JTM 60 Mod help!!??

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beaulieu
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JTM 60 Mod help!!??

Post by beaulieu » Fri Sep 07, 2007 9:41 am

I have a JTM 60 combo with 3 10 inch Eminance speakers.It sounds kinda ok but better with a 4x12 cabinet or 2.My question does anybody know of any mods I can do to this thing to make it sound better? It has a kinda iritating vrrrrrr sound to it. I don't really know how to explain it.It is very noticable when playing the 3 10s and you can still hear it waith a cab. I know its not broke but how this amp sounds.I guess it sounds kinda solid state. I am usin JJ 12ax7 tubes and Electro Harmonix 6ca7 tubes.I can't remember if it has LEDS for the distortion but if so would this be the culprit? I have several diffrent Marshall cab configs(25's,30Heritag,65,and 75 watt celestions) so I know its not the cab.Anybody have any ideas or should I just sell it and get something else? Image
69 SuperBass Plexi
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Gunner
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Post by Gunner » Tue Sep 11, 2007 4:50 am

Hi beaulieu!

I've done some serious mods on a JTM60. It took the amp from a boring, dull, paper sounding amp to a nice AC/DC rocker. I can give you details what I strongly recommend to change. But it depends on what sound you prefer.
First of all - you must be sure your amp is healthy. No modification should be done in an sick amp. From your description you might have some problems with the filter caps, giving the noise. But it can also be "normal" but unwanted ghost notes (not unusal in more modern amps with several gaint steps, my exeprience).
You should also know it's a thrill working in this pcb monster. The copper foil falls apart if you're not very careful. Still

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Post by deke99 » Wed Nov 14, 2007 12:39 am

Hey there Gunner,

I'm a newbie to this forum, but I saw your post and had to reply. I recently acquired a JTM-60 2x12 and I am in full agreement that in its stock form, it is a boxy sterile sounding amp. I would love to learn some more about the mods that you performed to yours. I am a classic rocker and those are the tones that I look for. I have an 18 watt JTM-45/Plexi style circuit that I absolutely love (both when dialed clean and dirty). It sounds like your mods may help me open up the tone of the JTM-60 into that territory. The band I'm currently in requires more power so I can't use the 18 for those gigs. Can I PM you for the details?

Thanks!!

Derek

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Post by d_wagar » Sat Nov 24, 2007 4:31 pm

I'm also new to the forum and am quite interested in the mods to a JTM60. I have the JTM622 (2-12) with the matching 2-12 cab.

I was toying with the idea of replacing the head with a kit, but I'd like to see what your mods can do first.

TIA

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Post by deke99 » Sun Nov 25, 2007 3:05 pm

Hey d_wager, welcome fellow newbie! I have the same model. On Friday I bypassed the diodes that are in the circuit for the distortion channel. I would say that is a very good start. It still has plenty of gain and the distortion is much more natural sounding. I got the details over on the Les Paul forum which I believe you posted on (dwagar). If I learn of any additional tweaks, I'll keep you posted.

D

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Post by d_wagar » Sun Nov 25, 2007 5:08 pm

Hi,
yep, that was me on the LPF. PM Gunnar here, he sent me a list of his mods, killer stuff.

I won't have a chance to try any of them for a month or so, but I am looking forward to it.

So, you liked the diode bypass mod?

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Post by deke99 » Sun Nov 25, 2007 9:10 pm

Hey D,

I did PM Gunnar, I'll look forward to hearing from him. The diode bypass is a big improvement. It makes the distortion more natural. I played a show with the stock amp and the distortion was just WAY too over the top for me. I had the gain around 3 or 4 and the distortion was nowhere near 'classic'. It sounded like a buzz saw. For both channels, I just couldn't get enough treble to make my neck pickup a 'toneful' selection. I had tried to improve the amp by swapping in JJ preamp tubes, but I'm finding JJ 12AX7s to be too midrangy for me. I like the stock Marshall tubes better . . . go figure.

I know this amp has a bad rap in general, but the one I have seems to be ok. I did put a few washers between the back screws and the top of the chassis to help it breath, heat is definitely an issue. If I can improve the overall tone to the point where I really like it (and want to keep it), then I'll go beyond the reversible stuff and install a vent on top. After the last show, the metal strap end nearest the power tubes was so hot that you couldn't touch it for more than a second or two!

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Post by Gunner » Mon Nov 26, 2007 6:00 am

Hi guys,

I've sent you both some detailed information and I hope you'll enjoy it :)

The JTM60 I've modified is owned by the second guitarist in one of my bands. We are playing classic rock, just some covers for fun. He use a nice strat and is a very decent big guy, but a mediocre guitarist... I couldn't stand hearing him pushing repulsive ugly muddy distorted sounds from his gear. I mean hey, we are actually trying to create some music here... So I asked him if I could borrow his JTM60-amp for a weekend 8)

Last saturday we had a gig at a club here in Stockholm, Sweden. And I must admit his sound was almost as good as my sound. But I'm playing on my hand wired gear with 300 dollar Mullard tubes and have spent hundreds of hours building, re-building, fine tweaking, ...

