Hi,
last Friday I came across with four pc. of apparently new (NOS) VALVO 4CX250B ceramic transmitting tubes (among many other things...). I know they're somewhat odd, due to their demand for high plate voltages, due to their open plates that also are heatsinks, due to their need of a »chimney« and forced cooling by a fan, and last but not least 'cause no one can see them glow.
Did someone yet, despites these drawbacks, have gained some experience with using them in AF power amplifiers? What from could one build a suitable and reliable (!!!) chimney? Acquiring the dedicated loctal sockets turns out to be not of any problem at all, but they're expensive.
Best regards!
last Friday I came across with four pc. of apparently new (NOS) VALVO 4CX250B ceramic transmitting tubes (among many other things...). I know they're somewhat odd, due to their demand for high plate voltages, due to their open plates that also are heatsinks, due to their need of a »chimney« and forced cooling by a fan, and last but not least 'cause no one can see them glow.
Did someone yet, despites these drawbacks, have gained some experience with using them in AF power amplifiers? What from could one build a suitable and reliable (!!!) chimney? Acquiring the dedicated loctal sockets turns out to be not of any problem at all, but they're expensive.
Best regards!
I gave mine away to folks who would use them as intended for RF. I just didn’t see the reward available using them for audio as there are many other and better tube choices.
I found 811s and 813s were better for audio and looked quite impressive.
I found 811s and 813s were better for audio and looked quite impressive.
Googling for 4cx250 power amplifier and 144 or 432 MHz will yield a lot of results.
(or 4cx350 / 4cx1000 for the bigger brothers) such as < https://la0by.darc.de/2m-hpa.htm >
Normally, a 4cx... amplifier will consist of 2 compartments. The upper one is for the anode circuit,
the bottom one for cathode/grids. The cathode/grid compartment is air-tight and pressurized
with a tangential blower. The air then goes THROUGH the socket and a chimney to the
anode fins. Silence is a different thing.
The chimney can be made from teflon foil. The high plate voltage will require an output
transformer with a high step down ratio for audio which may be hard to produce with a
flat frequency response, esp. since interleaving of the windings is hard to do with the
isolation gaps required.
The 4CX series works nicely at UHF, bad RF engineering will punished.
cheers, Gerhard DK4XP
(or 4cx350 / 4cx1000 for the bigger brothers) such as < https://la0by.darc.de/2m-hpa.htm >
Normally, a 4cx... amplifier will consist of 2 compartments. The upper one is for the anode circuit,
the bottom one for cathode/grids. The cathode/grid compartment is air-tight and pressurized
with a tangential blower. The air then goes THROUGH the socket and a chimney to the
anode fins. Silence is a different thing.
The chimney can be made from teflon foil. The high plate voltage will require an output
transformer with a high step down ratio for audio which may be hard to produce with a
flat frequency response, esp. since interleaving of the windings is hard to do with the
isolation gaps required.
The 4CX series works nicely at UHF, bad RF engineering will punished.
cheers, Gerhard DK4XP
Gerhard, I'm well aware that these tubes primarily were desigend for RF use. But my question was about AF, as shown in the datasheets.
I seem to recall that the now extinct Svetlana company of St. Petersburg produced 4CX250's and other ceramic tubes also and had published AF applications for them. But their website is down for a long time, as are the contents.
Anyway, it seems that I'd probably need to give them away to radio hams...
I was also given an Eimac JAN-8438, a Valvo QB3/200, both NOS with holders, and four Telefunken LS50's, also apparently NOS. The latter ones can well be used in AF power amplifiers.
Best regards!
I seem to recall that the now extinct Svetlana company of St. Petersburg produced 4CX250's and other ceramic tubes also and had published AF applications for them. But their website is down for a long time, as are the contents.
Anyway, it seems that I'd probably need to give them away to radio hams...
I was also given an Eimac JAN-8438, a Valvo QB3/200, both NOS with holders, and four Telefunken LS50's, also apparently NOS. The latter ones can well be used in AF power amplifiers.
