JTM45 mods to get Jimi mid grind, what else should I do?

[mike43202]

I have a JTM45 replica that I've been trying to get to sound more like a Jimi amp, with a lot more midrange grind to it. I hear the same mids quality in a lot of 12xxx amp clips. This amp is great but is just not there yet as far as that specific sound goes. I bought it second-hand but it appears to be a very good quality build, Mercury Magnetics transformers, Sozo caps, very clean.

So far I have changed the tone stack to 500pf/33k, clipped the bright cap, replaced the single 16uf filter cap with a dual 32uf, replaced the tube rectifier with a solid state plug-in replacement, and added the 10k resistor between the two parts of the dual 32uf. So as of now the preamp almost matches the JTM45/100 exactly.

Because that wasn't quite there, I also split the v1 cathode based on the 2.7k/.68uf 12xxx spec.

The output transformer for this amp is a Mercury Magnetics O45RS-L. Is this large OT the main reason this amp is not giving me the grind I want? I do turn it up to 10 when trying to get this sound ...

I've tried it through various speakers, Weber 1230-55 (G12H copy), Altec 417, Eminence Legends (the old greenback copy that I don't remember the model of). it seems like the tone was closest through the Weber speaker when it was in an open-back cab, I put it into a ported cab and it lost some of the quality I was looking for ...

What is the next thing I should try? Output transformer? At this point it seems like I should either reverse what I've done, put it back to stock, and sell it as a very good JTM45 replica and start over and build an amp from scratch, or swap out the OT and push on.

[JimiJames]

Welcome

Re: Is it easy to convert a SBass into a JTM45/100?

Both transformers are different, rectifier circuit is different, filtering is different.....I wouldn't call it easy. Or cheap.

george

VG also put a 4.7k in place of the 10k. (V1 - 2203) Said that most of the voicing comes from the tone stack though. The extra mids give it more of a perceived gain. I did this in my Cousin's MKII and there's no doubt you will here the difference.

I like shared with B.O.G. Tones and split for everything else.

With a 12 you can go through '67-'69 and it's SL/SB variables.

Creambacks are a great match for JTM's.

[Xplorer]

over my superlead 69 stock, my 45/100 stock is dead on bog. fatter, deeper , dead on bog. it depends how you set the fuzz, and the volume of the amp.

stock 270k mixer resistors , 16uf on the preamp, 56k 250pf ... 560pf , kt66 ...
that should be this if you want to sound more hendrixy. no doubt.

with the volume of the amp at arround 4, and the fuzz volume at max, the fuzz at arround 5-7 , and the strat volume rolled back a bit to arround 9, you're there. with a simple dunlop blue fuzz and a voodoo vibe, and imho, what i ended with was much closer to bog than even randy hansen. the megavibe didn't make it as well too. it'll show quite a difference from a set of pickups to another, with your strat. slider's 69 pickups are just right.

the saturation of the SL69 is much more aggressive, but also smoother i'd say. i like it a lot too. shared/split switch on it, allways using shared cathode.

maybe your transformer ? using a marstran perhaps, or a ******. mustards, nos silver mica, nos preamp tubes ..
16 ohms into 16 ohms G12H30 75hz.
if you're not hendrixy enough with all that, there's a problem somewhere

[mike43202]

XPlorer, if I have the fuzz on (MXR 108 Classic fuzz, trimpots adjusted to sound like the JH signature blue fuzz before I returned it) it definitely has more of a Jimi vibe, the question is how to get more of that grind without the fuzz on. I hear a lot of people playing through amps without fuzz and they are getting that midrange quality without it being the over-the-top fuzz tone.

[mike43202]

JimiJames, thanks, and thanks for that 4.7k resistor tip. Which of the two 10k's get swapped out? The going cross-ways in between the two 32uf cap connections?

I'm going to go pick up some parts today and swap that out along with the 470R resistors on the power tubes so I can maybe try EL34's. I also need to re-check my work from when I put the dual 32uF cap in, I might have done it wrong. There is an under the board connection that I don't remember checking, and I couldn't see it, since it is under the board.

One thing I did figure out though ... You know the story of Eric Johnson complaining about hearing the difference because his roadie put the wrong brand of battery into one of his effects, well this morning I greatly improved my tone by turning off the light in the hall.

(Not quite the same though. It was a flourescent bulb, and is apparently very buzzy when using single-coils.)

[Xplorer]

yes, ground loops. check for this.

please,k what do you call this "jimi grind midrange" ? any studio clip or live clip to illustrate what you mean ?
i could perhaps tell from there.
with your fuzz on, ok, just turn the guitar volume down a bit, for a cleaner tone, but still fatter than without it.

