Not another JTM45 build!

[jessemhopkins]

This build has been a long time coming. It’s only my second amp build but it’s been a bucket list amp to own since I first played through one of the limited edition offset reissues at a local shop almost 20 years ago (2002). I went with the Mojotone offset head and chassis since it most closely matches my favorite look, and thanks to Neikeel, I’m getting a coffin logo badge to pin on it. I also ordered Mojo’s small parts since it was easier than ordering everything a la cart, but I plan on ordering tubes and Sozo caps from CE Distribution, and transformers are still tbd.



So here’s where I need some input. I have seen some minor differences in schematics, mainly in the power filtering. I have circled all the values I have questions about in the attached schematic, but basically it comes down to the capacitor values in the bias supply (8 or 10 uf?), the values of the last two stages of power filtering (16 or 32 uf?), and the role of the 0.047 non-polar cap just before the standby switch. Even Mojotone’s other jtm45 kit has an additional 32 uf cap coming off of the rectifier. Is that necessary? As I said, this is only my second build so I’m still learning. Go easy!



I’m also looking for recs on transformers and preamp tubes. I’ve already decided on Gold Lion KT66s for the power tubes, but what do people like in the preamp? I’m only looking at new production, not planning to drop NOS money… yet. And transformers? I had decided on Classictone, but I’m open to suggestions as long as it’s something in stock somewhere I can order and have in a week or so.



This is gonna be fun! I’m looking forward to getting started in a few weeks when I have everything in hand and will be doing my best to take my time and get it right.



Cheers!

ps pics attached of some of my hoard

Schematic Quesions

• jessemhopkins
• Jul 26, 2020

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• jessemhopkins
• Jul 26, 2020

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• jessemhopkins
• Jul 26, 2020

 

[Mars]

bias supply: it does not make a particular difference if amp will be properly built and set-up
power caps: 16uF but I prefer 32uF for more headroom and clear sound, this amp is very bassy (I use humbuckers)
put the .05uF (on my schems its .05), regarding the 32uF I have seen it in many builds but I did not put it as it was not on the schematic I used (never had a problem).
For transformers I used merren's but it was a pita to have them and if you care the looks, well you will be disappointed... for preamp tubes in a JTM45 I like JJ, EHX, sovteks... tubes that does not shine in the midrange to maintain sound definition

good luck finding a NOS amber lamp.... its the rarest of the rarest, never saw an original one since I begun hunting vintage Marshall amps stuff 20years ago

 

[Far Rider]

I like Classictone for the transformers and Mullard for the preamp tubes. I also have Mullard KT66's in my JTM45. Nothing wrong with the Gold Lions though.

 

[william vogel]

Classictone has good transformers. Marstran are Heyboer, they are excellent. The Marstran power transformer is correct in about every way to the Drake in appearance and function, the output transformer is close in appearance and sounds great. Merren transformers are equal if not better than the Marstran but the power transformer has solder lugs for the heater winding which is more convenient but incorrect in appearance. I would use the Marstran power transformer and choke and the Merren output transformer if you want the best sounding and authentic appearance. The Classictone has flying leads and the wire colors are incorrect but they are good transformers they are also about 1/2 the cost.

 

[South Park]

I don’t trust all schematics That I find on line. When it comes to filter caps and choke go with original specks on all of it . People who design power supples always go bigger then needed it will last longer . As for tone caps I use a lot of Russian paper in oil caps . Up grade the pots to CTS or clearostat . Keep it all old school . My next build might be a slo 50 with a recto tube if the transformer can handle all the preamp tubes

 

[neikeel]

 

[coolidge56]

Are you teasing Mars? Well done! lol

 

[neikeel]

Are you teasing Mars? Well done! lol

Not really. Want sure if he meant this or the more common chrome bezel type.

Broader question is what type of JTM45 is the OP after. The offset cosmetics suggests RS OT and 5881 tubes. IMO the best sounding JTM45 is the 65-66 with Drake transformers with KT66s.
I agree with Mr Vogel that Marstran transformers are great so I would get his 1202-55 PT an his OT or a Merren OT. The choke is 3H 250mA with DCR of 100ohms and asking as you are that spec it is not too critical. That schem is all good. 10uF bias caps are fine and the 0.047uF snubber is a good anti pop mechanism.
I have built and restored quite a few of these and am firmly of the conclusion that Piher resistors and mustard caps are well worth using too.
I suggest TAD selected 12AX7s if you must go new production and get a burned in Gz34 with a warranty. Main issue is early failure with modern ones.
I always like a good build thread as I am rarely organised enough to make one myself as my building tends to be opportunistic as time permits.

