JTM45 cascading gain "one-wire" mod

[LarryLarry]

Can anyone explain to me how to do the one-wire mod to cascade the channels on a JTM 45? Preferably using the Ceriatone layout as a base.

[Tom Phillips]

Larry,
I don't have a direct answer for you but here is an interesting link that I saved. If you start there and follow the various links posted on the site, you may find the information you need.
http://raw-sewage.net/jtm45ri_mods.html
Tom

[LarryLarry]

Thanks Tom. I do have the reference hand drawn schematic (for the mod) but it is barely readable. I'm sure I'll figure it out, and when I do I'll take photos and post them!

[hasserl]

Here you go:

http://www.lynx.bc.ca/~jc/marshallAmps.html

[LarryLarry]

Yep, that's the reference I already have, thanks...

[Tom Phillips]

OK. Now I see what you are trying to do.
It's a bit more than just adding one wire. The Mod makes sense to me. If you want that insane gain that is.
Are you saying that you are not familiar with working from a schematic?
Tom

[LarryLarry]

No, it's just that all the writing on the mod schem is blurry.

Ok, so using the Ceriatone layout (cause it''s easy to read) I think I need to do this:

1. Unsolder the lead going to pin 7 of V1
2. Separate the 2 270k resistors (schem shows 470k) leaving the rightmost (Normal channel) still going to pin 2 of V2
3. Ground the 500pF capacitor (schem shows 220pF) that is across the leftmost 270k resistor
4. Solder a wire from the leftmost 270k resistor going to pin 7 of V1

Sound right?

[Tom Phillips]

I don’t have the Ceriatone layout so it’s hard to comment accurately.
The lefts & rights seem to be different and I think that the cap should be left open rather than grounded. You need to be careful when looping back the wire after the first gain stage. Probably best to use a shielded cable with only one end of the shield grounded.

[LarryLarry]

Here is the Ceriatone layout.

[Tom Phillips]

Do you have a clean scematic from Ceriatone?
That's the best way my brain follows signal flow.

[LarryLarry]

Ceriatone doesn't use schematics. I was just using the JTM45 reissue and original as references.

[Tom Phillips]

Larry,

I think you have it figured out.
I marked up your directions with my comments as I thought it through. My comments are shown in red below. All references are based on the Ceriatone layout. No guarantees but I think this is correct.

1. Unsolder the lead going to pin 7 of V1. (This disconnects the bright input jacks)
2. Separate the 2 270k resistors (schem shows 470k) leaving the rightmost Normal channel, ( The resistor without the capacitor in parallel ) still going to pin 2 of V2.
3. Remove the 500pF capacitor (schem shows 220pF) that is across the leftmost 270k resistor (The one wire mod instructions show this cap disconnected and left hanging. This makes for easy reversal of the mod and makes sure that an original vintage part is not lost. However, since you are working with a Ceriatone, I say just take the cap out for a cleaner mod result)
4. Solder a wire from the leftmost 270k resistor going to pin 7 of V1


Regards,
Tom

[Satamax]

You're talking about something like this?

[mikepukmel]

Larry,

I think you have it figured out.
I marked up your directions with my comments as I thought it through. My comments are shown in red below. All references are based on the Ceriatone layout. No guarantees but I think this is correct.

1. Unsolder the lead going to pin 7 of V1. (This disconnects the bright input jacks)
2. Separate the 2 270k resistors (schem shows 470k) leaving the rightmost Normal channel, ( The resistor without the capacitor in parallel ) still going to pin 2 of V2.
3. Remove the 500pF capacitor (schem shows 220pF) that is across the leftmost 270k resistor (The one wire mod instructions show this cap disconnected and left hanging. This makes for easy reversal of the mod and makes sure that an original vintage part is not lost. However, since you are working with a Ceriatone, I say just take the cap out for a cleaner mod result)
4. Solder a wire from the leftmost 270k resistor going to pin 7 of V1


Regards,
Tom

Hi Tom! This is an old post, decades old in internet years, but curious about this mod. In some amps (dang, can't find the schematics) sure it was Marshall, somebody cooked up a mod I think its just what you're describing except that they did it with a switch on the top of the amp, so the wires would be short. The switch would turn off one input, and route the output of the other stages volume pot to the input pin instead (also after the 270k resistor (shown 470 in the particular schm version above) to the input of the other half of V1. My question is: is the volume now like a gain pot? You can turn up the first volume pot to drive the second half of V1 harder, but use the other volume pot to keep the overall volume down.

Is this more likely to cause oscillation? Should the mod be done with extra grid stops on the input of the second (switchable) gain stage?

Another reason for asking is that ive read that we should be soldering the two 68k input stops right to the tube pins. So, this mod would also go through one of the 68k resistors, would that have a detrimental effect on the mod, or other parts of the amp?

Any other terrible things likely if a non electronics wiz tried this? Thanks! MP
(Have to cook up a reason to the MRS why Im posting and reading MEF forum instead of finishing the taxes, which are due Monday. Of course, cascaded gain stages are more important than taxes!)

[mikepukmel]

My poor drawing skills taken into account, something like this. (one half of the switch is drawn backwards, so its not like the standard ganged dpst drawing). As much as I could accomplish with MS Paint.



Since its a home made thing, I don't care too much about drilling the chassis for another switch. Wiring would need to be shielded I think? Not sure about the grid leak on the second half of V1. Would the switch create a very nice "pop" when its flipped? I don't see the need for switching it while playing, more like a play mode.