Why weren't Vacuum Tubes designed for higher currents

[T3sl4co1l]

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

Neat, I wonder what it looked like.

Secondary-emission tubes have a long pedigree:
http://www.r-type.org/exhib/aai0191.htm
https://www.radiomuseum.org/tubes/tube_1630.html

They were generally unpopular due to the higher operating voltage, current consumption (secondary emission counts twice!), and noise (in addition to pentode partition noise, there's multiplier noise too), as far as I know.

I remember something else about high power vacuum devices, but can't recall exactly what they were calling them.  Offhand I see keywords like "triggered vacuum switch", which appears to be a vacuum ignitron as it were (so, operation will be limited by sputtering of the electrodes providing the plasma).  These are often discussed in context of mains distribution handling (surge arresting, arc extinguishing, etc.; but probably not carrying load current, or turning off, for which a recloser is usually needed, AFAIK).  So, on the order of 50kV and about as many amperes peak.

Tim

[ZeroResistance]

What kind of heat losses is one looking at in vacuum tubes. If a tube is rated for 100kW, how much % of that would be wasted in heat?

[tszaboo]

If you place a conductor (wire) in vacum, the only way it can cool down is through conduction and emission. So your wires in the tube would get very warm if the current is high. Also, I dont have the number to back this, but high voltage low current is probably more efficient as high current low voltage. Then if you have a load witch requires high current, for example as 8 Ohm speaker, then you just run the current through a transformer. Which was the standard way of making these amplifiers.

[ZeroResistance]

I read that

"vacuum tube current is limited by space charge"

I understand that an electron cloud forms on the cathode but is space charge the effect of the electron cloud that is near the cathode repelling back the electrons emitted from the cathode?
How then do high current vacuum tubes mitigate this effect?

[bsdphk]

Some of the best articles about this can be found in Bell Systems Technical Journal (can be found on archive.org), in particular there are some good ones in the years after second world war and up to the transistor taking over.

The answer is that there are a large number of limiting factors, including the already mentioned cathode material issues, but many other more or less obscure effects.

[T3sl4co1l]

I read that

"vacuum tube current is limited by space charge"

I understand that an electron cloud forms on the cathode but is space charge the effect of the electron cloud that is near the cathode repelling back the electrons emitted from the cathode?
How then do high current vacuum tubes mitigate this effect?

Big fucking cathodes.

Have you read the numbers being thrown about in this thread?  Cathodes requiring 10s of kW have been mentioned.  That's a lot of white-hot tungsten!

What kind of heat losses is one looking at in vacuum tubes. If a tube is rated for 100kW, how much % of that would be wasted in heat?

Plate rating is plate rating.  Total input and output are whatever.  They can be higher (efficiency > 50% say for class C amps).

I've figured that a typical 30W sweep tube in class D can deliver 100-200W output, per tube.  Downside is, class D is very rough on the screen grid, which is typically rated for 3-5W for these types, but this service often dissipates >5W (depending on how carefully you've designed the inverter and its drive circuitry).

Still a far sight from the 90%+ of a MOSFET class D amp, but not bad considering.

They're still surprisingly competitive in terms of overall ratings -- it used to be a sweep tube (or the various other types: radar modulators, thyratrons, etc.) was about the only single device that could switch 5kV+ and nearly an ampere.  Now that 4.5kV 2.5A+ MOSFETs are cheap* and readily available, they really aren't good for much at all.

*I mean, they're like $20-30 each, but NOS sweep tubes are fairly in demand, easily running $60 each.  So, relatively speaking... not to mention all the savings from no screen or heater supply, and the lower drive voltages, of course.

Tim

[TimFox]

I read that

"vacuum tube current is limited by space charge"

I understand that an electron cloud forms on the cathode but is space charge the effect of the electron cloud that is near the cathode repelling back the electrons emitted from the cathode?
How then do high current vacuum tubes mitigate this effect?

Higher cathode current increases the space charge density until an equilibrium is reached with zero net field at the cathode and a finite current value less than the possible cathode emission at cathode temperature.  Increasing the applied field (from anode and grid voltages) increases that equilibrium cathode current in a nicely repeatable way.  The space charge controls the current, but does not kill it.

[ZeroResistance]

What is the significance of higher Anode Cathode Voltage, I believe that vacuum tubes were available in several volatges, starting at a few hundred volts to tens of kV's.
A high voltage would accelerate the electrons faster to the anode, wouldn't it.
So how does this high energy come into play? Because a higher energy electron slamming into the anode would manifest as heat once its velocity is put to a dead stop by the anode.
Is high volatge dependent on the anode cathode gap? I mean you need that amount of electric field to attract electrons towards you?

[Gyro]

What is the significance of higher Anode Cathode Voltage, I believe that vacuum tubes were available in several volatges, starting at a few hundred volts to tens of kV's.
A high voltage would accelerate the electrons faster to the anode, wouldn't it.
So how does this high energy come into play? Because a higher energy electron slamming into the anode would manifest as heat once its velocity is put to a dead stop by the anode.

It's like any component, P=V*A. For a given anode current, if you increase the voltage, the power dissipation (and available voltage swing) increases. Yes, the kinetic energy of the electrons hitting the anode is higher so the heat dissipation increases, just as you would expect.

Is high volatge dependent on the anode cathode gap? I mean you need that amount of electric field to attract electrons towards you?

No. For smaller gaps, the field gradient is higher and the electrons accelerate faster. For larger gaps, the field gradient is smaller but the electrons have a longer distance to accelerate in. The velocity with which they hit the anode is the same - you are accelerating them with the same amount of energy. Larger gaps simply make flashover between electrodes less likely.

[OldEE]

One of the problems with water cooling vacuum tubes is high voltage of the anode.  The first UHF TV transmitter that I babysat summers while in college had final amplifiers which ran at 6kV.  To minimize corrosion problems there was a 2ft diameter ceramic coil of purified water between the anode and the grounded plumbing.  How that thing was cast and fired always amazed me.  Once a day we check the conductivity of the water and it was usually 10-15 megOhms per cm.

Same job later had an RCA UHF TV transmitter which used 2 55kW klystrons for visual amplifiers.  These Varian klystrons were driven with 1 watt for 55kW out (47db gain) and consumed about 7A at 23kV continuous - not pulsed.  They were only 35% efficient so most of the power turned cooling water into steam which was condensed and recirculated.  The whole thing consumed just under a half a megaWatt.  The power company loved us.  Klystrons of this size were (are?) used on particle accelerators.

Larry