JTM 60 Mod help!!??

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Gunner
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Re: JTM 60 Mod help!!??

Post by Gunner » Sun Feb 08, 2009 3:09 am

d_wagar wrote: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?
Gain is never comming from the diodes. Gain alwyas comes from the tubes. And the word "gain" means amplifing a signal. It is very often gain is missunderstod as distortion. But distortion occurs when cutting a signal.

The diodes does cut the signal, they lower the gain a lot, more like a brake that "destroys" the signal into distortion. When cutting out the diodes you get a rather huge raise in gain in this area of the circuit, but less distortion. And now you have to produce the distortion in the tubes instead. You can use this raise in gain to push next tube into distortion. And this is why you have to balance and modify the circuit a lot when cutting out the diodes.

Knowledge is about asking questions. Go on!

/Gunnar

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Re: JTM 60 Mod help!!??

Post by d_wagar » Sun Feb 08, 2009 10:18 am

ah, now I understand. Thank you Gunnar.

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Re: JTM 60 Mod help!!??

Post by lp20th » Mon Feb 09, 2009 10:51 pm

In the JTM60 the 1st tube, V101 is shared between the normal and boost channels, each channel gets 1/2 of the tube, V101A is the normal channel, V102B is the boost channel. From there the boost channel has 2 more gain stages, V102A and V102B. V102A is where the clipping diodes are located. The signals come back together at V103 at the tone stack stage.

Clipping diodes do just what their name says, clip the signal. A pure sine wave has no harmonic content, a square wave is full of harmonics. If a pure sine wave was applied to the clipping diodes, the output would look more like a square wave, clipping off the positive and negative peaks of the original sine wave. Now the signal is full of harmonic content, both good and bad sounding. Its an easy way to get lots of distortion, or a more modern sound as opposed to the more vintage all tube overdrive.

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Re: JTM 60 Mod help!!??

Post by nicovlogg » Thu Sep 24, 2009 5:21 am

Hey all, just wanted to ask if anyone could repost the list of mods? It seems that Gunnar's list just stops halfway through. Any help would be appreciated!

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Re: JTM 60 Mod help!!??

Post by Gunner » Thu Sep 24, 2009 10:10 am

nicovlogg wrote:Hey all, just wanted to ask if anyone could repost the list of mods? It seems that Gunnar's list just stops halfway through. Any help would be appreciated!
Strange, don't know where the rest is. I might have it somewhere at home. Please send me an reminder email and I'll try to put the list together again. I might have a scheme (or could creat one...).

My mail is gunnar.elvin(at)glocalnet.net

Thanks, Gunnar

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Re: JTM 60 Mod help!!??

Post by Gunner » Thu Sep 24, 2009 4:44 pm

Well I'll hope this will make everything clear. And probably some loooon nights with the soldering iron :wink:

I do not claim my modification is the only way to make this amp sound better. But it is one way of doing it. A suggestion. And you can of course tweak the values until you're ready for mental hospital, without reaching the perfect sound. Sorry but I think that's the truth :?

I guess you already have the scheme? I fo not you find it att Schematic Heaven or Dr Tube site.

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.

My first idea was to convert the clean channel circuit sounding close to a normal 1987-circuit, a bluescrunchy tone with more fat bottom and crispy high end.

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. 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). But the lead channel prefer no cap here.


The second idea was to convert the lead channel from sounding as a pedal type of distortion to a more natural tube distorted tone, medium distortion, as in a Marshall 2204 or something similar.

This starts with cutting the diode clipping circuit and then we balance the circuit for the higher signal level.

I can’t give detailed instruction on exactly how I solved every little change on the pcb. But I’m sure you can find a way that works. This scheme shows the important changes.

Image

1) Locate the C105 component. Lift one leg or remove the component. Now the diodes are out of circuit.

2) Put a 2.2uF cap over the cathode resistor R123. This will focus the tone. Give a little more distortion, tighten the bass.

3) Remove the cap C120 and put it as replacement for R104. The C120 rolls off some treble. But moving it after V102A will make this tube distort in all frequencies. You need some treble roll off in the circuit, but it can wait until after this gain stage.

4) Lower R125 to 330k and replace R126 with a 220k. Now you have a voltage divider that will lower the signal a bit before it goes into next gain stage V012B.

5) You can put a 1uf cap over the R124 cathode resistor to get more and a little bit different distortion. But I preferred leaving this stage with no cap.

6) You might want to tighten up bass by lowering the cap C121 from 100nF to 22nF. But as I remember I let it be.

7) Now you need to take the signal down a lot after V102B. Certainly you have the volume pot VR6 here, but this pot now will work to sensitive. Just turn it up a notch and you’ll saturate the next stage. I installed an extra voltage divider before the pot. As I remember I put two 100k resistors (one to ground) at the back of the pcb, where the wire goes to the pcb with the volume pot. Then the signal to the pot from the point between the two resistors. I wish I had taken some pictures of this, but I’m sure you can find your own solution.

8. Finallly – I bypassed the filter before the gain pot VR5. Find the combo R4/C50 (470k/2n2). This is a high pass filter that might lift treble/mid too much. I didn’t like it and just put a wire across the resistor.

If you have done everything correct you will now have a very nice sounding rock’n roll amp in the old school.

/Gunnar

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Re: JTM 60 Mod help!!??

