[Boatanchors] Solid-Stating a Johnson Valiant Power Supply

[Richard Knoppow]

     I don't know about the old RCA diodes but if you decide 
to replace them use a series of diodes so that if one fails 
by shorting it won't short the whole supply.  Also, modern 
1kv diodes are very cheap so you could series enough to 
duplicate the about 15V drop of the original mercury tubes. 
That would also give you a _lot_ of HV margin.  The diodes 
must withstand several times the rated AC input from the 
transformer.
     If  you modify the low voltage supplies I strongly 
advise using a series resistor to drop the voltage to the 
original values.  Remember that increased voltage increases 
the dissipation of the resistive components by the square of 
the voltage difference.  You can do this with a simple 
series resistor or add a zener or other voltage regulator, 
which has some advantage of itself.  If done with some care 
the replacement of the vacuum tube rectifiers will reduce 
heat dissipation and transformer load.
     If you want to use tube HV rectifiers remember that 
there are gas regulators that replace the mercury vapor ones 
and don't need a long warm-up.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
----- Original Message ----- 
From: "Michael D. Harmon" <mharmon at att.net>
To: <boatanchors at mailman.qth.net>
Sent: Monday, September 10, 2012 4:55 PM
Subject: [Boatanchors] Solid-Stating a Johnson Valiant Power 
Supply


>I recently became the proud owner of a very nice Johnson 
>Valiant
> transmitter.  I've done the normal cleanup and tube 
> checking stuff, and
> the next step will be capacitor replacement.  The 
> transmitter has had
> the 866A rectifiers replaced by a pair of RCA CR-105 
> diodes epoxied into
> a couple of 4-pin tube bases.  Since the old HV rectifier 
> tubes are
> already gone (and I really don't want all that mercury 
> around anyway),
> I'm thinking about solid-stating the remaining power 
> supplies as well.
>
> I'm sort of ambivalent about this project.  I've always 
> been hesitant
> about making major mods to a piece of vintage gear, but 
> I'm planning to
> actually use the Valiant, so preserving its life as long 
> as possible is
> more important to me than maintaining its "vintage vibe".
>
> I've heard comments from both sides - I've been told to 
> solid-state all
> the power supplies, and I've been told to leave the power 
> supplies
> alone.  I know converting the power supplies will bring up 
> the B++, but
> I plan to recap the entire transmitter anyway.  I plan to 
> use poly film
> 630V caps for all the tubular replacements, and 450V caps 
> for the
> electrolytic replacements.
>
> By eliminating the filament wiring, I should relieve the 
> LV transformer
> of some filament current, as well as the HV running on the 
> filament
> leads.  I should also eliminate some heat in the cabinet.
>
> Other than the caps, What else can I damage by increasing 
> the B++?   Are
> there other areas I should be careful of?  I don't 
> anticipate any
> problems with the chokes or the mica and ceramic caps, but 
> there may be
> other things I'm not aware of which could jump up and bite 
> me!
>
> I would really appreciate any suggestions, tips, warnings 
> or helpful
> comments from anyone who has done the conversion to solid 
> state.  Oh,
> and BTW, I'm not all that sure about those CR-105 diodes 
> which were used
> to replace the 866A's.  I found them listed in a 1963 
> Allied Industrial
> catalog for $22.40 each, but there's no other 
> documentation on them.
> They say 5000 PIV, but they have to be at least 50 years 
> old.  I'm a
> little dubious about how well they would hold up if I 
> start using the
> Valiant a lot.  Should I replace them with some commercial 
> 866A
> replacement diode stacks or possibly a few 1N5408's with 
> 470K resistors
> and .01 uF caps across each diode?
>
> Thanks!
> Mike, WB0LDJ
> mharmon at att dot net
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