Advice: Marshall JTM45 kind of thing build

[mikepukmel]

Hi All,
Okay, admitted Im a bit of a hair brain, change my mind too often on stuff. I started a JTM 45 amp build a few years ago, and shelved it due to unreasonable work hours. Anyway I had bought a Mercury Magnetics Radiospares clone output transformer. I had a few bucks from a small bonus I got that year, so I have rationalized my temporary insanity.
So, the thing has a huge pile of output wiring that won't work with the rather expensive impedance switch I bought. And on and on.
Its never been installed, so was thinking about selling it and buying the one for this amp, that works with the impedance switch, and there are not all these extra windings that won't be used. And I could get the thing built and use it.
Based on some of your great advice on terminating the unused wiring, I have some shrink wrap on the unused taps, but its not soldered in.

I don't really like the aluminum chassis, but its mostly together. I started adding additional turrets so I could put a master volume control on the back, but read quite a few horror stories about people doing bad wiring (that I know how to do quite well) getting terrible oscillations and killing the output tubes, and I think these output tubes KT66's are not the most beefy tubes.

The only non-stock thing I wanted to do on this amp, is to not solder anything to the back of the pots. Or at least, not use the back of the pots as part of the grounding circuit.

Anyone have any preference for this particular transformer? Any benefits to using it over the more stock, late 60's JTM45 version?

Thanks
Mike

[loudthud]

It's been a couple of years, but do you remember seeing this ?: https://music-electronics-forum.com/...eluxe-ot/page3

It give you lots of options for the Radio Spares OT.

[mikepukmel]

It's been a couple of years, but do you remember seeing this ?: https://music-electronics-forum.com/...eluxe-ot/page3

It give you lots of options for the Radio Spares OT.

Oh thanks, way cool! Going to save the thread so I won't lose it again. Re-Reading . . .

[mikepukmel]

Re


Originally posted by Helmholtz View Post
Until solid evidence I tend to doubt that Marshall used the RS type OT in original Bluesbreaker combos. The reasons are 1)The KT66 equipped combos were produced only in the end of the JTM45 era ('65/'66), when most JTM45 tops already used the 8k Drakes. 2) The original schematic of models 1961/1962 shows an OT with a standard multi-tapped secondary. 3) The OT in the schematic has a 100V secondary winding, which only the Drakes had.

Oh man this is just so cool. So in my internet travels, I found a bunch of photos of a blues breaker someone took apart and repaired. The photos did show an original Radiospares, but that long ago, who knows what mods were done to that poor amp. The radiospares the original one was just a hideous mess. You probably know all this stuff, but I was very surprised to see the wierd board with solder balls so close to gether. They did say that it was famous for arcing. He talked about impedance stuff but ti was way over my head.

Going to print (to pdf) and re-read the thread more carefully. Im especially interested in talk about mismatch. on the KT66's. Im no wiz at this stuff, but have read modern tubes just don't hold up like the ancient's did. What Id like to do is set the amp up for a smaller 2 x 12 cabinet, but not sureabout which windings vs speakers would be best ( 2 x 16's wired for 8, or 2 x 8's wired for 16 or 4 or not even sure if I can get 2 x 4's wired for 8). Thanks again, this is all way cool stuff. Would be nice if I can get this working and not blow the tubes.

[Mike K]

I don't have any direct experience with the 45W RS OT but I do with the 18W versions. My advice, keep it. It's well worth the hassle.

There's probably a *bit* of hype around the RS OTs, but some of it is based in reality. They are good. Provided you can handle the volume and headroom of a 50W amp, otherwise running it at a few watts you're unlikely to notice anything spectacular.

[mikepukmel]

Thanks Mike