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I'm looking for a Parts List for JTM30...

7/30/1999 5:52 AM
Steve A.
I'm looking for a Parts List for JTM30...
    I finally tracked down a schematic for my Marshall JTM30 combo amp and it is as illegible as everybody had warned me... 8-(  
    I've cleaned up the drawings and stitched them together into two tabloid-sized prints, but many of the values are very questionable. If anybody has a parts list for this model I could add the real values to the drawings and post them on my site.  
Steve Ahola
7/30/1999 7:38 PM
Carl B.
You may just have to compare the schematic vs. what's on the PCB. I found that by putting a light source shining from behind the PCB, I could better trace my Park 50 watt (absolutely no schems for this particular one anywhere).  
I was really wanting to buy one of those JTM30s a while back, but now they seem to have disappeared. Are they still made, or have the new Marshall EL84 combos obsoleted them?  
An what's your take on the JTM30's sound? Any mods you've made?  
- Carl B.
7/30/1999 11:30 PM

I thought about buying a used JTM30, but when I tried it, it sounded very thin. The tone controls had only a minor effect, and the reverb was extremely weak.  
I passed.  
7/31/1999 0:31 AM
Michael Tousek

I'm kind of curious about your take on the sound, too, Steve. I've played one in the store for about five minutes and liked it -- seemed like a real neat little amp. Do you know how far the dirty channel strays from, say, a 2204 Marshall? I'm guessing there's some diode or LED clipping since the dirty channel gets pretty saturated.  
Michael Tousek
7/31/1999 3:38 AM
Steve A.

    My eyesight deteriorated noticeably after meticulously tracing out the pcb for my Pignose G40V last year, so I recently bought myself an early birthday present: a swing arm magnifier/lamp (I got a pretty good deal using my company's account over at WW Grainger- the round fluorescent tube burned out after about 60 minutes of use so they ordered me a replacement no charge). I'll use my Makita 9.6V rechargeable flashlight a lot for the back lighting that you mentioned and will even clip a cheap Target "miner's light" on my forehead for added lighting...  
    As for the sound qualities of the JTM312... well, I bought it before I got into working on tube amps (let's just ignore the nightmare mods I had done to my 65 Pro in the early 80's trying to remake it as a channel switching Mesa Boogie, okay? That's a sore subject around here!). In any case about 5 years ago I bought the JTM312 because its OD sound was a lot thicker than the stock Peavey Classic 30 and Classic 50 amps I had been using for awhile. I built a heavy duty shelf in my music/computer room so I had the Peavey Classic 30 on the top shelf for clean Fenderish sounds (especially with the Peavey Valverb for tremelo) and the Marshall JTM 312 underneath for OD sounds (I was mainly interested in the mildly distorted Bluesbreaker settings with the Pre gain set to ~11 o'clock.)  
    But that was all before I got into rewiring my guitar amps, starting with the Dan Torres SuperTexan mod for my already butchered-up 65 Pro. Although I eventually undid all of the ST mods (except for the channel switching relay) it did turn me on to the wide range of sounds available when you start messing around with a tube amp. (I had also worked through practically all of the applicable projects in the Torres books just to see how they would affect the sound and response of the amp.) Although I eventually rewired all of my other tube amps, I had been keeping my JTM312 dead stock, mainly to check my guitar mods to make sure that they sounded okay with a "store bought" amp. For the past few years I've been playing the amps I've rewired almost exclusively and there is just no comparison to a "mush-mouth" store-bought amp... But IMHO the OD channel on a stock JTM312 does sound a lot better than the stock OD channel on most modern mass-produced amps from Fender or Peavey. (I'm not including the Fender Blues Jr and Pro Jr amps because they don't have a modern OD channel that sounds like crap added on to a decent sounding Normal channel.)  
    Now that I have a basic schematic I'll probably pull out the board to fill in the blanks... unless I can locate a parts list for that model. (Perhaps someone can post a good number for ordering service info on Marshall amps and if I luck out they might fax me just the parts list...)  
Steve Ahola
7/31/1999 6:06 AM
David B

I've owned a JTM310 for about 4 yrs...also have the matching extension speaker which makes a great difference to the bottom end (the 10" speakers sound OK on their own) but when I'm gigging the exctra 12" speaker really expands the depth....also my amp is one of the earlier ones which came with 5881's instead of EL34's....the 5881's worked OK on overdrive but had a pretty harsh sound on the clean channel....a pair of Svetlana 6L6's fixed that ...made a big improvement on the clean channel....I agree with the other comments about the overall sound of the amp....the overdrive channel is great...I've also managed to get a decent clean sound (more of a classic clean blues sound) from the clean channel....would really like to see a schematic if you get one put together....also...a couple of this a fixed biased amp?..where is the bias adjustment? Also, is there a second fuse in addition to the external Mains fuse?....For the record, I've used this amp regularly for small gigs, recording and's been totally reliable and consistently has a good sound...I've heard comments that some of these amps were of highly varying quality...Let me know how you go with the schematic.  
7/31/1999 6:44 AM
Steve, again

    I got the 1-12" version because of the better bass response compared to the version with 3 10's. BTW a Mesa Boogie V-Twin preamp plugged into the Normal channel produces some of the best clean and mildly distorted blues tones I've been able to get out of any of my amps. There seems to be a certain magic to the combination.  
    I know that there is at least one fuse on the circuit board because I blew it out when I tried using a Power Brake and setting all of the controls up to 10... (Let me know if you've been able to dime all of the controls when using the extension cabinet.)  
    In comparing the construction to the similarly-sized Peavey Classic 30, the JTM310 has much larger trannies and a real Celestion speaker instead of a cheap wannabe. My biggest gripe is that the cover over the horizontally-mounted output tubes will not allow you to use the more common taller 6L6's. (I'm sure this will make everybody here cringe, but I couldn't find tubes that would fit so I took the expanded metal tube cover off and banged the heck out of it with a hammer to deform it enough to clear the tubes...)  
    As far as I know the bias is fixed and non-adjustable. (I get a headache looking at that schematic after editing it for a few hours.) Maybe I'll sip up what I have so far and maybe you can help me make heads or tails out of it. (If I zoom in on the original JPG files sometimes I can get a hint of what the actual number is and then I write that down on a work copy of the cleaned up GIF files.)  
Steve Ahola  
P.S. BTW the ext cab I had my eyes on was the matching one with 4-10's.  

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