5 ms·

> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requir

> 200 amps Something I’ve been curious about: is the current actually required for the thermionic effect, or just the heat? Could you lower the current requirement by thermally insulating the tubes?

doe_eyes 2y ago
They are insulated really well - by vacuum! I'm actually surprised by the figure, though. A small tube requires about 300 mA at 6 V, and the trick is that you can connect the heaters in series instead of doing it all in parallel and pumping out a ton of amps at a very low voltage. They could've done 10 tubes in series at a reasonably safe 60 VDC, and they'd only need 20 amps. Back in that era, because both valves and relays were expensive, it was also common to use them more creatively than just constructing standard logic gates. You'd try to make a full adder or a flip-flop cell as an analog circuit, breaking the abstractions we're now used to - but also saving components.
- userbinator 2y ago
  and the trick is that you can connect the heaters in series instead of doing it all in parallel and pumping out a ton of amps at a very low voltage. Yes, this is what a lot of tube equipment did, as they are naturally high-voltage, low-current devices; here is one notable example: https://en.wikipedia.org/wiki/All_American_Five https://en.wikipedia.org/wiki/All_American_Five
- hilbert42 2y ago
  "They could've done 10 tubes in series at a reasonably safe 60 VDC, and they'd only need 20 amps." Thermionic vacuum tubes of this type usually have a specified maximum heater/cathode voltage rating which varies considerably according to design. Exceeding that rating and one risks a short between the heater and cathode. For these types of tubes heaters can safely operate up to 200V negative with respect to the cathode and about 100V positive. In my post I suggested substituting a tube that's more common in the West—the 12AT7, it has the advantage of having a 'tapped' heater which means it can be wired in parallel mode to operate at 6.3V or in series mode at 12.6V. At 12.6V the current would be halved: https://en.wikipedia.org/wiki/12AT7 https://en.wikipedia.org/wiki/12AT7 (pins 4 and 5, the tap on pin 9).
  - bregma 2y ago
    When I was in high school (many decades ago now) we had a vacuum tube tester with a switch that allowed you to select the heater voltage. We quickly discovered, devils that we were, that if you stuck a simple diode with a 1.5 V heater in the tester and cranked the voltage to max it would launch like a little rocket. Eye protection recommended.
    - hilbert42 2y ago
      "We quickly discovered, devils that we were, that if you stuck a simple diode with a 1.5 V heater in the tester and cranked the voltage to max it would launch like a little rocket." Where I once worked we had several AVO Mk III valve testers† which we used in a nice little "demo" (for want of a better word) for both new employees and non-electronics types who'd occasionally wander into the engineering/electronics department. We'd take a 7 or 9-pin miniature valve (preferability 9-pin) and place it under water and break the evacuating seal on its top, being evacuated the valve would instantly fill with water. Now with suitable settings on the AVO we'd get the water to boil with steam bubbling out of its top. This all happened whilst we nonchalantly went about our business pretending that nothing unusual was happening. Sometimes the reaction from the newcomers/visitors was so funny that those of us who couldn't keep a straight face would quickly exit the lab and burst into hysterical laughter. That was party trick number one, there were more: half fill a CRT with water by the same process and put it back into the monitor for some poor unsuspecting tech to discover. Another was our famous CO2-powered valve gun which we'd use to shoot 7-pin and 9-pin valves at high speed across the carpark aimed at the door of the electricians' department with whom we were continually at war. The valves would embed themselves in the wooden door up to the full length of their pins and rarely would the glass break. Electricians would come in next morning to find our little gifts awaiting them. Yet another was the exploding electrolytic capacitor under one's seat. And there are many more to tell. Believe it or not, we were quite a professional outfit and our work output was excellent. But it was the funniest and most enjoyable place I've ever worked at. † https://www.radiomuseum.org/r/avo_valve_tester_mk3_mk_3.html https://www.radiomuseum.org/r/avo_valve_tester_mk3_mk_3.html
