JTM45

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crgfrench
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JTM45

Postby crgfrench » Mon 08/12/19 4:47 pm

I'm building a 1x12 JTM45 combo type amp for a client and am considering using a Marstran JTM45 power tranny. Has anyone had experience with Marstran? What did you like/dislike? Thanks in advance. If not Marstran I would probably source a Dagnall replacement for the Marshall 2245 reissue.

I'm going with Merren's Drake repro for the OT and a Classictone 3H choke.

My plan is NOS 5U4GB JAN Phillips rectifier, new production Mullard CV4004 preamp and PI, and a new matched pair of Marshall branded EL34s. It will also be able to take a GZ34 or a coppercap, and KT66s as well.
Last edited by crgfrench on Sun 08/18/19 2:04 am, edited 2 times in total.
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crgfrench
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Re: JTM45

Postby crgfrench » Sun 08/18/19 1:00 am

Man, I can almost see why people pay up for Mercury Magnetics.

I go to TubeDepot for a JTM45 OT, just to compare vs. the Merren I actually plan to buy for my next build which is a Plexi; I see:
"Built to the specifications of the original JTM 45 Output Transformer this tranny will give you the tone that factory replacements could never deliver. Proudly made in the USA. 4,8,16 Ohm Taps."
NO datasheet, NO Primary Impedance info, NO manufacturer name. NO reference to any tube types.

I go to Heyboer, I get literally NOTHING except a link to Mojotone.

I go to Mojotone, I see:
"British 45 Style Output Transformer. (Direct Replacement for the Marshall® JTM45) Output Taps: 2/4/8/16 ohm. X Mount with 2-1/2"" x 2-7/16" 4-hole mounting centers".
At least there is a 1-page black & white datasheet showing dimensions and indicating 3.4k CT primary. It has 2, 4, 8, and 16 Ohm secondaries. NO manufacturer name. NO reference to any tube types or anything else.

I go to Classictone, I see:
"This is a great output transformer for JTM45** style amps. It features 4/8/16 Ohm taps with a buried 100V tap. A great paper layer wound output transformer with 8K Ohm primary for use with KT66 tubes. It is also great for general replacement, new OEM amplifier builds and D.I.Y. amp projects."
There is a 1-page black & white datasheet showing dimensions and indicating 8k CT primary. It has 4, 8, 16 Ohm secondaries with an unused 100V line tap. NO reference to much of anything else.

I go to Marstran, I see a list of transformer model numbers with tiny little pictures that can't be clicked, with basic little wiring diagrams, most of which are black and white, omit primary impedances and none of which show dimensions.

I go to Hammond, I see commercial-grade high quality information on all their product lines, and in the Push Pull Tube Output Transformers section:
"Designed for push-pull tube output circuits. Enclosed (shielded), 4 slot, above chassis Type "X" mounting. Frequency response 30 Hz. to 30 Khz. at full rated power (+/- 1 db max. - ref. 1 Khz) minimum. Insulated flexible leads 8" min. Typical applications - Push-Pull: triode, Ultra-Linear pentode, pentode and tetrode connected audio output. Due to the unique interleaving of the windings BOTH secondary windings must be engaged to meet specifications (see hook-up diagrams below)."
I see, for every single model: RMS Watts, Primary Impedance, Max DC per side, secondary impedance(s). I see suggested tube types for push-pull from 2 tubes through 8 tubes. I see all the dimensions and schematic/hookup data. This is useful technical information that helps me make an INFORMED decision. However, I balance this against the lukewarm things I generally see in the amp-builder online communities regarding the tone of Hammond iron. (disclosure, my current "Blues Briefcase" build uses a Hammond OT but it's not finished yet so I have not heard any Hammond iron personally).

I go to Mercury Magnetics, I look at the O45RS-L get a complete 2-page color spec sheet showing 4 possible primaries (9k, 8k, 6.6k and UL, not that I would use the UL), and 4, 8, and 16 Ohm secondaries. I get the original Marshall part it was based on (RadioSpares De-Luxe Audio). I see all the wire colors, in color. I get mounting info -- horizontal or vertical. I get recommended tube types (KT66, 6L6, 5881, EL34, KT77, EL84, 6V6, EL84/6BQ5, 6550, KT88, KT90). I get a power range: "up to 50% more power than the original manufacturer spec." I get a great history of how this transformer was developed, and a brief history of Marshall amps and musicians who used this transformer in historical amps (they did misspell "John Mayhall" but what am I, the spelling police?). I even find out this tranny was used in the earliest AC-30s. This is what I appreciate, this helps me make an INFORMED decision. And there are a lot of high quality builders using it. Is it overpriced hype? I don't know. But at least they tell you the information you need to know just to spec out a component properly.

https://www.mercurymagnetics.com/2018/w ... 45RS-L.pdf
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Re: JTM45

Postby Rockinrocket » Sun 08/25/19 1:00 am

I dislike the Marstran because of the lug placements are not in historically correct places. The current is also overspeced at 250mA. Should be 150mA.

Merrens are great looking but his lugs protrude far off the transformer making it a PITA to fit in the chassis hole/bracket with out damaging the lugs (tearing them of the paper wind!)

Classic Tone very nice over all an when you get on with the brown paper insulation wrap they look excellent. Complaint is the wires are over sized compared to normal.

Merury Magnetics. Never used. End bells seem to always be in the nice conditon/paint job but crazy expensive. Way too may choices it makes it anoying to find something accurate.

Hop this is of help :)
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crgfrench
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Re: JTM45

Postby crgfrench » Sun 08/25/19 7:24 am

Oh and Weber. Lots of models listed, NO descriptions, NO data sheets NO links to anything, NOTHING.
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