 

[Amadeus91]

bias supply: it does not make a particular difference if amp will be properly built and set-up
power caps: 16uF but I prefer 32uF for more headroom and clear sound, this amp is very bassy (I use humbuckers)
put the .05uF (on my schems its .05), regarding the 32uF I have seen it in many builds but I did not put it as it was not on the schematic I used (never had a problem).
For transformers I used merren's but it was a pita to have them and if you care the looks, well you will be disappointed... for preamp tubes in a JTM45 I like JJ, EHX, sovteks... tubes that does not shine in the midrange to maintain sound definition

good luck finding a NOS amber lamp.... its the rarest of the rarest, never saw an original one since I begun hunting vintage Marshall amps stuff 20years ago

Mine

 

[jessemhopkins]

Not really. Want sure if he meant this or the more common chrome bezel type.

Broader question is what type of JTM45 is the OP after. The offset cosmetics suggests RS OT and 5881 tubes. IMO the best sounding JTM45 is the 65-66 with Drake transformers with KT66s.
I agree with Mr Vogel that Marstran transformers are great so I would get his 1202-55 PT an his OT or a Merren OT. The choke is 3H 250mA with DCR of 100ohms and asking as you are that spec it is not too critical. That schem is all good. 10uF bias caps are fine and the 0.047uF snubber is a good anti pop mechanism.
I have built and restored quite a few of these and am firmly of the conclusion that Piher resistors and mustard caps are well worth using too.
I suggest TAD selected 12AX7s if you must go new production and get a burned in Gz34 with a warranty. Main issue is early failure with modern ones.
I always like a good build thread as I am rarely organised enough to make one myself as my building tends to be opportunistic as time permits.

Hey Neil,

Yes, I suppose I’m mixing styles a bit with the offset cosmetics and the later KT66s, but hey, It’s a personal build and rules were made to be broken. Appreciate your thoughts on the transformers, I will inquire about lead times on those brands. As for resistors, I’ve already got a good supply of new production carbon comps that I will be using, and feel good about the Sozos, which I can always swap later if I can acquire some vintage mustard caps.

I do have a stash of unsorted vintage carbon comps I could use, what are your (or anyone else) thoughts on old stock? They're mostly NOS but some used. Presumably Allen and Bradley but I guess I can’t be sure. The seller I bought them from listed them as “tools” so they didn’t really know what they were or the origin. I seem to remember they acquired them from a relative who used to repair radio equipment. It also came with a ton of sprague orange drops, waxy Illinois Capacitor tubular caps, chocolate drops, etc, again mostly NOS but some used. Not planning to use them here, but also not sure what to do with them at all. Open to good ideas!

Thanks again!

Jesse

 

[neikeel]

I think the Sozos, Mallories and CDEs are all decent caps so see how you go.

I have had mixed results with carbon comps (even NOS AB or the RS ones) and have in a couple of amps had to take them all out as I could not tolerate the hiss.

See how you go. If you use split turrets and lay the components in the slots it will be really easy to swap them over later without the wrestling match that can be involved when you bend the wires and push them into hollow turrets, but that is just my opinion.

The other comment I would make is that if you are not experienced at building these (if you are I apologise!) then use the stock marshall colour codes (green for grids, yellow for cathodes, blue for plates and black for bus and grounds) and wire your bus above the board and use perforated board to loop the wires up to the turrets so you can see the wire colour to each turret.
Marshall used this method from late 68 onwards. You maybe very experienced and very confident and want the clean board look which is cool but thought I should mention before you start.

 

[Amadeus91]

Will always defer to Neil and the other more knowledgeable
members here, but I think the more carbon comps you use the more
noise you can introduce into the amp. The carbon comps are fine to use in a few selected positions. You also have to measure each one of the
carbon comps, as they can drift quite a bit. I have only ever used the
Military spec carbon comps, as they seem to have the tightest tolerances.

 

[Rotorcraft230]

I built a Valvestorm JTM45 about 4-5 months ago and used the Classictone PT and OT. The amp sounds absolutely beautiful. The Classictones should be in stock somewhere, I got mine at TubeDepot. The Merren and Marstrand has high lead times. Enjoy the build.

