khai123

I now have both of my amp heads in-operable. A '77 JMP heavily modded by Lee Jackson (way before I got it) and an '87 JCM 800. The JMP went about 4 months ago in my 1st attempt at daisy chaining, I played for about 5 minutes then a squeaky rumbling sound and the JMP fizzled. It's been sitting for those 4 months, and then maybe 2 weeks ago I started checking the amp over. Discovering that the only vidible problem was that the HT fuse was blown. So I replaced it with a 1A 250V that had been blown (the one I replaced it with wasn't blown) and it worked until the volume got past 3 or 4 and it made that same noise and all sound disappeared. But then I discovered that I've been using Fast Asting fuses (my dumb ass self) so I never even attempted the right fix apparently. Then just yesterday my JCM 800 went. I got it about a year and a half ago in LA for supeer cheap (like too cheap) and got it serviced by a local tech shop (The Tone Shop) they almost blew my dream amp by doubling the bias and cleaning everything with a weird solution that caused overheating, said the next, much better, tech; Damage Inc. It'e been working great and sounding great for about a year. I usually play with all the volumes on 10. Master-10 Gain Channel-10 Gain Knob-7/8 Clean Channel-4 Presence-6 Reverb-10. So after about 45 minutes of playing with the clean channel cranked (Ramones similar sound) I switched over to the gain channel which as already stated, almost permanently cranked and after about 15 minutes the sound slowly dissipated and the amp shut itself off. The Mains fuse was blown. I replaced with a correct 4A 250V, but a really old one. It blew as well. But the JCM 800's reaction is so calm, it doesn't even feel like anything's really wrong. It gets power, then I turn standby off and the sound comes on at set volume, then slowly turns to silence after turning itself off. The HT fuse remains unblown. Just as in the JMP the Mains fuse remains unblown. ANy help is greatly need and definitely appreciated. Sorry for such a long post, I'm stressed and looking to fix my only actual amps (which are my dream amps)

Thanks,
Eyan

Marshall Amps

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RussBert

You run the amps hard, and they are old. They need to be serviced regularly. That's just how it is

Sounds to me like a power tube went, maybe you blew a filter cap. You've had them serviced, right?

khai123

Not recently. Right after I bought the JCM 800 I had it serviced, cleaned, re-tubed, re-biased. Everything. But for the JMP, I bought it off of a local gigging player and well me and him kind of agreed "It sounds great, don't need to re-tube."

RussBert

There's more to it than just re-tubing.

khai123

Yeah, I know I should've but just as now as then. i don't really have the scratch for a Tech. On the JCM 800 could I remove two tubes, the bad one included, and run it as a 50 watt? I want it to work really bad, it's my favorite head.

RussBert

Why bother owning nice vintage amps if you're not willing to do what it takes to keep them running at their best?

I can make guess' all day long, but the amp needs to opened up and properly diagnosed

khai123

Woah, I'm not saying these amps won't get what they obviously deserve. You can't just call somebody out like that. As it stands right now, I don't have the money to have them reserviced, but they will be. I tremendously respect these heads, as they are the amps I grew up dreaming of having. I don't really have dispensable funds, though. I'll take them to a tech friend who gives free advice and occasionally free fixes. I'll see what the prognosis is.

Thanks,
Eyan

Micky

Unless you have the knowledge and tools to service the amp, you should take it to a tech. A lot of times you can do more damage to it and possibly yourself trying to do things 'on the cheap'.

There is of course, a certain amount of user service that CAN be accomplished with the proper tools such as a multimeter and a service guide. Things like fuses (ALWAYS replace fuses with the recommended replacement!) and certain voltages can be an indicator of specific problems when you have the knowledge and background to interpret them.

Asking others to repair your amp across the internet can sometimes be disastrous...

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fastdave

You need to some serious reading and self teaching if you're on the 'no money route'.
Try a read at my posts on JCM 900's - especially the biasing ones, see what a mess someone with a lot of knowledge, right out of practice can make, and you might find enough in there to try a fix.
Problems like these are so common, repair shops even know what components to carry, but you need a modicum of electrical knowledge and a great deal of common sense, and more than an ounce of practical flair.
Is this you?
If so - read - if not - save up for a tech shop to do the biz.
And again - if you are, and you get more specifics via measurements - then we can probably help - but read the following written by a clever guy.

