[Hammarlund] Johnson Electronic T/R Switch.

[Kenneth G. Gordon]

On 19 Feb 2007 at 14:36, Ken Hickman wrote:

> Hi Gang,
> 
> I obtained a Johnson T/R Switch, Catalog No. 250-39 about a year ago.
> Haven't had a set-up to use it until now. The case get too hot to
> touch after a short time on!

Yup. They are noted for that. Mine gets hot enough to fry an egg on.

> I used one many years ago and although it
> got pretty warm, I could pick it up.

You must have had tougher fingers then, or didn't have it on long 
enough. They have ALWAYS gotten hotter than hades.

> I seem to recollect the gating
> tube was a 12BL7.

No. 6BL7 dual triode. Good bottle.

> This one which looks almost new has a 6BL7 in it.
> Anyone have an manual and/or schematic? Help! Take care,

Hmmm...I have that somewhere...I'll take a look for it.

I mounted a 110VAC muffin fan from RatShack on top of mine and 
snaked the power for it out the same hole the power cord goesinta. 
That made a world of difference in the heat, but didn't change the 
operation of it, which I thought was pretty darned good.

As Peter says, replace the 6X4 with diodes and that will cut down on 
the heat...a little.

Ken Gordon W7EKB