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 that.
Bob
On Sat, Feb 25, 2017 at 11:57 PM, James Duffey [email protected] wrote:
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 [email protected] 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...
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-- KK6MC James Duffey Cedar Crest NM