71 JMP - screws for backplate

not jumped - just channel I

I'll work with jumping and blending. It's too bright in the room, but some of that is mic placement. When I recorded, I was using a JCM900 4x12 that I actually never miced before. After I get the head back from its wellness visit, I'll also experiment with different cabs and mic placement, and make some more clips. Just didn't want to go six months before I posted something :)

Another issue I think is that the pickup on my guitar is pretty hot (Gibson 57 classic - actually, you wouldn't think it would be hot, but it is), and so I'll work with turning it down / other pickups, etc. I'm almost certain the amp is working as designed, and I just need to learn it.

One other thing - what I'm hearing is much louder than what someone listening to the clips would probably be hearing. What's wild is that it doesn't seem loud until you do something like try and scream your lungs out and realize you can't hear a thing.


edit: and other thing that kind of freaks me out - it's still got the original power cable with two prongs. Now the studio room I got pushed into due to local politics in fact doesn't have grounded power anyway, but I'm worried that if I gig with it, I'd be taking a risk - do you all think it would be a reversible job to put a grounded plug on the thing? The tech will know, of course, but I thought I'd ask here, too.
 
OK - got the JMP back from the shop. I didn't get a chance to talk to the guy much yet, but I do notice that when I take it off standby, the sound fades in, where before it seemed like it just came on. Maybe that was just in my head. I had read that a lot of the 1971s (and 72s?) were miswired at the factory, such that the standby circuitry didn't actually protect the power transformer the way it was supposed to, and that a lot of the units had to have their power transformers replaced because they blew.

He did note that the power tubes were about blown, and he replaced them with some new Ruby EL34s and re-biased. The originals (maybe? maybe not really *original*) were 6CA7s that say made in Yugoslavia. He said that with the old tubes he was measuring 35 watts of output, but now it's 60. The amp is much louder than before, which is sort of a problem, but what're you gonna do. It's much too loud for me to record anything until the weekend - my house is about as soundproof as a sheet of toilet paper.

I did try jumping the channels - thanks guys. I actually tried also running another patch to my JCM-800, as shown below:
IMG_20130117_192609.jpg
not sure how well that worked - it seemed to be cutting the input strength to the JMP quite a bit - the JMP was making gwidgy sounds until I unplugged that cable, then back to normal. With the channels jumped, it's a lot less bright, but of course you can adjust that by setting the relative levels on the two volume knobs. I'll have to mess with that some. It's really like a different amp with the channels jumped, in a good way.

Another bonus - the amp guy put screws in the backplate! They look like they've always been there. I took the backplate off, and here are a couple of more pictures - this label is on top of the chassis - I can't read the handwriting
IMG_20130117_211212.jpg

And here's most of the insides. Not the real P to P insides, I'll get a shot of that at some point. Anyway, I took the cover off one of the preamp tubes - it's a Sylvania, and I had pulled one of the power tubes to see what they were. You can see the three transformers in the background. They all appear original. At the far left on the backplate, you can see an empty hole that's probably where there was a master volume mod that was later removed. I got my guy to put in a 3 prong power cord cause I don't wanna get killed. If not for the hole that was already in the backplate, I probably would have stuck with the original cord, but I figure since it already has that hole, I can live with the new power cord. I now have the old one separately that could be put back if need be.
IMG_20130117_211246.jpg
 
Cool stuff. Jealous. Classic Marshall powerhouses.


It's hard to tell from your pic, but I think you jumped the channels wrong, or at least not the usual way. I don't have too much recent experience with those amps, but when I did, I plugged the guitar into the upper left, then jumped lower left to upper right. Blend with the two volumes.
 
Ah - yeah, i did the mirror image of what you're saying - guitar into upper right, and jumped lower right to upper left. I'll try it the other way and see what the difference is. I'll also look into whether/how to run the heads together, even though I probably won't really do that much, or at all
 
antichef said:
Ah - yeah, i did the mirror image of what you're saying - guitar into upper right, and jumped lower right to upper left. I'll try it the other way and see what the difference is. I'll also look into whether/how to run the heads together, even though I probably won't really do that much, or at all
Click to expand...