Believe it or not, the factory recommended procedure on this amp is to put it on an AC current meter and adjust the bias control for 850 mA - 1A AC draw from the wall. (I have this secondhand from a Marshall warranty tech...) I'm going to have my bias-adjusting kits ready in a couple of weeks; it'll still cost you some bucks, but at least you'll have some test equipment when you've forked over your cash. If you're not familiar with the insides of a tube amp (or if you don't have any idea what high-voltage safety procedures entail) you'd be better off not to go poking around inside your amp. However... if you don't want to pay someone to do it for you, and you are determined not to send me any bucks for my bias-kit gizmo, listen up: What you'll need is two 1-ohm, 1-watt resistors, a DMM, and a soldering iron. If you look at the power tube sockets, you'll notice that pins one and eight are connected together, and that there is a wire leading from them to a ground lug that's attached to one of the tube socket mounting screws. (On a Fender, only pin eight will be grounded. DON'T ground pin one, or you'll get a big surprise...) CLIP this short wire from the ground lug and REPLACE it with a 1-ohm resistor on each socket. Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of either socket. You should see a negative voltage in the -35 to -50 volt range, if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s (or 6L6s). First, locate the bias trimmer. (Possibly a little square blue thingy with a screwdriver-adjust slot in the center, or a round black thing that stands on three legs, or, for an old Fender, a full-size pot with a screwdriver-adjust slot on both sides; newer PCB-type Fenders use three-leg horizontal trimpots, if they have a bias-adjust pot at all.) Next, adjust the bias control until you have MAX NEGATIVE voltage on pin FIVE. (In other words, rotate the bias trimmer until you obtain the highest negative voltage that the bias supply is capable of delivering.) Install your tubes (the amp is still on STANDBY, remember) and wait a few minutes for them to warm up. Take the amp off STANDBY and make sure your DMM is still set to the highest DCV scale; take a reading between the chassis and pin THREE on either power tube socket. Write this voltage down; you'll need it later. Now, set your DMM to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor(s). This reading can be interpreted directly in milliamperes, for reasons I won't go into here. It'll be pretty low, because you have the bias trimmer set to max neg voltage. NOW...adjust the bias trimmer until you get a reading across the 1-ohm resistor(s) somewhere in the 30-40 mV range. MULTIPLY the voltage you read on pin THREE earlier by the reading you just obtained from the 1-ohm resistor. (Example: 450 Volts times 35 milliamps, or .035 Amperes.) This will give you the STATIC DISSIPATION WATTAGE at which the tube(s) is idling. (It'll be wrong, but more on that later.) The above example gives a static dissipation of 15.75 WATTS, which is well within specs for an EL34 (fairly cold, in fact) or a 5881/6L6. Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the supply will sag somewhat. Multiply this new reading by the value you measured across the 1-ohm resistor(s); this will give you the idling wattage. If you have 5881s or 6L6s, you should shoot for 18 watts or lower. (The cooler you run the tubes, the longer they'll last. If you dig the way the amp sounds when the tubes are idling at 12 watts, fine...don't worry about it.) For EL34s, go for 21 watts or less. Remember, each time you adjust the bias control, you'll have to take a new reading from BOTH the 1-ohm resistor AND the plate (pin THREE) and multiply them to see what the tube is dissipating. You can play your guitar through the amp each time you adjust the bias, and see how you like it. You can even adjust the bias by ear, and then take readings as outlined above to see if the tubes are being operated within their ratings. If you find that you only like the tone when the tubes are operating near their limits, you may decide to trade some tube longevity for the tone you need. If you like the tone with the tubes running cold, you'll obtain significant extra tube life that way. It's your call! If you see a few milliamps difference between the two 1-ohm resistors, don't sweat it; this could be due to poor matching (not a factor if you bought 'em from me , differences in screen current between the tubes, or differing leg impedances in the output tranny's primary. (All of those things are fairly common in guitar amps.) If you see a large difference between them (say, 8-12+ milliamps) this means you need to find out why this difference exists. One thing you can do is SWAP the tubes into the opposite sockets and take new readings. If the readings are consistent on the SOCKETS, then you'll need to look at the amp and see why. If the readings MOVE with the TUBES, you can be fairly sure you have a poorly-matched pair. REMEMBER...THERE ARE VOLTAGES PRESENT INSIDE EVEN THE SMALLEST TUBE AMPLIFIER WHICH WILL KILL YOU JUST AS DEAD AS A HAND GRENADE WILL!! If you're not familiar with high-voltage safety, seek guidance from someone who is. BTW, an oven mitt (real men like me use welding gloves) will come in handy for handling hot power tubes if you need to switch sockets; you don't want to let the tubes cool off too much while you swap them before taking new readings.

With apologies to the writer - I solemnly promise to try to find your name, and feel free to call me a plageurist, but also to add your name - be flattered though, it's good enough to use anywhere.
Dave

SuperBeaver

Sounds like you have a 2210 JCM 800?
They are not as straight forward as a 2203 but as the other guys have said here running them flat out all the time you are gonna put a lot of stress on all the internal parts not just tubes.
Depending how often you play it if running everything pretty much dimed (reverb on 10??? ) you are going to need to have work done to that amp a couple or three times a year.

Good tone costs money man...

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RussBert

fastdave,

parentheses are your (and our!) friend! That's impossible to read

RussBert

AMEN!

RickyLee

Where are you located?