Output hookup on oddball OTs (Radiospares JTM45)

[skyboltone]

Greetings:
I had been driving myself nuts trying to figure an easy way to make the MM JTM 45 OT impedence selectable. The only way I could do it with a switch was to order a custome made 3 position 6 wafer job for about $150. It would have taken up a large chunk of the below chassis real estate. This solution came to me in a dream. I literally woke up with the solution one morning last week. An 8 conductor jones plug may make a better switching device so some poor fool doesn't plug one into a tube socket.

[Allynmey]

Or you can use a low profile 3 disc stack rotary switch and do the same thing. I would wonder about a tube base getting loose and blowing the OT. I'll look around. I may have some central lab ones laying around.

Allyn

[skyboltone]

Or you can use a low profile 3 disc stack rotary switch and do the same thing. I would wonder about a tube base getting loose and blowing the OT. I'll look around. I may have some central lab ones laying around.

Allyn

I would love to see the solution to this with just three disks. The 6 positions must align/disconnect differently in each of three positions, and the take off points change as well. I just couldn't figure it out.

Dan

[Ron]

Greetings:
I had been driving myself nuts trying to figure an easy way..

In the words of former President Bill Clinton, it depends on what the meaning of easy is. It's easy to mentally imagine a single, complex switch but hard to actually build one. On the other hand, it's easy to build the Jones plug in three versions. I have a Bogen amp that has an octal plug in the back; a male plug attaches a cord with a standard 1/4" phone plug on the other end to a speaker cab.


I really dislike the Radio Spares' arrangement. They boil down to:

* 16 ohms: AF + DE + BC (all in series with the same current direction in each winding)

* 8 ohms: [BC = DC] - AF (BC in parallel with DC, and the combination in series with a reversed AF]. This is really silly; they are bucking AF against the other turns to get this impedance.

* 4 ohms: AF = [BC + DE]. Here AF is in parallel with the series combination of BC and DE. All currents are in the same direction.

This seems so British to me. I worked with people in the UK nuclear weapons program for four years and their engineering efforts were innovative (to use a positive term) but overly complex.

As an example, AF must go from series to parallel with other winding combinations and must reverse polarity in one, so a single switch must of necessity be complex. I think it's funny that when the vintage bug hits RS transformer replicas that MM decides to duplicate the crazy design.

[Normster]

Or you could just try this...
http://www.dst-engineering.com/TN.php?n=OTWiring
...using a DPDT switch. 8 & 16 Ohm only, but it IS simple.

[Ron]

Yep, that works for the two most useful impedances to me.

[skyboltone]

Thanks Normster, I hadn't seen that. Also an elegant solution. And thanks for a robust application of Clintonese Ron. That's kind of the way I saw it. I'll make a holder that attaches to the cabinet back plate with two octal sockets in which to store the unused plugs.

Dan

[Allynmey]

Bob hit that one on the money!

Allyn