To run the 2 heads in parallel you would need to plug the guitar into a distortion or effect pedal that is not true bypass so that you have a buffer. That way you won't be splitting the guitar pickup impedance. Then run it the way Greg suggested.
 
well, shoot. Came home from work and plugged in, turned up to about 4/6 and had about 8 seconds of awesomating, then the volume dropped to nothing. One of the new power tubes looks scorched. I tried replacing them (and each preamp tube in turn), but nothing. Back to the shop...
 
ocnor said:
Wow that really sucks!
Click to expand...
Yeah - such is the way of amps that are [almost] as old as I am, I guess. Thanks for the post about the buffer. You'd think after 30 years of playing I'd know that stuff, but I was always a no effects guy (but not anymore - I'm starting to use them. All mine are true bypass at the moment, I think)
 
Alright - it was just a bad tube (which blew a fuse) - got it back today. (whew!!!!)

Here's another clip - I think I may have gone too far to the edge of the cone on this one, and I also switched microphones (to a SM57) and preamps (to a GR MP2-NV) - I think the other preamp may have been my main issue for lack of bass and brightness, but I'm not sure - I'll figure it out later. My biggest problem is that the thing is so loud that I can't really experiment, which I need to do. Maybe I can figure out how to get the best sound at a lower volume, then turn up, but that doesn't seem like a 100% certain approach.

This is dry with the channels jumped - the guitar going into channel I, with channel 1 volume on 2 and channel 2 volume on 4.



I gotta find somewhere to really open this thing up with out law enforcement involvement.
 
Cool man. With more experimenting you'll get better and better with it, but that certainly does sound old school Marshally.

The problem with those non master volume amps is that if you dial it in and then turn it up, it'll just get more and more overdriven. More volume = more crunch with those amps. It's not like a master volume amp in which the tone stays somewhat the same the louder you go.

You might be good candidate for an attenuator or a PPIMV mod (post phase-inverter master volume). Your amp tech should be able to do that mod pretty easily if you wanted to go that route. I don't know how to do it, but everyone says it's easy and reversible if you ever wanna resell it (to me).
 
Yup - gotta be loud. It's actually a nice clean sound at low volume, but not all that remarkable.

I have a Power Brake attenuator, and I've been debating on whether to try it out with this amp. My amp tech (the same I guy I just got the amp back from) warned me that he'd be replacing the output transformer in my JCM-800 if I used an attenuator with it. I didn't tell him which model I have (or that it was made by Marshall), and his reaction was quick, and he's pretty old school, so *maybe* I can overlook the warning, but it still freaked me out. The JCM-800 sounded really strained with it when I did try it, but that was before a much needed bias correction, so maybe it had something to do with that.

I might try it for 90 seconds or so - but of course if [heaven forbid] something happened to the output transformer on this old amp, that would be a capital offense.
 
I don't know dude. Lots of people use attenuators all the time. It's your call though. There used to be a guy here that had a 1959SLP and he used a THD Hotplate on his all the time. Worked and sounded great.

On the flip side, you might not get enough attenuation to be quiet enough for neighbors while still getting a decent tone. I can't imagine that amp would sound too good squashed down to conversation levels. Probably not even loud stereo levels.

Another option is to keep it relatively low volume and just slam the shit out of the front end with a clean boost or overdrive pedal. That works well on those amps. I played a gig recently with a guy that had a 50w 4-holer like yours and even without an attenuator and at reasonable club gig stage volume he had really nice tone with his pedals pushing the front end.
 
The Power Brake will work fine as long as you don't turn the amp up to 10 and try to attenuate it down to bedroom level. You"ll be able to knock down the volume a few Db but it's still going to be loud.
 
Maybe I'll get the balls to try the Power Brake soon - makes sense about not turning it down too much.

On the front end front, I had been trying to figure out what to do with this pedal:
13 - 1.jpg

I picked it up at an Austin pawn shop just based on the faceplate. It seems to be a slightly dirty boost, and I've used it to good effect (heh) on another amp in between the preamp and power amp. It's clearly home-made,but with decent components. The lights in the eyes are always on or always off, which I've been trying to fix without success anyway -

here's the amp with channel I at 0.2 and channel II at 0.5 - very bedroom level. Then comes the sponge box - what do you think?



I think it sounds a lot better than anything I can do with a tube screamer or that fulldrive 2 or DS-1
 
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