Better late than never - my JTM 45 build

[Elad E]

Hello,

after a few months of silent lurking I decided it was time to dive in and ordered a JTM 45 kit from Valvestorm.
I'm a bass player but started getting into guitar about 4-5 years ago when I had the chance to participate a blues jam on guitar and found out I equally enjoy the skinny strings side.

since then I've had three combo amps, the last of which is a Fender PRRI which I still own.
while I enjoy the Princeton (and Fender tones in general) one of my strongest childhood memories is my dad playing the Brothers in Arms album (on a wonderful stereo hi-fi rig which is still in his possession) a lot in the evenings and in the weekends.
while I love virtually any track on that album I remember finding the title track most striking even as a kid. it's that muscular yet dynamic Les Paul tone through a JTM 45 that I held as the ultimate guitar tone in my mind through the years (beside Knopfler I also heard a lot of late 80's/early 90's Gary Moore and some Cream too, though I guess Clapton was using Super Bass/Lead amps mostly by then).

as I turned 30 this year I decided I need to build an amp.
I was pretty much decided on building a Tweed Deluxe (I also love Billy Gibbon's tone) but then thought I'd build a JTM 45 anyway somewhere down the line and as I'm not going to be building a second amp in the near future why not take a chance on my own personal holy grail?!

so, 4 months later - here's my build:
trannies are ClassicTone
coupling caps are from Sozo
everything else is from Valvestorm (Robert is just super!)

had some problems such as the mounting holes for the choke not fitting the ClassicTone Choke (a drill solved that), a bad Valve Art output tube which had me reworking the solder joints on the board (and fixing my purple and blue wire mix up on the preamp tubes to board wiring) and setting me back a week to wait for a pair of Gold Lions.
a week ago I received the new tubes, plugged it into one of my bass cabs and there it was - that muscular tone (though I need to use my pedals to get it in BiA territory) alongside a few dozen other tones, all of which were dripping with tonal goodness!!

all in all I enjoyed the build process very much, I believe it took me around 16-18 hours of actual build time. the only thing I'm a little disappointed of is that I don't feel I learned that much about the amp's design - the guys here in the forum and reading various older posts taught me much more than actually building the amp.

so here's some pics (I'll try to upload sound bits too later next week):

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[Elad E]

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[Elad E]

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and now to building a head case...hmmm - British Green?

[Elad E]

on another note, I switched the purple NFB wire from the 16 Ohm tap to the 8 Ohm tap - now the amp is all I wanted it to be! it's got all the warmth and gain I associate with JTM45s and with my pedals gets me anywhere else I want it to go (I don't go any further, gain-wise, than VH's Unchained tone and make do with much less for most of what I do and like).

[OnTheFritz]

Very nice build! I've used the M.C. Classic Tone 50 watt OT and their PT's, but have not tried the 45 8k OT.
On my to-do list, and have been waiting to hear some comments from people regarding said OT.
Anything more you can add regarding the OT and the tone? Feedback, harmonics, etc?
Appreciate any "feedback"!

[Elad E]

Very nice build! I've used the M.C. Classic Tone 50 watt OT and their PT's, but have not tried the 45 8k OT.
On my to-do list, and have been waiting to hear some comments from people regarding said OT.
Anything more you can add regarding the OT and the tone? Feedback, harmonics, etc?
Appreciate any "feedback"!

hey, thanks for the compliments.

I haven't tried any other OTs so I can't really say what my amp would sound like with other OTs and what changes would replacing the OT yield.
however, I can tell you the amp sounds very muscular, with the NFB wired to the 16ohm tap it sounded pretty much like a beefed up Twin - punchy, glassy and super clean (though the amp only having around 10 hours of use may also contribute to that), very similar to recordings of JTM45's equipped with the Radio Spares OTs (though the ClassicTone OT is a Drake replica). when the NFB was moved to the OTs 8 ohm tap (on the impedance selector) - everything you'd expect from a Marshall came out.