Still - don't think it is magic. Just a good basic amp that amplifies a strat without any unneccesary freqs roll off. The bad constructions compromises and failures are removed. Good sound in, good sound out, as it were back in the good old days...

/G

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Post by d_wagar » Mon Nov 26, 2007 8:47 am

Thank you Gunner.
It's almost impossible to find mods for these amps. Which is a shame, as I quite like mine. I've had it since new ('96?), haven't had any of the overheating problems, and always thought there was probably a bit more in there but couldn't find any info on getting it out.

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Post by deke99 » Tue Nov 27, 2007 8:00 pm

I second the thanks! I'll be getting my parts in hopefully before the weekend so I can start the tweaking process. I'll let you know how they turn out.

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Post by Tyler Savage » Thu Dec 13, 2007 6:48 pm

Wow! I've been waiting for this thread - I was thinking of tinkering with my jtm60 myself.

Gunner, when you say open'er up, do you actually mean break connections, or loosen up the component values?
if I could get in on these instructions, I would be eternally greatful

My amp is half decent - set of new tubes and a maxon od808 in front of the clean channel keeps me satisfied, but it's not pushing any borders here, so I'd like to see if I could add a bit more of the monster to it.

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Post by Gunner » Fri Dec 14, 2007 2:32 am

Since this topic seems to be of general interest I give you all information here. As I've said to everyone - this is no magic, just some basic re-work.

My idea was simply to convert the clean channel circuit close to a normal 1987-circuit, a bluescrunchy tone with more fat bottom and crispy high end. Since the trannies are good in this amp, my small mods is doing good for the sound, it has effect!

I guess you already have the scheme? I fo not you find it here: http://www.schematicheaven.com/marshall ... jtm600.pdf

Be very careful with the copper foil on the pcb. Be sure the component mounting holes are clean and empty when you put in the new component. This is often the moment when the copper foil comes loose.

1) Change R118 on the V101A input grid to 22-33k. This mod is more important than it seems. What you cut at the input (treble) you can never restore later in the circuit. But, it might be a god idea to cut the copper foil that goes to V101B and put another 33k from R118 and to the tube input grid at V101B. The high gain channel does like the treble roll of with the higher value 68k (33+33k).

2) Cap C116 is stock 2n2, cutting bass. Raise to 22n. A Mallory or any polyester cap will do fine. A Xicon polypropylene works to (better fit on a pcb), it does not affect the sound at all, very transparent. But I like the sound from Mallorys (very subtile differences of course).

3) Cap C114. Do you want your amp to have a fat bottom, blues crunchy and not so stiff, then raise the value from 1uF to lets say 22uF (you might already have a 22uF here, it seems to be stock value in some amps). The lower value rolls off a lot of bass.

4) R129 just before V103 (V2). This resistor rolls off some treble freqs. Here a compromise is necessary. The clean channel might like a treble bypass cap here, a shelf filter, in the range of 120-500pF depending on how low freqs you want let bypass the resistor. The higher value, the lower freqs will bypass. The high gain channel might not like this at all. But my main concern is the clean channel, so the high gain channel has to take what it gets... You choose your way!

5) V103. R128 is 220k, lower to normal 100k. In the same time you have to lower the cathode resistor R131 to 820 ohm (or 1k will do). This makes V2 run cleaner, a more classic tone. If you want the typical Marshall upper mid/treble lift, put a 0.68uF cap over R131 (just solder to the legs of R131). If you don

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Post by d_wagar » Fri Dec 14, 2007 9:31 pm

Fantastic info Gunner, thanks again!

I'm going to get my JTM-622 home over Christmas (I keep it where we practice) and try these mods out.

I'd be interested in what you do to the OD channel too.

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Post by Tyler Savage » Sat Dec 15, 2007 9:33 pm

Thanks so much, this saves me lots of trials and circuit checking. I'm an engineer by trade, I really appreciate the attention to detail.

I'm going to apply most of these mods this week and follow up with my results. Right now my amp is running through a marshall 4x12 with greenbacks (it's the 3x10 version though) , and sounds "great", (granted it was rebiased from stock with new tubes), but I think it could sound fantastic.

Thanks again gunner,

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Post by jtm60 » Thu Dec 20, 2007 6:32 am

Hi,I'm new on this forum,i'm very interested about the discussion Jtm60 mods..I have a marshall jtm60 (3X10") and I connected it to cab with(2X12") I play acdc and i would to mod the overdrive channel if it possible, because it's "glassy" :cry:
I have read that someone has modified this channel (e.g:removing o bypass diode clipping and other some changes).Someone can help me?
Thanks!!
Sorry if my english isn't correct.I'm italian..
:D

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