Surgery Time! (JTM45 Modding Content)

[LLL]

So the story goes...

- Back in 2002 I purchased a bone-stock, used Marshall JTM45RI for about $600.00
- It sounded like ass and was nothing like a mid 60's JTM45
- In 2004 I installed a PTP kit and lots of fixin's (MM OT, NOS Mullard pre's, VA KT66 power, etc)
- It sounded great!

Fast forward to 2013...

- I wanted MOAR GAIN and the ability to use 6CA7s & variac down, so I modded it again
- swapped grid resistors from 470 to 1K
- swapped 27K NFB resistor to 47K
- moved NFB wire from 16ohm tap to 8ohm tap
- changed tonestack to Superlead values
- tried a trebley-er bright cap but yuck... took it off (500pf stayed)
- got a pair of EHX 6CA7s... very nice - clank and crunch
- it sounded closer to what I wanted...

Here's what it sounded like (and currently sounds like):



So fast forward to 2017...

It's time to finish the job. The amp is still extremely thick and needs to have some more parts changed
to get things exact. I also gotta figure out something to go from shared cathode to split (turret board
is designed for shared only).

I've gone through the amp and labeled everything (huge size, so had to shrink it; probably unreadable).



My ever-faithful assistant is ready to lend a hand...

 

[Darg1911]

Re: Surgery Time! (JTM45 Modding Content)

Cant really see but I assume your 1 meg grid (to ground) resistors are at the input jacks? You could move the grid stoppers to the input jack which would free up space for the additional resistor and cap for split cathode. You may want to add another turret to keep it neat. Obviously you'd have to do some rewiring. Not ideal but my build is quiet with the resistors at the jacks. I did use shielded wire to the V1 grids.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

Cant really see but I assume your 1 meg grid (to ground) resistors are at the input jacks? You could move the grid stoppers to the input jack which would free up space for the additional resistor and cap for split cathode. You may want to add another turret to keep it neat. Obviously you'd have to do some rewiring. Not ideal but my build is quiet with the resistors at the jacks. I did use shielded wire to the V1 grids.

Good points... geez, it's a relief to know there are other Marshall circuit-heads here.

I think what I'm gonna do is get a very small terminal (w/ turret) and affix it to the phenolic board (superglue works). There's no wiring underneath so it's all good.

 

[Ayrton]

Re: Surgery Time! (JTM45 Modding Content)

What year is it? I ask because the earlier RI came with Drake iron and that was supposed to make all the difference

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

What year is it? I ask because the earlier RI came with Drake iron and that was supposed to make all the difference

I believe a '96.

In 2004 I put a Mercury Magnetics O-45JT in it.

Those early re-issues were really lame. I had the 1959S as well.

 

[Ayrton]

Re: Surgery Time! (JTM45 Modding Content)

I bought an ‘89 JTM45 RI that was a complete basket case (yes it was cheap), but has the Drake (or is it Dagnall?) iron. I will eventually get around to sending to Dave Friedman when I stop being lazy.

 

[Myaccount876]

Re: Surgery Time! (JTM45 Modding Content)

Cant really see but I assume your 1 meg grid (to ground) resistors are at the input jacks? You could move the grid stoppers to the input jack which would free up space for the additional resistor and cap for split cathode. You may want to add another turret to keep it neat. Obviously you'd have to do some rewiring. Not ideal but my build is quiet with the resistors at the jacks. I did use shielded wire to the V1 grids.

It’d actually be better to move the grid stoppers to the tube socket rather than the input jack. To give a short version of the reason - close proximity of the grid stoppers to the actual grid can help prevent/reduce blocking distortion. I’ve had it happen to me before (though it’s not garunteed to happen), but that amp is a lot messier internally than this one.

And cute dog.

 

[Darg1911]

Re: Surgery Time! (JTM45 Modding Content)

It’d actually be better to move the grid stoppers to the tube socket rather than the input jack. To give a short version of the reason - close proximity of the grid stoppers to the actual grid can help prevent/reduce blocking distortion. I’ve had it happen to me before (though it’s not garunteed to happen), but that amp is a lot messier internally than this one.

And cute dog.

Also oscillation and RFI. Installing them at the socket is not very elegant, and there are no open pins to use as a tie-off/connection point and depending on the resistors used, they leads of 1/2 watt resistors are usually wimpy. Locating at the jacks isn't a thing of beauty but more secure. I was planning on moving mine to the socket if it was necessary, but using small terminal strips as connection point. It wasn't necessary.
Anyway, at the socket is definitely another option, and the best one ... electrically anyway.

 

[Myaccount876]

Re: Surgery Time! (JTM45 Modding Content)

Yes, but you could easily add small tag strips to make it neater/more secure. There’s room in the picture.

 

[Darg1911]

Re: Surgery Time! (JTM45 Modding Content)

Yes, but you could easily add small tag strips to make it neater/more secure. There’s room in the picture.

There is an echo in here.

 

[Joey Voltage]

Re: Surgery Time! (JTM45 Modding Content)

Straight to the grid with shielded cable and little to no lead from the pin to resistor body is always the most preferred place for a grid stopper else they may not be stopping much of anything, and they are usually plenty sturdy like that. If you are worried about the lead strength, this is one of the few places a carbon comp resistor is preferable since they generally have thicker copper leads not cladded steel, and have non magnetic bodies, both of which high impedance grid nodes can be sensitive to. Since no current flows through a grid stopper, you also don’t have a lot of the extra noise issues that are associated with carbon comp.