I don't know to what degree is the OT responsible for that but I can tell you the ClassicTone trannies work great for me - I don't feel I could possibly make things any better by spending twice as much on trannies, though YMMV.

[OnTheFritz]

Thanks for the info. I liked the NFB on the 8 ohm with the amps I've built. Usually end up with a 47-68k resistor.
Have to give that OT a shot. If it sounds great, you can't beat the price.
Plate voltage? Using the high or low option?

[Elad E]

Thanks for the info. I liked the NFB on the 8 ohm with the amps I've built. Usually end up with a 47-68k resistor.
Have to give that OT a shot. If it sounds great, you can't beat the price.
Plate voltage? Using the high or low option?

interesting, how does your JTM45 sound with 47K NFB resistor on the 8 ohm tap?
I have a spare 100K allen bradley resistor - I thought about changing the NFB to 100K, without moving it from the 16 ohm tap, which I reckon would be the same as using a 47k on the 8ohm tap. I was just afraid I'd loose the glassy top end and punchy bass with the 100k. I also wanted some amount of clean headroom.

I didn't notice any option for high or low voltage. voltage on the plates of the output tube sockets was 490v DC without tubes. thats much higher than George's spec sheet but from what I gathered from the experts around here - the old 45s ran in the 500v range so I guess that's ok.

I've noticed George's 12 series Super amps have a low voltage mode on the standby switch which I believe is meant for achieving the VH variac tone.
did you mean something like that?

[Elad E]

I now realized the ClassicTone JTM 45/100 PT has optional low B+ taps alongside vintage correct high B+ taps.

their JTM45 PT only has taps for the lower B+ setting though I don't know if the original 60s JTM45s (not the 100 watt Super models) actually had plate voltages of 560v or whether this was the spec for the 45/100 amps only.

I guess the 560v option would cater mainly to those lucky ones who stocked on original GECs or can afford the outrageous prices these commend nowadays.

[OnTheFritz]

I now realized the ClassicTone JTM 45/100 PT has optional low B+ taps alongside vintage correct high B+ taps.

their JTM45 PT only has taps for the lower B+ setting though I don't know if the original 60s JTM45s (not the 100 watt Super models) actually had plate voltages of 560v or whether this was the spec for the 45/100 amps only.

I guess the 560v option would cater mainly to those lucky ones who stocked on original GECs or can afford the outrageous prices these commend nowadays.

Here's the ClassicTone PT I've been using:
http://www.classictone.net/40-18054.html" onclick="window.open(this.href);return false;
With the lower voltage option on a SS rectified amp I get about 420-25 ish on the plates. Tube rectifier, get about 400 or a tad more, depending of course on the wall voltage and the quality of the tube rec.
Higher voltage option I was getting about 455-65 with a tube rectifier. My wall voltage runs a tad high at 121-22, so those numbers are pretty much what you'd expect for era correct voltages.
450 VDC on the plates is period correct for a 65' era JTM-45. The Supers were a whole different animal.

As far as the NFB, my current amp is a 50 watter. I find the 47k/8 Ohm tap to be smooth, yet still aggressive enough without being harsh. It's just preference and trial and error. 68k and 87k are great as well, just a subtle difference.
At the extreme end of the spectrum, 100k/4 ohm tap to me sounds too harsh (for a plexi), but it's standard for metal panel amps, like the infamous aggressive 72' 50 watter with the chicklet caps.
There's no right or wrong, mix and match, find what suits your ears unless you're trying to absolutely nail the tone of a specific amp/year.
It's all good!

[Elad E]

I contemplated getting the 18054 PT for the supposedly superior M6 core but eventually decided to stick with the 18033 version.
I wonder - Magnetics Corp's site seems to recommend 18054 for both JTMs and JMPs. is it the lower voltage tap which makes it suitable for EL34s?

I think I may ultimately just add a 3 way switch alternating the NFB wire between the 16, 8 and 4 ohm tap or just an "NFB off" instead of the later.

for now though I'll keep it unchanged save for the 27K/8ohm wiring.