Hi,
I get this question every now and then and have put together the information I earlier posted at the Metro Forum. Here it is. And a scheme for the JCM600, it is the same amp as JTM60, only the head version. I think you can find it on the net.
Well I'll hope this will make everything clear. And probably some loooong nights with the soldering iron
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.
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 R116 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.
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 too 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