Ductapemaster 2y ago
Current into a low-resistance "heater" element is used to produce the heat required for Thermionic Emission [0] in a vacuum tube. You only need the heater/emitter to be hot, and insulating the tubes would just spread the heat around to everything inside of it — at some extreme, making everything into an emitter, instead of elements that control the emission. [0] https://en.wikipedia.org/wiki/Thermionic_emission https://en.wikipedia.org/wiki/Thermionic_emission
- doe_eyes 2y ago
  This is not really accurate. To get meaningful emissions from normal electrodes, you need to heat them up to about 2000 °C. Vacuum tubes operate at 700 °C or something like that. The trick is that one electrode is doped with special rare-earth additives that greatly increase electron emissions. The same treatment isn't applied to the rest of the device. So, even if all internal components have the same temperature, a vacuum tube can still work (to some extent).
  - Ductapemaster 2y ago
    100% correct and I appreciate the additional details! I couldn't come up with a good analogy to explain you want the emitter as a separate and unique element from everything else involved in a tube — oversimplified in the process.
mrob 2y ago
Perhaps the inside of the tube could be coated with a thin layer of gold using evaporative deposition before the grid/plate/filament/etc is added, like on spacesuit visors for IR reflection.
deleted 2y ago
[deleted]
mannykannot 2y ago
I did a quick search for the specs, and, if I am reading it right, Wikipedia [1] gives the filament current as being 350mA at 6.3V, a dissipation of 2.2W. As that current sums to 196A for the 560 tubes, I suspect this is what the 200A figure refers to. As the tubes are at high vacuum, they are already well-insulated, so I imagine that most of the heat loss is via infra-red radiation. I have a very vague recollection that, in thermionic tubes, the anode has to be kept reasonably cool so that it is not emitting electrons itself. I would be surprised if there are any low-hanging fruit to be plucked here, especially given that vacuum tubes were important technology for a half-century. [1] https://en.wikipedia.org/wiki/6N3P https://en.wikipedia.org/wiki/6N3P
an_aparallel 2y ago
The high voltage from my understanding is to be able to handle the high inrush current to the heaters on power up, I believe after that the requirements are lower (parroting what I've read written by Eric Barbour on his metasonix vacuum tube synthesizers)
Prcmaker 2y ago
Really it's both ways in terms of required current. The thermionic effect requires filament heat, which since being a filament, take some amount of current, often around half an amp, some more some less. A filament could, in theory, be run at any current though so long as the power through the filament stays the same and the voltage is kept low enough not to arc to adjacent parts. There is likely also a minimum current requirement (since you need a source for those free electrons), which often then implies some non-linearity at the low end. As others have mentioned, the 200amps of this case could be reduced substantially by running filaments in series (can be done with 6.3v heaters as the error from a common 5v or 9v supply is more than if you pair them up and use a 12V supply) though this introduces the failure mode of old Christmas tree lights. Source: I make vacuum tubes.
