JTM 60 Mod help!!??

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Gunner
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Post by Gunner » Thu Dec 20, 2007 7:55 am

Hi jtm60! I'm sure your english is much more correct than my "swenglish" :D

OD-channel is more complicated to modify IMO. The most interesting and

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Post by jtm60 » Thu Dec 20, 2007 8:25 am

Wow!!
Thanks a lot for your invaluable advice! :D
I will remove the diode clipping (...lift/cut the cap C105 and R104...) as soon as I can.

Jtm60

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Post by lp20th » Wed Jan 02, 2008 11:04 pm

I've been a lurker on this site for awhile wishing and waiting for the day I can afford a Metro kit. In the mean time I have my JTM60.

Some great info here on what can be done to the JTM60 amp. I was the one over at the Les Paul forum who suggested bypassing the clipping diodes.

One thing that some of you may want to consider if you want to do this mod is to put a feedback resistor in place of the diodes and not just take them out of the circuit. If you look at the schematics the clipping diodes are fed by the output of V102A and they in turn feedback into V101B. This is a negative feedback loop. The diodes clip the signal, creating more of a square wave to the signal. A square wave if full of harmonics, thus the thick distortion. This negative feedback loop controls the gain. If you simply take the diodes out of the circuit you lose any negative feedback and are moving towards open loop gain. This may be why Gunner is seeing so much gain. I put a 390K resistor in place of the diodes to maintain a feed back loop without having any clipping diodes distorting the signal. Below is how I did the mod to control the gain and get pure tube overdrive.

I lifted the leg of C105 out of the eyelet where it junctions with D103 and D104. I soldered one leg of a 390K resistor to the lifted leg of C105, keeping it off the PWB away from the eyelets and other components. I used a little RTV silicone to hold the body of the 390K resistor to the PWB, right at the top edge of the board. The other leg of the 390K resistor was soldered to the eyelet where D101 and D102 come together. This effectively takes all 4 diodes out of the circuit and puts the 390K resistor in there place, it also makes it very easy to put it back to the stock setup if desired. I played with the resistor value to try keep the sound level about the same as when the diodes were in the circuit. Too little or too much on the value changes the gain. I settled on 390K.

Also the tip on playing with the value of the R118 and R119 is varying the B+ voltage level that feeds V101A and V101B. Increasing the value lowers the B+ voltage putting the tubes into a more non-linear area of operation and this opens up the tone of the amp a lot. I go this tip from TennesseJed over at the Marshall Amp Forum.

lp20th

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Post by jtm60 » Thu Jan 10, 2008 2:11 am

sorry, can i use a 390k resistor 1/4w 5% to do this mod?
Thanks

This is the schematic circuit of my jtm60 is correct where i put the 390k?
also I must to cut/lift where is red?

Thanks a lot!

Jtm60
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Post by Gunner » Thu Jan 10, 2008 3:29 am

Aah, this was interesting!

The scheme as well as the amp I've modified hasn't got this local feeback solution at all. The diodes just goes down to ground. The feedback solution is of course one alternative way of keeping gain down a bit.

jtm60 - as I understand lp20th you shall not cut the path where you have marked with red. Instead of using the diodes in the feedback path you now have a 390k resistor (I guess you can try different values, or have a trim pot here, but if 390k works well, go for it).

There can't be any high voltage or current here so a 1/4w would probably work well, but I always use 1/2w or more in my amps, just to be sure.

/G

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Post by jtm60 » Thu Jan 10, 2008 5:06 am

Thanks for your reply.
Does my jtm 60 combo have PPIMV? :oops:
Sorry i'm a bit confused about this...

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Post by Gunner » Thu Jan 10, 2008 5:30 am

No. This amp has a standard Marshall pre phase inverter master volume. Works ok for a high gain pre amp.

A post phase inverter mv is possible to install, but it is a little bit complicated in a pcb amp.

The advantage of a post pi mv is that the phase inverter is working un-attenuated when turning mv-volume down. The pi will still distort as if you put the present master volume at max. And on low mv settings it will distort even more since low mv settings means very little nfb back to the pi. It will run "hotter", distort more, and more aggressive.