Best regards!
I know some enthousiasts use these tubes using the screen grid as plate. The actual plate is NOT used. This seems to work very good, no very high voltage needed at all.
Regards, Gerrit
Regards, Gerrit
Yes, maybe, but at a maximum screen dissipation of 12 W there are better and more simple solutions 😉 .
Best regards!
Best regards!
If you have the tubes, sockets and chimneys it may be worth trying. After all 12 Watt dissipations isn’t too bad.
Regards, Gerrit
Regards, Gerrit
I only have the tubes atm, need to buy the sockets etc. A plate dissipation rating of some mere 12 W also applies to e.g. 6BQ5's or 6AQ5's, hence there's no need to acquire the €€€ 4CX250 sockets.
Best regards!
Best regards!
LS50, designed towards the end of WW2!
Reminds me at the books by Ingenieur Steinhauser.
I still have a 500W coaxial transmitter for TV and new tubes for it.
I wanted to give them away but no takers.
Even I would build a LDMOS PA instead.
But then, there was a time when we ran 2 pcs. QB5/1750 in
parallel on shortwave. Clearly forbidden, but safe now to admit.
I was invited to a tour through the travelling wave tube production
at Thales, most interesting! First it looked like a mechanics workshop:
grinding, drilling, electrocutting, glass blowing, all looked sooo retro...
Then, some doors later: 60 GHz vector network analyzers by the dozen
and ladies shifting little magnets to adjust the beam 🙂
Plate dissipation = 250W, not 12!
Gerhard DK4XP
Reminds me at the books by Ingenieur Steinhauser.
I still have a 500W coaxial transmitter for TV and new tubes for it.
I wanted to give them away but no takers.
Even I would build a LDMOS PA instead.
But then, there was a time when we ran 2 pcs. QB5/1750 in
parallel on shortwave. Clearly forbidden, but safe now to admit.
I was invited to a tour through the travelling wave tube production
at Thales, most interesting! First it looked like a mechanics workshop:
grinding, drilling, electrocutting, glass blowing, all looked sooo retro...
Then, some doors later: 60 GHz vector network analyzers by the dozen
and ladies shifting little magnets to adjust the beam 🙂
Plate dissipation = 250W, not 12!
Gerhard DK4XP
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I have kicked around the idea of using some 4x150s that I have. Triode curves look pretty good. You may be able to use them with a little less airflow if you really do not need to run up to 250Watts of plate dissipation. Considering there is no supressor grid, you can expect to run them as a triode to a fairly high voltage without worrying about the screen voltage limits. That limit is really for the case where the plate swings below the screen voltage and causes high screen current.
Also you would need to obey the 30 second heater warmup time.
Also you would need to obey the 30 second heater warmup time.
Attachments
My fault, of course, it's a screen dissipation rating of some mere 12 watts. I won't use them in the way Gerrit suggested, because it would mean triode operation with restricted output power in comparison with a 6BQ5 operated as a pentode. Not too economical.
A pair of LS50's can give an output power of 120 watts when operated in class AB2.
Best regards!
A pair of LS50's can give an output power of 120 watts when operated in class AB2.
Best regards!
Gerhard,
Yes, it’s the G2 with 12 Watt dissipations. I’ve used them too with more than 2000 Volts on the plate, but that’s a different story (and not audio related).
I used to run TB4/1250’s in parallel or a TBW6/6000 (water cooled) on short wave. Again not audio related.
4cx250’s with G2 as plate seem to sound very well, but I haven’t tried this myself.
Regards, Gerrit
Yes, it’s the G2 with 12 Watt dissipations. I’ve used them too with more than 2000 Volts on the plate, but that’s a different story (and not audio related).
I used to run TB4/1250’s in parallel or a TBW6/6000 (water cooled) on short wave. Again not audio related.
4cx250’s with G2 as plate seem to sound very well, but I haven’t tried this myself.