creer 2y ago
There are tubes made which share one heater. For an exotic example with a working web page, see this tube where two triodes and one pentode share the same one heater filament. https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.html https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.... This one has two diodes and two triodes. https://vinylsavor.blogspot.com/search/label/6AY11 https://vinylsavor.blogspot.com/search/label/6AY11 And in this very design, they chose "6N3P valve contains 2 triodes around a single heater, halving the physical size and power requirements." There may have been tubes made where the triode function can be pretty rough (sufficient for a digital circuit) and several of them could share one enclosure. In the tubes shown above, apparently the limit was the number of pins on the socket - but also that all these active elements do not share any pin. Insulating the whole thing would run into issues like burning wire insulation.
hilbert42 2y ago
"Could you lower the current requirement by thermally insulating the tubes?" The thermionic effect is very interesting, if the right material is used to coat the cathode then very large emissions can be had. Combinations of oxides such as barium, strontium and others can have both low work functions and high emissions. Currents in the region of over 100A/sq cm can be achieved. Thus, valves/tubes could be designed to be much smaller and have much smaller currents. For a digital application such as this only a very small cathode current would be needed, this then would mean a much smaller heater could be used. In the past, miniaturizing vacuum tubes was desirable but wasn't a major priority and further development was stopped when the transistor became available. That said, in the 1950s portable tube radios were available that used much less heater power than their mains-operated counterparts, for example tubes like the 3V4. It has a directly-heated cathode and a filament/ heater voltage of 1.4V and current of only 100mA (in series mode it operates at 2.8V at only 50mA).
- jhallenworld 2y ago
  The directly-heated cathode (meaning that the heater is the cathode) tubes have another advantage: they are nearly instant turn-on, no warm-up needed. Same as vacuum fluorescent displays. Reminds me: there are new tubes based on VFD: https://www.korgnutube.com/en https://www.korgnutube.com/en (only 12mW heater power) Very old 1920s tubes also used direct cathodes (but used a huge amount of heater power). If you look at the circuits, they had to jump through hoops to have the desired grid to cathode bias while at the same time providing the heater current. I think this would be easier for logic gates: set all of them to ground.
  - hilbert42 2y ago
    "Very old 1920s tubes also used direct cathodes (but used a huge amount of heater power)." Yeah, and not-so-old ones too (that is, ones designed in the 1940s). I've a couple of 100TH power triodes whose directly heated thoriated tungsten cathodes consume just over 30 Watts (5V @ 6.3A) yet their plate dissipation is only around 100W. That's pretty miserable efficiency. (They look very pretty when working though.) You're right about jumping through hoops, circuis become messy and contorted. Also, there's the messy business of eliminating AC filament hum, thus the commonplace practice of using a humdinger circuit on directly-heated triodes such as the 2A3. Nearly instant turn on can also be a disadvantage when they're used as rectifiers. Tubes like 80, 5Y3G, 5R4G, etc. supply HT long before loading occurs from the indirectly heated ones. In unregulated or poorly designed power supplies it can put additional strain on the PS's electrolytic capacitors. I've often wondered why rectifiers, especially low power one like those mentioned, remained so popular for so long—or why it took so long for indirectly-heated, unipotential cathode tubes such as the 6X4 to become popular. Cost and ease of manufacturing I suppose.

