a bad sign. that means someone was trying to "Run(ing) it with the Devil". there could be other damage to the amp from blowing out transformers. im sure thats exactly what you wanted to hear. But there also may be no other damage. tube Amps get really hot. If your Power trans. (the bigger one) is too hot to touch or you can touch it for only like 2-3 seconds then something IS wrong. Your output trans shouldnt get hot.
three questions for troubleshooting
1.does the buzz get quieter when you touch the strings on the guitar?
2. have you tried a different cord/guitars?
3. have you tried new or different preamp and power tubes?

I know these sound like dumb questions but people bring me amps to fix and 50% of the time its something along these lines

other questions
is the amp biased?
the noise from standby- is it just a pop or a rushing sound? a rushing sound could be loose or bad caps.

an extreme hum or buzz could be a loose anything or a worn/bad resistor

I was playing a active pickup jazz bass last week through a blackface showman and it started acting crazy (volume started to lower with alot of noise). after changing the preamp tubes with no luck I started to sweat knowing that not long ago I did a tune up on this amp and got it to sound wonderful and clean. I unplugged the cord and pop. the jazz bass needed a new 9 volt.

if youve troubleshooted with the three questions and no change in results its time to call your local tech. beware God did not create all techs equal. I became a tech because i love guitar but my local tech loved money and telling stories. he would actually call me up to see if he could work on anything for me. what an asshole...

anyways..it is possibe that your ground came loose. or you may need filter caps. If they look to be the original filter caps then you should change them anyway.

[rolandk]

I could be totally off base here- and if I am then I apologize, but...... how good is the tech who worked on your amp? Seems very odd to have to replace both transformers at once. Did they give any specific reasons? Did they do any other work to it?
If it was me, I'd take it (or send it off) to a super reputuable amp tech and have them evaluate it. If the tubes and transformers are new then I'd guess it couldn't be a whole lot more money to get it fixed right.

Good luck! Sounds like a sweet amp thats well worth getting back up to top shape.

[jmpizzoferrato]

i havnt been able to troubleshoot the amp yet- and probably wont for a while as i am just too busy. when i get around to it, ill definitely try those three tests.

i might just bring it to someone else to have it reevaluated. im keeping my fingers crossed here. i love this amp. it was free but i dont want to get rid of it. it sounds soooo damn nice!

thanks

[jmpizzoferrato]

the transformer doesnt seem to be heating up. the power tubes seem to running very hot and one of them has a faint purple glow.

i think im just going to bring it to reputable tech and have him diagnose it. thanks for the help!

-jp

[0xeneye]

Get thee to a tech. Grounding problems, hum problems, glow problems can all be related and dangerous. You will be blowing tubes until you get someone to check voltages through the caps, verify the grounding, and verify the DC filament voltages at various points in the amp.

I have a JMP, sounds like godhead, runs great, going on 12 years with the same Mullards! It's not the design of the amp, it's the proper maintenance.

If the transformers were replaced, then make sure they are correct, else you will be blowing speakers too.

Good luck

[lacquer_monkey]

A faint blue or purple glow in an output tube is quite normal. What you need to be worried about is if you see the plate, that big chunk of sheet metal in the tube, glowing even dull red anywhere when the amp is idling, That dull red glow would indicate inadequate bias to keep the tube under control. That is the condition that blows stuff up. The envelope of a tube gets pretty damn hot, easily 200C. Using the old 9/5 + 32 conversion that's 392F. That's really pretty hot. The underbiased condition will cause excess D.C. current in the output transformer as well as excess draw though the power transformer. This would cause both of them to run hotter than normal. You can look at the power tubes in a dark room to see if this condition may be your problem.

jd

[Coco]

Good advice given in the above posts. I had a Hiwatt that had a small mod done to it before I bought it. Someone put diodes accross some of the pins on the power tubes. I gues this is a stock thing in 5150's to keep power tubes from blowing, but I am not sure. Anyway all those shitty little diodes did was fry when I cranked the amp. Long story short, I had it put back to stock and it sounds beautiful. Sometimes all these mods do is make the amp run and sound like shit, especially in my Hiwatts.

[llamaj]

couldnt agree with you more Coco
the diode (tube saver) mod does work with minimal change in tone on some amps
but most ive tried it on didnt like it. it made the amps sound like some of the new mass produced amps without character.
there is another mod using a varistor (130V) between the power transformer leads
that works with no apparent change in tone. eats up spikes. might be a good alternative for the Hiwatt if you play it hard all the time or gig with it.
i had a opportunity to buy a Hiwatt 50w head and a Guild Les Paul style guitar for 350 (150 for the guild 200 for the amp) bucks back in 1991. I didnt have 350 bucks. Im still trying to get back to that sound i heard then

[ctmsound]

In the early 80's there was a JCM800 model that was a 100 watt Non-master. It wasn't point to point but was still well made. Mine was modded with channel switching, master volume, and ran on a pair of 6L6s and a pair of EL34s at the same time. It sounded like God but after 2 years of touring one of the power tubes gave up the ghost.

I took it into three different amp guys and all three of them swore that it never should have run with that mix of tubes and that they couldn't/wouldn't duplicate it. I had it switched back to stock and eventually sold it. It just never had the same sound again.

Have someone that knows what they are doing check it out. Tube gear is not something you want to mess with on your own.

http://www.thdelectronics.com/products/flexi.htm

You'd like this amp. Read up on it. You can use any tubes in any combination. THD makes some great shit too!