Regards, Gerrit
I got a pair of NOS 4CX250's with the Eimac chimneyed sockets and the 2 meter RF amp that was wrapped around it all probably 20 to 30 years ago at a hamfest. It was a DIY push pull design that used sheet brass suspended over PC board for the plate transformer, built from plans that were published in a ham radio magazine or ARRL handbook, don't remember which, and never worked right. The original article included several warnings about keeping the heater voltage at or below 6.0 volts. The cathode is also heated by the RF and DC current flowing through it. My audio experiments consisted of connecting a large power transformer up as an OPT by using the HV secondary as the primary and wiring all heater windings in series for the speaker output the 115 volt primary is not used. The amp did make sound, and very loud sound at that. It also had a propensity for RF oscillation. I discovered big TV sweep tubes and found them easier to deal with, so I never got back to the 4CX250's.
I had planned to rebuild it all into a nice box but a coworker bought it from me, built it back like it was, only followed the plans a bit better and put it to use on 2 meters. My experiments were done on about 1000 volts. The new owner used about 1800 volts on 144 MHz.
I had planned to rebuild it all into a nice box but a coworker bought it from me, built it back like it was, only followed the plans a bit better and put it to use on 2 meters. My experiments were done on about 1000 volts. The new owner used about 1800 volts on 144 MHz.
Many radio amateurs use that tube in their linear amplifiers. There's a popular commercial amplifier, the AM17, that uses them. They should be easy to sell for good money.
About 25 years ago, 4CX250 stuff was very plentiful and dirt cheap at hamfests around here - likely FAA surplus that had been dumped; nos Raytheon 6293's were also super abundant.
Everybody loaded up on 4CX250 parts to build KW amps, but no one ever did. I eventually gave all mine away, but a couple of years ago I found some 4X150's and sockets in the back of a drawer.
Best I recall, the chimneys were just some fiberglass like material. They might be 3d printable.
Win W5JAG
Everybody loaded up on 4CX250 parts to build KW amps, but no one ever did. I eventually gave all mine away, but a couple of years ago I found some 4X150's and sockets in the back of a drawer.
Best I recall, the chimneys were just some fiberglass like material. They might be 3d printable.
Win W5JAG
A hurricane jar, a single or both ends glass cylinder used for putting a candle in at outdoor parties. Ebay, Ikea, Amazon, etc sell them in a range of sizes. A capacitor clamp could be used to hold it in place.
I gather the getter is usually a zirconium rod activated by the heater not the hot anode so a low dissipation may work well.
Getting air flow to pass through those thins requires pressure not volume. A squirel cage centrifugal blower is better for this role. The ones they use in central heating boilers work well and the speed reduced with a transformer to make them quiet but they're bulky. Computer fans don't have the pressure.
I gather the getter is usually a zirconium rod activated by the heater not the hot anode so a low dissipation may work well.
Getting air flow to pass through those thins requires pressure not volume. A squirel cage centrifugal blower is better for this role. The ones they use in central heating boilers work well and the speed reduced with a transformer to make them quiet but they're bulky. Computer fans don't have the pressure.
Post 15 : "FAA surplus" - please explain, as far as I can see this is an international
forum, but Federação Angolana de Atletismo is not what you mean ?
Topic was : how can the original poster use his 4CX250B for audio purposes.
forum, but Federação Angolana de Atletismo is not what you mean ?
Topic was : how can the original poster use his 4CX250B for audio purposes.
If you want to part with them, I have a HF PA that needs a pair.. drop me a PM if you're interested..
We probably need to ask how much power are you looking for? It may change socket and cooling choices.
You should be able to get a solid 200 Watts ultralinear with a fairly long tube life. Getting 100 Watts ultralinear would be fairly simple. You can go tetrode but be ready for that knee when plate voltage approaches the screen.
You should also be able to to SE if desired.
Also you could go push-pull triode.
You should be able to get a solid 200 Watts ultralinear with a fairly long tube life. Getting 100 Watts ultralinear would be fairly simple. You can go tetrode but be ready for that knee when plate voltage approaches the screen.
You should also be able to to SE if desired.
Also you could go push-pull triode.
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