JTM 45 questions:

[Raoul Duke]

Calling all Marshall experts for a few questions regarding the amp that put Marshall on the map so to speak.

After really digging in to the 2204 and 1987 circuits - I’ve begun studying the Marshall lineage. I’m now really jumping down the JTM45 rabbit hole and have a few questions for the trusted group here. Seems the power section of this circuit is sort of a pot luck kind of thing; so I’m wondering what a good example entails…

1. Which power tube is considered the benchmark for the JTM45 sound? Having evolved through 3 types in the first 4-5 years of production seems to muddy the waters quite a bit.

2. A related but somewhat separate issue given the “mix and match” situation during early production: which output transformer is the benchmark? I understand the text book way to determine this - but see that there are examples of “cross-pollination” between tube types and “correct” primary impedance ranges during the evolution of this model.

3. Same question for the power transformers; seems there were two models that put out B+ about 50 or so volts apart.

After reading a bunch here and the Marshall forum - if I were to build one right now trying to capture a “benchmark” example this is what I think I would use:

1. KT66
2. 6 to 6k6 output transformer
3. 305-0-305 power transformer (mainly due to modern wall voltage effect on power output equation)

Would I be in the ball park? Are the “mixes and matches” an integral part of the sound? Will any of the combinations produce a good example in the sense that there are many shades of a primary color?

Looking for opinions from folks who’ve fixed/built/played any of these variations and hoping to understand this a little better.

Thanks all!

[katopan]

Apart from the KT66 the other big difference to the JMP or JCM is I always understood the JTM45 to have an 8K OT. But I'm now reading that depends on the era like the different output valves used.

[cdemike]

I can’t speak to what was most common in terms of OT or B+, but for me a big part of what distinguishes the JTM’s from JMP’s is use of KT66’s. I think there are a number of Marshall experts who would agree, especially since historically the use of tube rectification lasted past when the KT66’s made way for EL34’s when the transitional “JTM50” spec was standard prior to the more familiar diode rectified version.

I’ve messed around with a cathode biased lead-spec amp (admittedly different than a JTM), and the difference between the tetrodes and EL34’s was significant in terms of what the amp was like to play. KT66’s gave considerably more headroom, broader frequency response, and a more solid bottom end. Again though, very different circuit than a JTM.

[Suckseeder]

my early 66 JTM 45 (KT66) has 440v on the plates. Prim. Imp. Is 8k

[Roe]

Calling all Marshall experts for a few questions regarding the amp that put Marshall on the map so to speak.


1. Which power tube is considered the benchmark for the JTM45 sound? ...

2. ...which output transformer is the benchmark? ...
3. Same question for the power transformer...
...

1. KT66
2. 6 to 6k6 output transformer
3. 305-0-305 power transformer (mainly due to modern wall voltage effect on power output equation)

1 GEC kt66
2 RS dlx, nominally 6k6, in reality from 5k7 and up
3 no you need higher voltages. try the 1202-55 with it the original 350-0-350v

[Raoul Duke]

Thanks All!

I’m seeing some consensus here - which is very helpful. There’s so much rhetoric and hyperbole out there surrounding the evolution of “the benchmark JTM45” that it’s hard to narrow down. I always get good perspective on this board and suspected you guys could clear this up some for me.

Very much appreciated!

Any other relevant information is welcome also, for sure

[Roe]

We had an ampfest yesterday, with three jtm45s, incl. one original one. The original sounded quite powerful but someone had mismatched the load, lowering the 8k primary greatly

[martin manning]

...someone had mismatched the load, lowering the 8k primary greatly

Playin' it safe, maybe? Here is where an [edit] 6k6 load line sits with Vg2 = Va = 450V.

[Roe]

...someone had mismatched the load, lowering the 8k primary greatly

Playin' it safe, maybe? Here is where an 8k load line sits with Vg2 = Va = 450V.

Isn' that 6k6, not 8k?

[martin manning]

Isn' that 6k6, not 8k?

Yes, sorry; grabbed the wrong plot. For 8k, Class B LL 0V current is 0.225A.

[Raoul Duke]

We had an ampfest yesterday, with three jtm45s, incl. one original one. The original sounded quite powerful but someone had mismatched the load, lowering the 8k primary greatly

This is what I’m referring to regarding the rhetoric. There’s a fairly popular line of opinion “out there” that says the KT66 into 8k mis-match is the secret - but that doesn’t make sense to me. The 5.7k - 6k and change makes more sense to my novice understanding - which is part of my confusion. I was wondering if there was something else that I missed that made that combination make sense.

Of course - the possibility that the “out there” opinion may come from those who haven’t owned or built one of these is a consideration as well. The inter-webs is famous for that, lol.

Either way - I appreciate all the info guys!

[pdf64]

...
3. 305-0-305 power transformer (mainly due to modern wall voltage effect on power output equation)
...

Seems lower than vintage spec, more like the modern 2245?
Real JTM45 mains transformer should be more like 700V at 150mA loading, winding resistance about 130 ohms, with a primary winding specified for your typical wall outlet voltage.
The Hammond 290MAEX seems well specified for this application https://www.hammfg.com/files/parts/pdf/ ... 1747400469

I found that the 1k shared, 470R individual screws grid supply resistor arrangement produced a different, smoother, overdriven tone, compared to 1k or 470R individual resistors only.
The difference may be more significant than the the difference between KT66 and other 6L6 types.
https://el34world.com/charts/Schematics ... tm45tr.gif

[pdf64]

.

[pdf64]

You may be interested in this thread https://marshallforum.com/threads/build ... st-2642043

[nuke]

Interesting thread.

I'm not a Marshall expert. The output transformer for the JTM45 has always been confusing to me.

I have in hand, a "Drake" with the sticker, re-issue JTM45 transformer, genuine Marshall re-issue part with the bar-code paper label on the bottom. It measures at 7371-ohm when I ring out the windings.

Is there a definitive history of the early Marshalls and transformers specs?