doe_eyes 2y ago

They are insulated really well - by vacuum! I'm actually surprised by the figure, though. A small tube requires about 300 mA at 6 V, and the trick is that you can connect the heaters in series instead of doing it all in parallel and pumping out a ton of amps at a very low voltage. They could've done 10 tubes in series at a reasonably safe 60 VDC, and they'd only need 20 amps. Back in that era, because both valves and relays were expensive, it was also common to use them more creatively than just constructing standard logic gates. You'd try to make a full adder or a flip-flop cell as an analog circuit, breaking the abstractions we're now used to - but also saving components.

userbinator 2y ago
and the trick is that you can connect the heaters in series instead of doing it all in parallel and pumping out a ton of amps at a very low voltage. Yes, this is what a lot of tube equipment did, as they are naturally high-voltage, low-current devices; here is one notable example: https://en.wikipedia.org/wiki/All_American_Five https://en.wikipedia.org/wiki/All_American_Five
hilbert42 2y ago
"They could've done 10 tubes in series at a reasonably safe 60 VDC, and they'd only need 20 amps." Thermionic vacuum tubes of this type usually have a specified maximum heater/cathode voltage rating which varies considerably according to design. Exceeding that rating and one risks a short between the heater and cathode. For these types of tubes heaters can safely operate up to 200V negative with respect to the cathode and about 100V positive. In my post I suggested substituting a tube that's more common in the West—the 12AT7, it has the advantage of having a 'tapped' heater which means it can be wired in parallel mode to operate at 6.3V or in series mode at 12.6V. At 12.6V the current would be halved: https://en.wikipedia.org/wiki/12AT7 https://en.wikipedia.org/wiki/12AT7 (pins 4 and 5, the tap on pin 9).
- bregma 2y ago
  When I was in high school (many decades ago now) we had a vacuum tube tester with a switch that allowed you to select the heater voltage. We quickly discovered, devils that we were, that if you stuck a simple diode with a 1.5 V heater in the tester and cranked the voltage to max it would launch like a little rocket. Eye protection recommended.
  - hilbert42 2y ago
    "We quickly discovered, devils that we were, that if you stuck a simple diode with a 1.5 V heater in the tester and cranked the voltage to max it would launch like a little rocket." Where I once worked we had several AVO Mk III valve testers† which we used in a nice little "demo" (for want of a better word) for both new employees and non-electronics types who'd occasionally wander into the engineering/electronics department. We'd take a 7 or 9-pin miniature valve (preferability 9-pin) and place it under water and break the evacuating seal on its top, being evacuated the valve would instantly fill with water. Now with suitable settings on the AVO we'd get the water to boil with steam bubbling out of its top. This all happened whilst we nonchalantly went about our business pretending that nothing unusual was happening. Sometimes the reaction from the newcomers/visitors was so funny that those of us who couldn't keep a straight face would quickly exit the lab and burst into hysterical laughter. That was party trick number one, there were more: half fill a CRT with water by the same process and put it back into the monitor for some poor unsuspecting tech to discover. Another was our famous CO2-powered valve gun which we'd use to shoot 7-pin and 9-pin valves at high speed across the carpark aimed at the door of the electricians' department with whom we were continually at war. The valves would embed themselves in the wooden door up to the full length of their pins and rarely would the glass break. Electricians would come in next morning to find our little gifts awaiting them. Yet another was the exploding electrolytic capacitor under one's seat. And there are many more to tell. Believe it or not, we were quite a professional outfit and our work output was excellent. But it was the funniest and most enjoyable place I've ever worked at. † https://www.radiomuseum.org/r/avo_valve_tester_mk3_mk_3.html https://www.radiomuseum.org/r/avo_valve_tester_mk3_mk_3.html

Ductapemaster 2y ago

Current into a low-resistance "heater" element is used to produce the heat required for Thermionic Emission [0] in a vacuum tube. You only need the heater/emitter to be hot, and insulating the tubes would just spread the heat around to everything inside of it — at some extreme, making everything into an emitter, instead of elements that control the emission. [0] https://en.wikipedia.org/wiki/Thermionic_emission https://en.wikipedia.org/wiki/Thermionic_emission

doe_eyes 2y ago
This is not really accurate. To get meaningful emissions from normal electrodes, you need to heat them up to about 2000 °C. Vacuum tubes operate at 700 °C or something like that. The trick is that one electrode is doped with special rare-earth additives that greatly increase electron emissions. The same treatment isn't applied to the rest of the device. So, even if all internal components have the same temperature, a vacuum tube can still work (to some extent).
- Ductapemaster 2y ago
  100% correct and I appreciate the additional details! I couldn't come up with a good analogy to explain you want the emitter as a separate and unique element from everything else involved in a tube — oversimplified in the process.

mrob 2y ago

Perhaps the inside of the tube could be coated with a thin layer of gold using evaporative deposition before the grid/plate/filament/etc is added, like on spacesuit visors for IR reflection.

deleted 2y ago

[deleted]

mannykannot 2y ago

I did a quick search for the specs, and, if I am reading it right, Wikipedia [1] gives the filament current as being 350mA at 6.3V, a dissipation of 2.2W. As that current sums to 196A for the 560 tubes, I suspect this is what the 200A figure refers to. As the tubes are at high vacuum, they are already well-insulated, so I imagine that most of the heat loss is via infra-red radiation. I have a very vague recollection that, in thermionic tubes, the anode has to be kept reasonably cool so that it is not emitting electrons itself. I would be surprised if there are any low-hanging fruit to be plucked here, especially given that vacuum tubes were important technology for a half-century. [1] https://en.wikipedia.org/wiki/6N3P https://en.wikipedia.org/wiki/6N3P

an_aparallel 2y ago

The high voltage from my understanding is to be able to handle the high inrush current to the heaters on power up, I believe after that the requirements are lower (parroting what I've read written by Eric Barbour on his metasonix vacuum tube synthesizers)

