Recommended JTM45 mods

[southernblues]

Hey all,

I've got a JTM45 RI head coming this week. 2003 model. I've been listening and liking the Bray mods for the JTM45 but wanted to see if there were any others out there that have been pretty positive. Specifically looking for maybe a PPIV mod (mast vol) because there are times I can't torch the volume like at church..


Thanks
Travis

[John Phillips]

Crossline (Matchless type) PPIMV, exactly the same as for the Marshall 50 in the other thread.

One pot, six inches or so of wire, ten minutes' work. No alteration to the circuit or tone when it's up full. Sound good? :)

[southernblues]

Crossline (Matchless type) PPIMV, exactly the same as for the Marshall 50 in the other thread.

One pot, six inches or so of wire, ten minutes' work. No alteration to the circuit or tone when it's up full. Sound good? :)


Sounds good to me -- have you done this mod yourself or could a novice electrical engineer (not really but sounds fancy) like myself pull this off?

[John Phillips]

I've done loads of them. It's incredibly easy - if you can operate a pair of wire cutters, a soldering iron and a small spanner, you can do it.

It's so amazingly simple that I find it difficult to believe how little-known it seems to be.

Have a look in the Marshall MkII 50W thread for a detailed description :).

[southernblues]

THanks John!

[BBQLS1]

Metro amp drop in turret board. Metroamp.com

[acorkos]

jtm45 mods:
http://www.raw-sewage.net/jtm45ri_mods.html

[Blue Strat]

Crossline (Matchless type) PPIMV, exactly the same as for the Marshall 50 in the other thread.

One pot, six inches or so of wire, ten minutes' work. No alteration to the circuit or tone when it's up full. Sound good? :)

I've only heard this type of MV in one amp (early '70s Marshall 50 watter) and thought it sounded horrid.

[John Phillips]

I've only heard this type of MV in one amp (early '70s Marshall 50 watter) and thought it sounded horrid.
It can do (but not always), when turned down too far. It depends critically on the matching of the PI to work well, since it works purely by cancelling the two halves of the PI output against each other... as I'm sure you know!

I've sometimes had it sound great right down to very quiet (most recently on a SF Bassman), but more normally it goes a bit spiky and harsh towards the lower end. Still, given that it doesn't involve an actual mod to the amp circuit at all, and is completely out of it when turned up full (especially if you use that nice pot that Fender unaccountably put in their guitars :)), I think it's a pretty good compromise - and it's so simple to fit that almost anyone can do it.

[donnyjaguar]

I gotta admit that putting a master volume *inside* the negative feedback loop just strikes me as wrong. I do however recognize that many have done this mod. What exactly does it do to the sound when set to various levels?

[John Phillips]

I gotta admit that putting a master volume *inside* the negative feedback loop just strikes me as wrong. I do however recognize that many have done this mod. What exactly does it do to the sound when set to various levels?
It may be that it works well (when it does :)) exactly for the reason it sounds like it shouldn't... because by progressively defeating the NFB as the volume goes down, you add back a little of the dynamics and sparkle that cutting the volume takes away.

[donnyjaguar]

Okay, I'll go along with this. Does it sound any different from just dialling the NFB away by putting a 250k pot where the NFB resistor used to be? (Which is what I've done on one of my amp's).

[Blue Strat]

Okay, I'll go along with this. Does it sound any different from just dialling the NFB away by putting a 250k pot where the NFB resistor used to be? (Which is what I've done on one of my amp's).

A lot different plus it allows distortion at any volume.

[BrownIsound]

The JTM45 was perfected in 1962. No mods are necisarry.:crazyguy

[Plague Dog]

I have Mark Cameron doing a mod on my JCM 800, I asked about the ppimv and he told me absolutely not. He hates them, called it a tone killer.

My $.02

[samtheman]

I had JTM45 and I used w/o any mods, just good pedal front of it, it sound good for me.

Stock JTM45 and Xtreme pedal (http://www.vilo.net/mp3/ampsamples/Marshall/jtm45xtone/)

/sam

[Mickey_C]

1) Do a PPIMV correctly
The PPIMV circuit, as commonly done, is IMHO not a good idea. By the insertion of a couple of capacitors, you can make it work correctly and safely too, isolating the pots from the bias voltage. It will sound better too.

