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Why weren't Vacuum Tubes designed for higher currents

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TimFox:
The series-regulator tubes were optimized for (relatively) high current at low plate voltage, but required high heater power.  For the 6C33, one could obtain 600 mA at only 50 V (conservative), with both cathodes and approximately 40 W of heater power.  The tube can make more current, but 600 mA is the continuous limit. With careful design, multiple devices can be connected in parallel for higher current, and to help heat your location during a Siberian winter.

Gyro:

--- Quote from: TimFox on August 13, 2019, 04:57:09 pm ---If you really need high current, there are low-mu triodes designed for series regulator service, starting with the 6AS7G.  The Soviet Union produced some big ones that have been used recently to make audio power amplifiers.  Look for the 6C33C (actually Cyrillic for 6S33), which can handle roughly 600 mA and 60 W dissipation.

--- End quote ---

Former Soviet military NOS tubes are a low cost goldmine for making audio equipment (for those of us who are into that sort of thing  ;)). I use GU50s for push-pull output tubes, very cheap and rugged (Pa 40W) with hot-swap handles on top - designed for battlefield RF transmitters (Hams like them too).

I believe the 6S33S was used in  low voltage series regulators in MIG fighters (discovered when a pilot defected irrc). If you look at the datasheets for some of the former Soviet tubes you'll find some very impressive specs for shock, vibration, altitude etc. Also EMP resistant of course.

EDIT: https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25

--- Quote ---The majority of the on-board avionics were based on vacuum-tube technology, not solid-state electronics. Although they represented aging technology, vacuum tubes were more tolerant of temperature extremes, thereby removing the need for environmental controls in the avionics bays. With the use of vacuum tubes, the MiG-25P's original Smerch-A (Tornado, NATO reporting name "Foxfire") radar had enormous power – about 600 kilowatts. As with most Soviet aircraft, the MiG-25 was designed to be as robust as possible. The use of vacuum tubes also made the aircraft's systems resistant to an electromagnetic pulse, for example after a nuclear blast
--- End quote ---



--- Quote from: ZeroResistance on August 13, 2019, 05:07:11 pm ---The max I found on this page https://en.wikipedia.org/wiki/List_of_vacuum_tubes is 10kV, 20A switch.
I was wondering why they don't make tubes above 30amps+, looking at some of Gyro's posts above maybe I'm wrong and high current tubes (> 30Amps) do exist?
But I guess its difficult to find them>

--- End quote ---

Once you get into pulsed operation or gas filled tubes, ratings get very high. For instance, multi-phase mercury vapour (liquid cathode) rectifiers were used to rectify the 600V traction current for railways! I'm sure there are some still in service somewhere. Also Thyratrons for RF heating and welding equipment.

Gyro:

--- Quote from: ejeffrey on August 13, 2019, 05:14:01 pm ---I was wondering about this and thought maybe you could make a vacuum tube that used electron multipliers dynodes like a PMT to increase the anode current beyond what a heated cathode could apply.  Yes: a "grid controlled electron multiplier" is a thing https://ieeexplore.ieee.org/document/1472778

--- End quote ---

That's impressive! I was aware of such structures in photomultiplier tubes of course, but not for high power, high current applications.

hfleming:
TWT’s are high power. 180kW, 40kV, 16A are not uncommon.
the big boys are high-power magnetrons, like 5MW peak, 50kV anode voltage, and 250A.
Then there are deuterium-filled thyratrons, 35kV, 10000A.

Just browse through the datasheets of a manufacurer, like Teledyne, and you will see some really interesting and high-power vacuum tubes.



ZeroResistance:

--- Quote from: hfleming on August 13, 2019, 07:10:35 pm ---TWT’s are high power. 180kW, 40kV, 16A are not uncommon.
the big boys are high-power magnetrons, like 5MW peak, 50kV anode voltage, and 250A.
Then there are deuterium-filled thyratrons, 35kV, 10000A.

Just browse through the datasheets of a manufacurer, like Teledyne, and you will see some really interesting and high-power vacuum tubes.

--- End quote ---

Thanks for the reply, I appreciate it.
I understand that gas filled tubes  like Hydrogen filled Thyratrons are capable of higher currents due to the "gas ionization" causing a electron multiplier effect.
I would like to know how high a current can a vacuum tube handle. Would it be safe to say that a Vacuum tube > 50A was never built till date and the only tubes that can handle this current and above is a gas filled tube?

Magnetrons though would be vacuum tubes correct? and you iindicated a current of 250A ? Was that the main anode cathode current?

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