[amsat-bb] Solid State conversion (tube equivalents)?

Robert Bruninga bruninga at usna.edu
Sun Feb 26 15:20:12 UTC 2017

You know, you are right.  The three tubes would need 6.3v at 1 amp.  I
could get the plate voltage from a simple voltage doubler from 60 Hz..
Since 5v 1A switching wall-warts are throwaways, they would be a great
filament supply except that 5v on the filament vs 6.3 is like a 37%
reduction in filament power.  Humm... Ill have to look for a 6v DC/DC
supply... Our use two 5v's in series and use a series regulator down from


On Sat, Feb 25, 2017 at 11:57 PM, James Duffey <jamesduffey at comcast.net>

> This is probably not the answer you are looking for, but why bother
> converting to solid state? The LM/BC-221 frequency meters are, as you
> note,  very nice frequency meters/sources. They don’t drift when warmed up
> and are very solid mechanically. They age well and you are likely to have
> performance now very close to what they were new. The cal books are usually
> only off by a small amount these days. The tubes are available and although
> not cheap, are not outrageous either. I do not think that the tubes are
> highly stressed and should last a long time. The picture you linked to
> shows a cal sticker from 1976, which demonstrates the commercial/military
> utility of the unit well into the solid state age. It would probably make
> sense to put a digital readout on it though. That would take care of the
> cal book being off a bit, if it is in fact off, and make reading the
> frequency a simple project.
> If you insist on converting it to solid state, here is one article
> describing how:
> < http://www.hanssummers.com/images/stories/bc221t/bc221.pdf >
> with more information here:
> < http://hanssummers.com/bc221t.html >
> including schematics and the manual.
> But I doubt performance will be substantially better, if any, than the
> tube unit you start out with. The original was designed for thermal
> performance assuming the thermal mass of the tubes and the heat that each
> produces. I think switching to something that does not dissipate the same
> amount of power would alter the thermal performance substantially and it
> may be that thermal equilibrium may never be reached and drift performance
> may be worse.
> Now if you really want to talk sacrilege, that unit has one of the nicest
> variable capacitor/vernier units you will ever see which will make a jim
> dandy VFO or tuning cap for a home-brew transceiver. Somewhere on the web
> are plans for making a regenerative receiver out of an LM/BC-221. But you
> really shouldn’t do either unless the unit has already been butchered or
> hacked. There is a nice High Q 1MHz crystal in there too. Several would
> make a nice crystal IF filter, but you don’t want to do that either.
> Enjoy it. Technologically it is a wonderful thing to have just the way it
> is. It is still a very useful laboratory quality instrument.  - Duffey KK6MC
> On Feb 25, 2017, at 2:07 PM, Robert Bruninga <bruninga at usna.edu> wrote:
> > About 40 years ago, the topic of tube-to-FET conversions was popular for
> > making 1-for-1 conversions of old tube stuff to solid state.
> >
> > Anyone feel good at doing this these days?  Or am I just wallowing in
> > nostalgia...?
> >
> > Today at a small hamfest, (40 tables only) there were at least 4 of the
> old
> > high quality FREQ meters (9" cube boxes) that could give frequency
> accuracy
> > to 0.01% anywhere from 125 KHz to 20 MHz.
> > https://www.pa3esy.nl/military/us/meet/LM20/pics/LM20-front.jpg
> >
> > Going rate was about $10.  What an absolutely astonishing piece of 1930
> > engineering when you open it up. , with 3 tubes. (6A7, 76 and a 77)
> >
> > I can do just about anything with bi-polar transistors... (my coming of
> > age...) but I PRE-dated FET's which are good direct substitutes for tubes
> > since they are both transconductance devices .
> >
> > There is an excellent 1969 QST article on doing tube conversions, but it
> > assumes one already knows more about it than I do.  And one of the tubes
> is
> > a penta-grid tube which does oscillator and mixer all in one...
> > http://www.qsl.net/kh6grt/page4/tubesters/MOSFETs%20for%20Tubes.pdf
> >
> > I would have thought that a solid state conversion of this simple 3 tube
> > but very useful device would be out there somewhere...  But haven't even
> > found a schematic on line...
> >
> > Thought I'd ask before all the old fuds are gone...
> >
> > Bob, Wb4APR
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> --
> James Duffey
> Cedar Crest NM

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