2) Power Scale your amp
London Power has a list of certified Power Scaling installers (http://www.londonpower.com/links.htm#pscale). Power Scaling the amp will let you get the cranked sound at much lower volumes. If you power scale the whole amp (no reason you cannot, if you don't have an effects loop or other non-scalable features) you won't even need the master (or limit) control. He also maintains a list of amps that are factory Power Scaled (http://www.londonpower.com/whichamp.htm). See option 4 ;-)

3) Mercury Magnetics transformers
Having the correct primary impedance for running the KT66 tubes, and having the correct voltages, will make the amp sound better. This is a highly recommended upgrade for the RI.

3) Just get a different amp.
Of course, hacking up a good amp may not be your thing. Maybe you should shop for something that works better. Or, maybe go DIY, and get a kit from George at Metroamp and build one the way you want it to begin with.

[retro]

I've been working on my JTMRI head for about a year.

A couple of things I've learned FWIW is to take enough time to get to know it well before modding. And take little steps.

A big thing of course is what you may be looking for tonewise. Because you may find mods changes are not exactly better as much as just different.

In my research and efforts I found the raw sewage link info for example was not at all what worked for me.

In some instances for example I've found the stock 5881's were a better choice then say Tung-Sol 5881 RI's. Didn't like KT66's either. But I never did try NOS.

EI Gold nor Tung-Sol 12AX7's were a no go also. NOS in PI yes.

Lowering the filtering also was the wrong direction for me.

It wasn't that the changes were necessarily bad, but I lost too much clean headroom. I got a great crunchy amp but...

New iron however, was a huge difference. Still the stock iron offers some things the new iron doesn't.

Next step for me is NOS Tesla EL-34's and some more NOS testing in the V1 and 2 slots. And NOS Rectifier. With help from Blue Strat.


Once I get the tubes sorted...I think the next logical step for me would be Metro's board.

I can't personally see using a PPIMV with the JTM45RI. As I've found I just barely have enough clean headroom as is. I don't know what loud may be to you or in church but I would say give it a try first. It's not very loud at 4. Add an OD and you might be there.

My best limited advice would be to try tubes first. Find out what works for you.

You may find different tubes are all you may need. And cut the clean headroom at the same time. The first tube change I made reminded very much of the Bray clips.

And as sametheman posted it's pretty damn good as is.

It's all a trade-off IMHO. There are some things about the stock iteration I really like.

[Southbay Ampworks]

I have Mark Cameron doing a mod on my JCM 800, I asked about the ppimv and he told me absolutely not. He hates them, called it a tone killer.

My $.02
Hmm, well then I guess I'll bring down my JTM 45 amp modded with a PPIMV tomorrow night to the Crest. It has NOS tubes (Mullard, Bugle Boy preamp tubes and Siemens EL 34's), and Mercury iron in it, so maybe that's why the PPIMV sounds so good...at least to me.

But you be the judge. See ya then.

[retro]

Maybe it's the speakers...;).


I'm wondering if you would be kind enough to characterize how the EL34's are working for you?

[Southbay Ampworks]

Maybe it's the speakers...;).


I'm wondering if you would be kind enough to characterize how the EL34's are working for you?

The Siemens sound great, but since I test things to destruction (just that part of me that wants to know as much as possible) I've just purchased two pairs of Mullard EL34's (xf 2, NOS), two pairs of NOS Amperex Bugle Boy EL 34's, a pair of NOS Telefunken EL 34's, NOS Tesla EL 34's and I've got some current production models here as well.

I'll be giving them all a sound check next week, and hopefully be able to record samples, too.

Current preamp tubes are a Mullard ECC83 in V1, Amperex 12AX7's (Orange Globe logo in V2, Bugle Boy in V3), although I have a few Telefunkens and more Bugle Boys as well as current production preamp tubes, too.

How this all affects the PPIMV mod will be interesting.

Right now I'll be running the Mullard/Amperex/Siemens set for tonight's blues jam appearance.

[retro]

Thanks...

Looking forward to clips...

Hopefully I'll have EL34's in mine by next week.

[JimmyR]

The best thing I ever did to my JTM45RI was install a Metro board kit with Sozo caps. Really brought the amp to life. I then used a MM JTM45 O/T and was very disappointed. It really didn't add anything to the amp; in fact I ended up putting the stock O/T back in.

Tried it with KT66s and EL34s. I actually prefer the EL34s in this amp. I also left the stock filtering because I'm not a huge fan of less filtering.

But changing the board - wow, what a difference! Highly recommended. The Metroamp stuff is top notch.