Prcmaker 2y ago

Really it's both ways in terms of required current. The thermionic effect requires filament heat, which since being a filament, take some amount of current, often around half an amp, some more some less. A filament could, in theory, be run at any current though so long as the power through the filament stays the same and the voltage is kept low enough not to arc to adjacent parts. There is likely also a minimum current requirement (since you need a source for those free electrons), which often then implies some non-linearity at the low end. As others have mentioned, the 200amps of this case could be reduced substantially by running filaments in series (can be done with 6.3v heaters as the error from a common 5v or 9v supply is more than if you pair them up and use a 12V supply) though this introduces the failure mode of old Christmas tree lights. Source: I make vacuum tubes.

creer 2y ago

There are tubes made which share one heater. For an exotic example with a working web page, see this tube where two triodes and one pentode share the same one heater filament. https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.html https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.... This one has two diodes and two triodes. https://vinylsavor.blogspot.com/search/label/6AY11 https://vinylsavor.blogspot.com/search/label/6AY11 And in this very design, they chose "6N3P valve contains 2 triodes around a single heater, halving the physical size and power requirements." There may have been tubes made where the triode function can be pretty rough (sufficient for a digital circuit) and several of them could share one enclosure. In the tubes shown above, apparently the limit was the number of pins on the socket - but also that all these active elements do not share any pin. Insulating the whole thing would run into issues like burning wire insulation.

hilbert42 2y ago

"Could you lower the current requirement by thermally insulating the tubes?" The thermionic effect is very interesting, if the right material is used to coat the cathode then very large emissions can be had. Combinations of oxides such as barium, strontium and others can have both low work functions and high emissions. Currents in the region of over 100A/sq cm can be achieved. Thus, valves/tubes could be designed to be much smaller and have much smaller currents. For a digital application such as this only a very small cathode current would be needed, this then would mean a much smaller heater could be used. In the past, miniaturizing vacuum tubes was desirable but wasn't a major priority and further development was stopped when the transistor became available. That said, in the 1950s portable tube radios were available that used much less heater power than their mains-operated counterparts, for example tubes like the 3V4. It has a directly-heated cathode and a filament/ heater voltage of 1.4V and current of only 100mA (in series mode it operates at 2.8V at only 50mA).

jhallenworld 2y ago
The directly-heated cathode (meaning that the heater is the cathode) tubes have another advantage: they are nearly instant turn-on, no warm-up needed. Same as vacuum fluorescent displays. Reminds me: there are new tubes based on VFD: https://www.korgnutube.com/en https://www.korgnutube.com/en (only 12mW heater power) Very old 1920s tubes also used direct cathodes (but used a huge amount of heater power). If you look at the circuits, they had to jump through hoops to have the desired grid to cathode bias while at the same time providing the heater current. I think this would be easier for logic gates: set all of them to ground.
- hilbert42 2y ago
  "Very old 1920s tubes also used direct cathodes (but used a huge amount of heater power)." Yeah, and not-so-old ones too (that is, ones designed in the 1940s). I've a couple of 100TH power triodes whose directly heated thoriated tungsten cathodes consume just over 30 Watts (5V @ 6.3A) yet their plate dissipation is only around 100W. That's pretty miserable efficiency. (They look very pretty when working though.) You're right about jumping through hoops, circuis become messy and contorted. Also, there's the messy business of eliminating AC filament hum, thus the commonplace practice of using a humdinger circuit on directly-heated triodes such as the 2A3. Nearly instant turn on can also be a disadvantage when they're used as rectifiers. Tubes like 80, 5Y3G, 5R4G, etc. supply HT long before loading occurs from the indirectly heated ones. In unregulated or poorly designed power supplies it can put additional strain on the PS's electrolytic capacitors. I've often wondered why rectifiers, especially low power one like those mentioned, remained so popular for so long—or why it took so long for indirectly-heated, unipotential cathode tubes such as the 6X4 to become popular. Cost and ease of manufacturing I suppose.