You may like it, but if you want to run your normal channel rather clean the present mv is ok.

If you use the high gain channel and a post pi mv on low settings you'll get VERY much distortion since the pi is screaming from all the gain (high signal level) it has to handle.

/G

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Post by jtm60 » Thu Jan 10, 2008 5:55 am

ok thanks i have understand.. :D
When I play my guitar, I usually set the Master volume to Max, and I turn just overdrive channel volume.
Is it right use the amply in this way??

Thanks!

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Post by Gunner » Thu Jan 10, 2008 6:15 am

That's at least how I think you get the absolute best sound from the normal channel. And you can even get som nice crunch when pushing that channel hard.
With mv on max, or at least very high, the pi is working non-attenuated, and do probably add a little amount of distortion in itself.
If you turn mv down and normal channel up (max) you also get some distortion, but it doesn't sound so good to my ears. This distortion is produced in only one tube (V103), the tube driving the tone stack.

Since you have both gain and volume in the od channel it is possible to set your amp that way.

You're amp might be a little bit noisy with mv on max. That's normal. If you think it is a problem, then turn down mv a little bit. A "necessary" compromise.

/G

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Post by lp20th » Thu Jan 10, 2008 8:01 pm

Gunner is correct. I didn't cut any traces and a 1/4 watt resistor will work fine. In the preamp stages there isn't a lot of current flowing in the signal yet.

Interesting that Gunner's JTM60 clipping diode circuit is setup different. I have seen some other Marshall schematics do it different ways too.

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Post by lp20th » Sun Jan 20, 2008 9:43 am

If anyone is interested here is a photo of my clipping diode bypass mod.

I never realized that there were two versions of the clipping diode circuit in the JTM60. One utilizing a feedback loop and the other with the diodes tied to ground. Mine is the feedback loop version, so I have replaced the diodes with a 390K resistor.

As you can see I pulled C105's leg out of the eyelet and tacked one leg of the 390K resistor to it. The other leg of the 390K ties to the D101- D102 junction.
I tacked the body of the 390K onto the board with a little RTV to hold it in place. This is an easy mod you can do without even removing the board.

LP20th[/img]
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Post by d_wagar » Fri Mar 14, 2008 2:58 pm

I like being able to see it in pics.

Thanks man.

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Re: JTM 60 Mod help!!??

Post by d_wagar » Sat Feb 07, 2009 1:29 pm

talk about procrastination, I see it took me about a year to finally get around to trying the diode bypass mod.

the picture was exactly the same as my circuit board, so it was very easy.

Does this only affect the normal channel, or does it affect the gain channel too?
If it does affect the gain channel, does turning up the gain now hit the preamp tubes as opposed to hitting the diodes, just as it does in the 800?

I don't understand schematics. On my 800 (a 4010, same as a 2204) as I understand it, the difference between the Low and High channel is an extra gain stage through the 12AX7s. By bypassing the diodes on the JTM60, is it basically doing the same thing?

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Re: JTM 60 Mod help!!??

Post by Gunner » Sat Feb 07, 2009 1:52 pm

No not at all. It is not the same thing.

In a JTM60 you have to separate channels, mixed together before the phase inverter. In a JCM800 2204 you one channel and the low input just bypass the first gain stage. It has nothing to do with cutting the diode circuit in a JTM60. A 2204 has no diodes at all.

The modification does only affect the od/gain channel, not the normal/clean channel.

Sorry man, no offence, but if you don't understand schematics at all this mabye is nothing for you to go into.

/Gunnar

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Re: JTM 60 Mod help!!??

Post by d_wagar » Sat Feb 07, 2009 7:52 pm

that's okay, no offense taken.
you gotta start learning somewhere though, don't you?

when I asked this:
as I understand it, the difference between the Low and High channel is an extra gain stage through the 12AX7s
and you replied
In a JCM800 2204 you one channel and the low input just bypass the first gain stage
that is what I was getting at.

I understand a 2204 has no diodes. What I was asking was, by cutting the diode circuit out of the JTM60, is the gain then coming from the 12AX7?

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