[mike43202]

The Jimi mid grind is the whole unique characteristic of the Marshall midrange tone, which is a driving gritty midrange overdriven tone, but not the over-the-top tone you get when you click on the fuzzface.

I hear it a lot on Band of Gypsies (like the intro to Machine Gun before the fuzz kicks on and the notes get never-ending) and on a lot of other things. It is not a full wide-range sound containing lots of lows and lots of highs, it is midrange focused. (I know the univibe is on during that bit but I am trying to concentrate on the amp rather than the effects.

I also hear it in the sample videos of the 12000 series, which surprised me. The youtube video "Metropoulos 12000 Series and Aracom PRX150-DAG with Lance presented by DAG" has that grind in spades. That is the classic Marshall grind that I wanted to get when I first bought this JTM45 replica and it isn't there yet, and maybe that is because the JTM45 doesn't deliver that kind of sound (though Clapton managed to get it) but I am trying to figure out what needs changing to get the tone.

[Xplorer]

ok ok. there are several different Jimi's tones, from different eras. clips would be appreciated, thanks.

about the bog tone, in the intro, don't be misleaded, this uses the fuzz, but the volume of the guitar, once again as i said before, is rolled down to a cleaner tone. or without the fuzz it would sound thinner, with a different personality, and you'd have to cranck the amp, but again, for another kind of tone.
i made some clips of it. in the original BOG record, you don't hear a fuzz tone in the intro and it's very clean actualy, but the fuzz is there anyway.
with the never ending notes, he just cranck the guitar volume. fuzz is always ON.
also, once again, the volume of the amp, for this tone, is surprisingly low, at about 4 on 10. that's my conclusions with my 45/100, and it's dead on bog to my ears ( even if i could improve some acoustic stuffs etc ... but it's difficult to reproduce the enhancing acoustic of the fillmore east )

did you try it ?

how does your amp sound, at ten ? at 5 ? saturation ? at least a bit of break up ?
what pickups do you use ? which guitar ?
you should play with the neck pickup.

anyway, your amp should sound midrange anyway, depending on the settings, without effects. maybe post some clips from your amp ? it would help us.

here are some low end/mid range tones. 45/100 stock ( 16uf 250pf/56k 560pf ) - dunlop blue fuzz stock - voodoo vibe stock - slider's 69 pickups on a strat - amp's volume is only at 4. fuzz is working, from clean to wild, thanks to the guitar volume control.

for a non BOG tone, but mid range, please show me some clips of what you'd like, and of what your amp sounds like, without effects. thanks.

[Xplorer]

other clips here in my build story.

http://forum.metroamp.com/viewtopic.php ... &start=435" onclick="window.open(this.href);return false;

[neikeel]

A lot of what you have done is in the right direction.

I think that to get the best, full sound I would keep the dual 32uF with 10k dropper, use 27k/16 ohm tap for NFB, stock tone stack 56k/250pF. I would put it back to shared cathode on V1. Keep the 820R on V1 but change the bypass cap from 250uF down to 4n7 - 25uF.

The RS type OT is usually cleaner, bit chimier and slightly more hi-fi with flatter EG response than the Drake 784-103 type which is more know for its mid range grind, but try this other stuff first.

When you have done this plug into Hi 1 (no bridging) and turn volume 1 upto 8 and vol2 upto 5, bass on 2, mids and treble on 8-10 and presence on 4-6 (to taste).

A heavy magnet speaker sounds best IMO and for that percussive thunk a fully enclosed decent size cab with G12H or similar is best (of course old Marshall 4x12 is always preferred ).

[Xplorer]

+ 1 , actualy quite stock values !

[mike43202]

I think I'm a lot closer now.

When I replaced the single 16uF with a dual 32, apparently I didn't move the 10k resistor along the side to the opposite lug. Then when the tone wasn't quite there I went forward and split the cathode.

(Does anybody know what effect that mistake should have had on the tone based on looking at the circuit?)

I've now reversed the split and fixed the 10k resistor connection. It is sounding a lot more familiar now. There is less noise, and the tone is a lot closer. I didn't make any changes that required other parts because I got busy today and couldn't go get them, but I've got a stock JTM45/100 Jimi-style preamp now and it seems to be doing well.

Once I'm able to test this at full volume we'll see where we are, but so far it is definitely an improvement. The amp by itself sounds more familiar and musical with more grind when I turn it up, with a fuzz and univibe it sounds like it is working as it should, and I'm a lot happier.

Thanks for all of your suggestions so far. I will create some clips soon.

[parkhead]

if you are adding filtering you are adding headroom and moving away from distortion FWIW

p

[neikeel]

if you are adding filtering you are adding headroom and moving away from distortion FWIW

p

I agree but the extra 10k dropper to V1 is what will give a bit more distortion by lowering the V1 plate voltages.