[R-390] 6DC6 Replacement

Wed, 16 Jan 2002 11:38:37 -0500

Hi Steve:

I confess! I've poked JFETs into both 26C6 mixers on my R-392, and those
24-V stages work really well. I used a Dremel tool cutter to scribe the
glass of 7-pin miniature tubes, cracking them open and removing the innards.
I then soldered the JFETs with drain source and gate connected to the old
plate, cathode, and grid leads, respectively. After inserting a label with
my callsign on it, I then glued the glass envelopes closed. I popped 'em
into the R-392, where these solid-state jobbies perform flawlessly. I can
elaborate about the glass cutting procedure if you're interested.

As per the 1977 QST article, last night I fired up an 1800-volt bipolar
junction transistor in the relay circuit of an old Drake TR4. The device is
a TV horizontal output transistor.

I pulled the relay driver tube and put my little solid-state jobbie (fabbed
on a scrap of circuit board) in there with some clip leads, with my VTVM
hanging in to see what's happening with the switching levels. I also put a
reverse-biased diode across the driven relay in order to quash any possible
counter-EMF that might do in the transistor. The xstr that drives the relay
is in turn driven by a cheap N-channel JFET such as an MPF-102 or MPF-105,
which derives its Vdd from a diode and a 300 uF cap hanging off the filament
line as a simple halfwave rectifier/filter. The FET gate is extremely
high-Z, and sees the control voltage at a VOX/anti-VOX summing point. Works
like a champ and offloads the filament line.

Now on to RF "replacements." Finding suitable high-voltage FETs for the
cascode circuits may be a problem. The Idss of the output FETs in these
pairs has to complement the input FET. As for dual-gate MOSFETs, I know that
Dan's Small Parts has some.

Your thoughts?

Vy 73, AI2Q, Alex in Kennebunk, Maine   .-.-.

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Steve Goode
Sent: Wednesday, January 16, 2002 10:41 AM
To: [email protected]; Roger L Ruszkowski
Subject: Re: [R-390] 6DC6 Replacement

Alright!  I must confess that I have had similar evil thoughts.

WARNING---SAND STATE THOUGHTS TO FOLLOW!!!!

I love the way my R-390A performs.  Right now the only sand in it is the
Navy approved power supply changes.  It even has a working ballast tube.
But I want this receiver to work forever!  So what happens when I run out of
tubes?  My thoughts are that anything behind the mechanical filters is fair
game since that should have minimal performance hits in dynamic range,
sensitivity, etc.  So I was thinking of looking at the IF, detector, agc,
calibrator circuits to start experimenting on.  What I have found so far in
discussions with others who have actually done solid state conversions is an
article from the April 1977 QST (it is not the April fool article).  This
article shows how to make plug in solid state replacements for any tube
using JET or MOSFETs in cascode with high voltage transistors.  This is as
far as I have gotten.  Anyone else willing to confess?  Anyone actually
succeeding in doing a solid state conversion?

Steve, K9NG

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