[Johnson] Johnson T-R Switch

Sun, 2 Mar 2003 14:34:58 -0600

I was lucky, mine worked and has ever since I got it. But I did build in a
tiny 12V 1-1/2" muffin fan inside to blow some air around the tube, makes it
run a LOT cooler. Can't even hear the little fan unless you stick your ear
right on the cabinet. I used a diode and small electrolytic to tap some
roughly-rectified DC off the tube's filament voltage. Has worked fine for
about 5 years now, fan runs about half speed so it should last nearly
forever, I hope. FWIW, the Johnson literature says don't worry, these things
are built to run hot. I think they were engaged in some wishful thinking at
the time. With the fan running, I can actually put my hand on the top of the
cabinet without blistering it.
73, Jerry W5KP

----- Original Message -----
From: "Charles Ring, W3NU" <[email protected]>
To: <[email protected]>
Sent: Sunday, March 02, 2003 12:08 PM
Subject: [Johnson] Johnson T-R Switch

> I have three of these, with one good transformer and one good 6BL7 tube
> between them. While replacing the power cord on the one I got running
> again after many years, I also installed an internal fuse holder. I see
> no sign that they ever had a fuse, not even the infamous fused AC plug.
> Is this correct? What was Johnson thinking of to omit a fuse? Was the
> transformer intended to burn out like two of mine did, hopefully with
> the excess heat contained in the metal case? The normal working heat
> around the 6BL7 is already enough to worry about even though the manual
> says that is normal.
>
> I'd like to get one more of these in working order. Any suggestions on
> finding a transformer? The good one measures 425 VCT under load plus 6.3
> filament winding, but i'd like the HV winding a little lower in order to
> use silicon rectifiers instead of the 6X4. Also lower plate voltage
> should reduce the operating temperature - losing some of the unit's 6dB
> gain is acceptable, but i wonder if it's basic function would be
> degraded too.
>
> 73 de W3NU
>
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