 

[jessemhopkins]

I’m going to give Marstran the old college try, but if I can’t get them by the end of August I’m just going with Classictone. I used them in my deluxe reverb and they sound great.

 

[Chris-in-LA]

The Metro-spec iron that Metropoulos uses are supposed to have a good reputation as well. I just ordered the same kit as @Rotorcraft230 but went with the Metro-spec. They are more expensive though, about $400 for the whole set. They say they ship in 2 weeks. I ordered everything on 7/22 but have not heard a peep from either dealer.

 

[Big Mike]

Classictone are fine, but just fine IMO. I prefer the Metro Heyboar or the Merren OT. Haven't tried Marstran but am anxious to do so.

Worse case CT for the power is fine, IMO no tonal difference there, but the lugs on the Merren sure make it easier.

Those are all pretty much 'drake' era. Consider merren or marstran or even Mercury if you want the 'radio spares' early era JTM, which is cleaner and more tweedy.

Caps..Mallory are fine. I actually like them better at higher gain than a lot of things.
I wouldn't waste money on nos mustard or anything like that.
I do tend to prefer ceramic disc caps over silver mica, but that's because I've had a few mica's fail.

 

[Big Mike]

The Metro-spec iron that Metropoulos uses are supposed to have a good reputation as well. I just ordered the same kit as @Rotorcraft230 but went with the Metro-spec. They are more expensive though, about $400 for the whole set. They say they ship in 2 weeks. I ordered everything on 7/22 but have not heard a peep from either dealer.

They sound good, I built that kit stock from Metro when George was doing them. Several modifications later I smoked the OT cuz I hooked something up wrong. LOL.

I liked it with a 5Y3 and 6V6's as well, but in general I'm not a huge JTM45 fan, I end up switching the OT's out and making them JMP 1986's. mainly it's the 270K mix resistor spec I dislike.

 

[Chris-in-LA]

They sound good, I built that kit stock from Metro when George was doing them. Several modifications later I smoked the OT cuz I hooked something up wrong. LOL.

I liked it with a 5Y3 and 6V6's as well, but in general I'm not a huge JTM45 fan, I end up switching the OT's out and making them JMP 1986's. mainly it's the 270K mix resistor spec I dislike.

What is it about the 270k mix resistor that you don’t like, too clean?

 

[Big Mike]

What is it about the 270k mix resistor that you don’t like, too clean?

Wooly. It's probably the only reason I like the EL34 era better. They don't have that mid bark 'keraaang' the 470's do, even the 1986 bass specs.
Just shifts the mid frequency I guess, but the JTm's always feel kind of muddy and weak low end for my taste.
Plus really my rig/cabs are built around the 50 watt JMP..Lead or bass works fine, (lead is my fave). buckers, scumback M75's and a lightspeed or klone pedal.

The JTM for my ear, I end up liking different speakers, cab, etc. Changes everything. I did run it straight JTM with the 470 mixers and liked it a lot.

But tinkering got the best of me...

For discussion sake...here are the 3 I built on a real 87 50 watt jubilee.
The GT350 Shelby logo'd one is a JMP 1987 spec. 100ki NFB, but metro kit stock layout for the rest (for the most part).
I think I have a .0068 at V1B instead off .022 or .0022. I usually use a 100pF PI fizz cap on all of mine. Subtle but I like it.
The lead spec has 100pF brite cap, the other two none.
The germino 'isn't. It's currently in pieces becoming a 2204. But was my first kit, Metro JTM45, but now a classictone OT, EL34's, full 1987 board.
The mercury was a JTM45, but now is a full purf'ed board, (My wiring ain't pretty like these guys) 1986 Bass spec with Merren OT running 425volts on the plates with a CT power trans.
So my 2nd, 3rd and 4th attempts to like a JTM (The other was a rockytop, good amp).
I always end up making them JMPs.
https://www.instagram.com/p/BjyTSwClIA4/?utm_source=ig_web_copy_link

My favorite all time amp is a Greer Cam18. Sort of a 6V6 spitfire / ac type thing but def has its own sound. I like chime.

 

[Chris-in-LA]

Valvestorm shipped the JTM45 kit, ordered on 7/22, supposed to be here by 7/29. Seems pretty fast to me.