Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jrgtwo on June 14, 2022, 02:39:39 pm

Title: JTM45 Radiospares Impedance selector
Post by: jrgtwo on June 14, 2022, 02:39:39 pm

Hey All, wrapping up a JTM45 build and looking to see if anyone has worked with the Mercury Magnetics Radiospares or original Radiospares OT.  Looking to see if it's possible to make a selector switch or something to connect the 6 different OT secondary wires for 16 and 8 ohm operation.

I tried searching this forum and the internet at large but found very little on the topic.

I have some 8 pole jones plugs that may work but they are quite large and will be difficult to fit into the back panel.  I have also thought of maybe using an octal tube plug and socket. Jumper the six wires accordingly and two poles for the speaker load connections.

I can't make it work in my head using a 3pdt or dpdt switch but hoping someone may have hacked something together in the past

At worse I can just hardwire the jacks to 16ohm



OT spec and secondary wiring doc attached

Thanks all!
-Jonathan

Title: Re: JTM45 Radiospares Impedance selector
Post by: JB on June 15, 2022, 12:21:05 am

Yes it’s possible to do this. I was given an original RS Deluxe and use a DPDT to switch between 8 and 16 ohms. I’ll try and find my drawing later or tomorrow. 

Title: Re: JTM45 Radiospares Impedance selector
Post by: jrgtwo on June 15, 2022, 11:21:33 am

Thank you!

Look forward to seeing how you did it with the dpdt switch

-Jonathan

Title: Re: JTM45 Radiospares Impedance selector
Post by: pdf64 on June 15, 2022, 11:42:36 am

There’s 3 windings, a half winding and 2 quarter windings.
The 2 quarter windings are switched to be in series for 16ohm or parallel for 8ohm.
Just the same as wiring a twin coil pickup for series or parallel.
The half winding is wired in series with the above.
Unfortunately I’m away from my notebooks for a couple of weeks, so no further details for a while.

Title: Re: JTM45 Radiospares Impedance selector
Post by: jrgtwo on June 15, 2022, 12:13:21 pm

Oh interesting, thank you

I see that the secondary has multiple windings, but your description helps to better understand the secondary wiring in general.  I'll see what I can figure out.

Thanks alot!
-Jonathan

Title: Re: JTM45 Radiospares Impedance selector
Post by: pdf64 on June 15, 2022, 01:47:19 pm

As I recall, A - F is the half winding.
The windings have a fairly high resistance, so it should be feasible to check which is which.
Don’t switch impedance with the amp operational.

Title: Re: JTM45 Radiospares Impedance selector
Post by: JB on June 15, 2022, 03:39:47 pm

Hi,
Here's how I did it:
(https://live.staticflickr.com/65535/52148389837_8e886cc25c_w.jpg)
As PDF says above the transformer has a 4 ohm and two 1 ohm secondaries.  With that arrangement I'm measuring approx 9 ohms and 16 ohms.  I used a DPDT switch with the two halves strapped.

Title: Re: JTM45 Radiospares Impedance selector
Post by: jrgtwo on June 15, 2022, 03:45:00 pm

Y'all are the best

Thanks so much for the help!!
 :worthy1:
-Jonathan