Post by TarheelMarshall » Tue Oct 06, 2009 11:59 am

Hey, i"m new to the site and I've got a JTM-622 with 2 x 12 combo . I look forward to Gunners mods but How do you bias this amp? I've got new groove tubes to go in but I'd need to bias first. Any help would be appreciated .

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Re: JTM 60 Mod help!!??

Post by nicovlogg » Thu Oct 08, 2009 3:42 pm

Gunnar, thanks a lot for all that info! I don't know whether to happy or not, because now I just know I'm going to spending a lot of time getting my fingers burned and smelling solder!

Cheers

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Re:

Post by Fullenglish » Sat Mar 13, 2010 4:51 am

lp20th wrote: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]

Hi Ip20th

I have my JTM600 apart to fix a broken pot. Then I came across your post and photo on the diode bypass mod. I have never used the distortion channel as it sounded awful and it had a weak bass response.

My JTM600 has no C105 in place but a jumper wire. I was hoping you could advise what to do here. I don't read schematics so pics are a great help. I have seen Gunners post but it incorporates other mods.

I had already fitted a fan to cool the el34s so was pleased to see other owners had done the same thing. Also I have fitted an external bias pot which makes valve changes a doddle

Regards

Fullenglish
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Re: JTM 60 Mod help!!??

Post by lp20th » Sat Mar 13, 2010 3:57 pm

1st you need to determine which version of the clipping circuit your amp has. There were at least 2 revisions of this amps circuit that I was aware of and now that I see your amp has C105 jumpered out, make it at least 3. I know you say you don't read schematics but just to get an idea take a look at a couple of earlier posts in this thread. The 1st is posted by JTM60 on 1/10/08 and the 2nd posted by Gunner on 9/24/09. The schematic in the 1/10/08 post is the version I have, you will see the diodes labeled D101 and D102 lead back to a previous tube, where as the schematic in the 9/24/09 post shows the same diodes coming together, dropping straight down and connecting to a horizontal line below. This is a ground line.

If you determine you have the diodes connected the same as my version you should be able to do the mod as I did. The jumper wire you have in place of C105 would be treated just like it was the capacitor. C105 in the circuit limits some of the lower frequencies from going through the clipping circuit. With a jumper wire in its place the whole spectrum of frequencies flow through the clipping circuit.

In my past post dated 1/20/08 there is an attached image. As it shows I pulled the top leg of the C105 out of the eyelet, tacked one leg of the 390K resistor to that leg, keeping it out of the eyelet and the other leg of the resistor was tacked to D102. This takes the clipping diodes out of the circuit and replaces them with a 390K resistor. To achieve the diode bypass on my version is much easier as all I had to do was play with the one resistor value to balance the gain to where I liked it. I settled on the 390K.

The version I have feeds back the diode clipped signal (distorted) to a previous gain stage, by removing the diodes and replacing them with a resistor now an unclipped signal is fed back. Any over drive must now come from the tubes by cranking up the gain knob where before the diodes provided clipping at much lower levels. To me it's not as sweet or natural as pure tube overdrive.

Good luck and be careful, the printed circuit card on these amps are not very robust, it doesn't take much heat to accidentally lift a trace and damage the board.

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Re: JTM 60 Mod help!!??

Post by Fullenglish » Sun Mar 14, 2010 4:38 pm

Thanks for reply

I done the diode bypass and really pleased with the result. I used a 390k resistor as you said. Originally tried a 500k pot but couldn't make a decision on the resistance so went with your suggestion. I now have another channel I can use for a lead boost with a bit more gain and volume and same bass response as clean channel.

Thanks again and if you have any more mod suggestions or links to them for JTM60/600 I'd be very interested in them.

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Re: JTM 60 Mod help!!??

Post by Fullenglish » Mon Mar 15, 2010 9:15 am

Having looked at schematic it seems that my circuit is the same as Gunners. So will have to change the mod slightly. Will changing the Boost Channel Vol Pot sort out the big increase in volume ?

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Re: JTM 60 Mod help!!??

Post by d_wagar » Tue Jun 08, 2010 12:34 pm

just thought I'd mention, did anyone notice in the Oct 09 issue of Vintage Guitar Mag
Andy Brauer's article on 'Sleeper Amps' is the JTM60.

and, he suggested two mods, the fan, and the diode bypass.

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Re: JTM 60 Mod help!!??

Post by jcsifu » Sat Dec 24, 2011 4:42 pm

Gunner wrote: 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).
I know this is an old thread, just wanted to clarify something before I try it. Does this mean to change both R118 AND R119 to 33k? That would drop the B+ on both sections right?
thanks for any help.

jc
Last edited by jcsifu on Sun Dec 25, 2011 9:25 pm, edited 1 time in total.

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Re: JTM 60 Mod help!!??

Post by Gunner » Sun Dec 25, 2011 5:45 pm

jcsifu wrote:
Gunner wrote: 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).
I know this is an old thread, but we just bought a jcm600, got it for $250 on CL, to go with an LP special I bought as a Christmas gift for my nephew and wanted to clarify something before I try it. Does this mean to change both R118 AND R119 to 33k? That would drop the B+ on both sections right?
thanks for any help.

jc
Sorry but the grid resistor should be R116, not R118. We are not talking about changing the B+ here, only the treble roll off at the input. Maybe there are different JTM60/600 schemes, and at one of them the 68k input grid resistor is referred to as R118, or maybe I just have made a misstake (most likely...). Sorry again.
/G

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