 

[Chickenwings]

Re: Surgery Time! (JTM45 Modding Content)

purty

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

After much researching...

Got some Sozos (let's see if the hype is true; although they will need a little break-in time) and I'm going to be replacing select resistors with carbon film type.

Got some turrets... I'll affix one so I can do split cathode.

I've got spare filter caps so I'm gonna replace those too (only due to age - filtering is 16+16 pre, 32+32 power)

Doing the .68uF bypass on V2a.

I'm also gonna see the difference between 47k NFB resistor + 8ohm tap, and 100k NFB resistor + 4ohm tap...

Order should ship soon :smoker:

 

[Darg1911]

Re: Surgery Time! (JTM45 Modding Content)

I did the "one wire mod". Swapped normal volume pot for a push pull (Bourns 1 meg) so it's switchable. But I have a post PI master volume and my amp isn't a Marshall. Not sure I'd drill a Marshall for an extra hole for a MV. Although a master could be installed in the "low" normal input jack or one of the speaker jacks. Also can be done without the master volume. Switched on, it kills the normal input jacks and the normal volume pot acts as a gain control. Switched off, everything works as normal. I guess there are some reported, potential "noise" issues. But, I used shielded wire and have no oscillation or feedback or other issues, even with it cranked. It sounds pretty good but really, I hardly ever use it. I like to use an ABY to mix both inputs and it's easier just to tap one of the pedals on rather than messing with the switch. But it's still a pretty cool, easy mod and easily reversed.

 

[Joey Voltage]

Re: Surgery Time! (JTM45 Modding Content)

After much researching...

Got some Sozos (let's see if the hype is true; although they will need a little break-in time) and I'm going to be replacing select resistors with carbon film type.

Got some turrets... I'll affix one so I can do split cathode.

I've got spare filter caps so I'm gonna replace those too (only due to age - filtering is 16+16 pre, 32+32 power)

Doing the .68uF bypass on V2a.

I'm also gonna see the difference between 56k NFB resistor + 8ohm tap, and 100k NFB resistor + 4ohm tap...

Order should ship soon :smoker:

you mean carbon comp? You have carbon Film in there now. FWIW New carbon comp wont give you the same "pink noise" effect older ones 60 years ago would. manufacturing processes have changed, and modern carbon comp are much more reliable, and wont necessarily produce this effect. They still have poor temperature drift in comparison to film types, and thus their value will increase with heat, but nowhere near what they used to.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

you mean carbon comp? You have carbon Film in there now. FWIW New carbon comp wont give you the same "pink noise" effect older ones 60 years ago would. manufacturing processes have changed, and modern carbon comp are much more reliable, and wont necessarily produce this effect. They still have poor temperature drift in comparison to film types, and thus their value will increase with heat, but nowhere near what they used to.

The 68k pairs (x2) at the inputs and the 270k pair are carbon comp.

I'm especially getting rid of those CCs at the input; in fact I want to get them all out and do carbon film in their stead.

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

For my amp surgery, I'm going with 820+330uf (normal) & 820+.68uf (bright).

So that, including the other changes I'll be doing will make the amp a:

'68 50 watter with a tube rectifier, variac'd to 100v... kind of Marshall. :lol:

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

I've done a good amount of the changes, but still waiting for some parts from Mouser.

Again, the overall goal is going from mid 60's JTM45 specs to a 1968 50 watter w/ tube rectifier.

So far I've got the meat of it done:

- installed turret for split cathode
- installed 820Ω + .68uF for bright channel (old was shared cathode 820Ω+250uF)
- installed .0022uF coupling cap for bright channel (old was .022uF)
- installed 2x 470KΩ mixers (old was 2x 270KΩ)
- installed V2a .68uF bypass cap (old did not exist)
- installed 2x .022uF output coupling caps (old was 2x .1uF)

The rest is just gravy. I've never been a fan of the .005uF bright cap in later Marshalls, but I'll try it here (per spec).
I'm running the amp at "9" so it's mostly out of the circuit anyways.

I've still got a cap job to do on my EMU-1820m before I can record anything (thanks for making me wait, Mouser :smash: ).

But I have plugged in and tested... and I love the results!

The mud has been cleared up, but it's still a thick tone compared to later 70's Marshalls... thick 'n' crunchy.

There's also more gain on tap. :yeah:

Turning up to "9" doesn't make everything mush as per the old spec; still defined.

 

[Joey Voltage]

Re: Surgery Time! (JTM45 Modding Content)

The 68k pairs (x2) at the inputs and the 270k pair are carbon comp.

I'm especially getting rid of those CCs at the input; in fact I want to get them all out and do carbon film in their stead.

Thats actually the one place carbon comp excel. They are naturally non magnetic, don't have magnetic end caps, and mostly have tinned copper leads wich are all ideal things for a grid stopper like the input 68K resistors. There is also ideally no current that flows through that resistor, so they wont suffer from some causes of excessive resistor noise

 

[LLL]

Re: Surgery Time! (JTM45 Modding Content)

Thats actually the one place carbon comp excel. They are naturally non magnetic, don't have magnetic end caps, and mostly have tinned copper leads wich are all ideal things for a grid stopper like the input 68K resistors. There is also ideally no current that flows through that resistor, so they wont suffer from some causes of excessive resistor noise

I'll leave them in there. I was mostly concerned with RFI at the inputs but can't say I've noticed any.