Mercury Magnetic JTM45-RI-P won’t fit.... Help.

[mikepukmel]

Or if you're not a drill and tap guy, give Patrick Selfridge at MM a shout, ask him what chassis makers make a chassis that this transformer fits and maybe exchange the chassis? I realize shipping is the bear here. Bought more than one thing that seemed to expensive to ship back, so my shelf of "unused things" has grown a bit.

[Chuck H]

I doubt anyone (other than Marshall) is making a JTM45 chassis cut for the reissue transformers. I would expect any after market chassis to be cut for a clone transformer which would mount on an original chassis. That Mercury transformer is made as an upgrade for a reissue amp, so it fits the reissue amp chassis. Unless you can buy a reissue chassis I think using it with any other after market chassis is going to require retrofitting.

Mercury might make an Axiom PT in the stock size. Good luck getting them to swap trannies though.

[The Dude]

The adapter plate is a great idea and certainly a better one. That said, if you don't have the tools or skills to make the plate, I would think the right size washers with the rounded in corners would be plenty sturdy.

Edit: I thought I'd edit Chuck's drawing to illustrate what I'm talking about. (washer indicated in red X4)

[Chuck H]

I don't see any reason that couldn't work. In fact, there's not much stress relief by making the full sized adapter plate since the long tweener sections have no real support. May even rattle.?. Fender washers would be fine for this. You might need to add some kind of spacer (hard rubber?) between the unsupported end of the washer and the transformer to keep the washer plane parallel to the chassis for the best stability. The adapter plate solves for this by itself, but is a lot more trouble to fabricate.

[Bloomfield]

Filing out the corners like Chuck suggested sounds like the best idea. Washers would probably be fine, but you could make a couple of straps out of flat bar and it would give you most of the advantages of the adapter plate while being much simpler to make. If it's a top-mounted transformer in a head there isn't much stress on the mountings anyways.

Andy

[cluster]

Thanks everyone for your excellent help. What would you think about just rotating the PT 90 degrees and drilling new holes. See attached pictures. May be the easiest solution at the moment. I just wonder if rotating the PT would adversely the magnetic field of the OT sitting next to it. Thoughts??

[Helmholtz]

Thanks everyone for your excellent help. What would you think about just rotating the PT 90 degrees and drilling new holes. See attached pictures. May be the easiest solution at the moment. I just wonder if rotating the PT would adversely the magnetic field of the OT sitting next to it. Thoughts??

The PT orientation as shown in the left picture is not advisable as it will increase hum injection into the OT. The leakage field of the PT is max. at right angles to the winding, i.e. at the coil's window.
For minimum coupling the magnetic axes of PT and OT should be at right angles.

[mac dillard]

Try the old headphone trick to check transformer hum in different locations.

[cluster]

please explain..

Try the old headphone trick to check transformer hum in different locations.

[Chuck H]

Try the old headphone trick to check transformer hum in different locations.

THAT! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Big +1. If you commit to using that PT regardless of mounting method then there's nothing to lose trying it both ways to see if it hums in the other orientation. It may for the reasons Helmholtz stated, but often times it works anyway. The obviously worst orientation would be both transformers upright with the lams parallel. THAT'S the one to really avoid. So try it in both orientations and drill the new holes if it doesn't hum. If it does you're just back where you started.

[cluster]

so basically, I would have to assemble the entire amp to working spec and then test to see if it hums? then rotate PT 180 degrees if it does?

THAT! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Big +1. If you commit to using that PT regardless of mounting method then there's nothing to lose trying it both ways to see if it hums in the other orientation. It may for the reasons Helmholtz stated, but often times it works anyway. The obviously worst orientation would be both transformers upright with the lams parallel. THAT'S the one to really avoid. So try it in both orientations and drill the new holes if it doesn't hum. If it does you're just back where you started.

[Chuck H]

Sort of. The PT would only be tack soldered in, no wire routing yet and it would not be fixed to the chassis. The extra 2/3 inch of standoff from the mounting screws probably won't make a significant difference. Then you would try it with the PT in both orientations. Once you know which way you want to mount it you would retrofit accordingly. That is, drill four holes or round in the corners. It's not as bad as it seems to work the chassis once it's a built amp. Just tape some paper in place to keep metal shavings and dust out of the chassis. A bigger issue might be if you needed to troubleshoot some other build problem. Trying to work on the amp without the PT mounted properly could be cumbersome.

[mac dillard]

Headphone trick....http://www.ax84.com/faq.html#hp-tri see #16

[Helmholtz]

so basically, I would have to assemble the entire amp to working spec and then test to see if it hums? then rotate PT 180 degrees if it does?

It is not necessary to assemble the amp. Just place the transformers on a table at the same distance as on the chassis, connect the PT primary to mains, insulate all other wires, connect headphone to the primary of the OT, other wires open - and change orientation.
I have never seen an exception to the rule I gave above.

[Chuck H]

I have never seen an exception to the rule I gave above.

The JTM45 reissue chassis I looked at actually do have the PT mounted 45* rotated compared to the originals. I've never heard one so I can't say if they hum.