Author Topic: Why weren't Vacuum Tubes designed for higher currents  (Read 1209 times)

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Offline ZeroResistance

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Why weren't Vacuum Tubes designed for higher currents
« on: August 13, 2019, 03:35:16 pm »
I see that Vacuum tubes are typically rated for medium to high voltage (100's of Volts to few kV)  but currents are typically in mA upto a few Amps.
What factors limit their use only upto a few Amps?

TIA
 

Offline MagicSmoker

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #1 on: August 13, 2019, 03:45:08 pm »
Thyratrons, mercury arc rectifiers and quite a few other exotic tubes were (are) capable of 100s to 1000s of amps.

Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically).

 

Offline Tom45

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #2 on: August 13, 2019, 03:47:05 pm »
I don't know the answer, but that isn't going to stop me from speculating.

In the linear region of operation a device dissipates E*I. For transistors, E is typically under 100 volts. For vacuum tubes E is more commonly in the 100s of volts. For a given power dissipation, a tube operating at a voltage 5 times that of a transistor, will max out at a current 1/5 of the transistor having a same power rating.

And the tube's vacuum makes it harder to get rid of the power it is dissipating.

There are water cooled tubes for high power operation. But these weren't seen in consumer electronics.

For example: http://www.tubecollectors.org/eimac/archives/8974.pdf
 
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Offline ZeroResistance

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #3 on: August 13, 2019, 04:03:56 pm »
Thyratrons, mercury arc rectifiers and quite a few other exotic tubes were (are) capable of 100s to 1000s of amps.

Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically).

Agreed! however I wanted to know regarding vacuum tubes.
I guess that Thyratrons and the other devices that you stated are "Gas Filled Tubes".
 

Offline ZeroResistance

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #4 on: August 13, 2019, 04:05:43 pm »
Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically).

What are these space charge limits? I mean is there a known current (amps) limit beyond which vacuum tubes cannot be used?
 

Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #5 on: August 13, 2019, 04:21:36 pm »
For a given cathode material / coating and temperature, there is a limit to the number of electrons that it can emit per unit area. Under non-biassed conditions, these electrons form a cloud (the space charge) around the cathode.

Once you bias the anode positive and the tube starts conducting, these electrons flow towards the anode. If you pull too much current, the space charge is exhausted and you start physically 'ripping' electrons from the Cathode surface, irreversibly damaging it (particularly if it is an oxide coated one for higher emission). The space charge also helps protect the cathode surface from positive ion bombardment.

Pure Tungsten or Thorated Tungsten directly heated (filament) cathodes are much more resistant to damage, they will effectively turn into constant current sources (again determined by surface area and temperature) when you try to pull more than the available density of electrons.

EDIT: The other physical limits are the maximum sustainable thermal dissipations of grids and anodes determined by voltage and current.
« Last Edit: August 13, 2019, 04:28:49 pm by Gyro »
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Online IanB

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #6 on: August 13, 2019, 04:29:46 pm »
I understand there are "big" tubes used in broadcast equipment that can handle many kilowatts. I'm guessing they may also handle larger currents due to their physical size?
I'm not an EE--what am I doing here?
 

Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #7 on: August 13, 2019, 04:37:23 pm »
Yes, large cathode area (often in a 'basket' form for maximum surface area) which need correspondingly high currents to heat them to emission temperature.

I believe large broadcast transmitter tubes were/are bright emitter Tungsten cathodes to withstand ion bombardment.

The other way they get so much power is to increase the anode voltage (a lot) of course.


P.S. Peter Millett's tubebooks.org site is an excellent resource for downloadable books on the subject... http://www.tubebooks.org/
« Last Edit: August 13, 2019, 04:41:23 pm by Gyro »
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Offline TimFox

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #8 on: August 13, 2019, 04:38:57 pm »
A normal vacuum tube has two limiting operating conditions:   emission-limited (“saturated”) and space-charge limited.  This is a simplified description of an ideal diode or triode:
If the anode-cathode voltage is high enough to attract all of the electrons emitted by the hot cathode, then (to first order) the diode current is independent of the voltage and is a strong function of the cathode temperature.  When the cathode current forms a space-charge cloud near the cathode, the negative charge cloud reduces the field at the cathode and controls the current.  In the limit of full space-charge control, the current behaves as the famous 3/2 power of the anode voltage, and does not depend on the cathode temperature, so long as it be hot enough to produce the current.
 When a control grid is added to make a triode, the 3/2 power law is applied to a linear combination of anode and grid voltages.  Here, I refer you to Wikipedia or any textbook.  This operating condition is very useful, since it doesn’t depend directly on the cathode temperature, so long as it is hot enough.
The vacuum tube structure is very easy to scale up for high power, but the space-charge equations and cathode current density limit this to relatively high voltage and low current, compared with a semiconductor device at similar power levels
 

Online filssavi

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #9 on: August 13, 2019, 04:43:18 pm »
As other have already said there are plenty of limits, but I think the main reason high current tubes were not around is that simply there was no pressure from the market to develop them in a pre semiconductor world. Let’s keep in mind that the higher the current the higher the joule losses in the whole system...
 

Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #10 on: August 13, 2019, 04:55:43 pm »
A search for 'broadcast transmitter valve' images brings up some very pretty pictures, eg.

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Offline TimFox

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #11 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.
 

Online T3sl4co1l

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #12 on: August 13, 2019, 05:00:50 pm »
Cathode emission is relatively weak.  A typical sweep tube needs 15W of heat for about an ampere of emission, and that's supposed to be pulsed at that; a bigger transmitter tube requires hundreds or thousands of watts.

And yeah, between the space charge neutralization property of a plasma, and the much greater emission due to ion bombardment, the current density can be much higher.

We can go even further and consider a stationary matrix of ions; this further reduces the energy levels required (~eV to reach conduction band, versus ~10s eV to reach vacuum state), and greatly increases the current density.  Unfortunately, transit time is significantly impaired, because transport is via diffusion, at quite low velocities considering the energy levels and an electron's [free] rest mass).  On the upside, the breakdown voltage is also enhanced, so we can make very thin junctions, and this more than makes up for transport.

In other words, from a sufficiently high level, we can look at tubes and transistors as the same general thing: some exchange of charge carriers at the electrodes, with transportation inbetween, subject to applied fields.  For tubes, we use the ballistic transport of electrons, and space charges; for transistors, we use diffusive transport, and recombination.  Solving the equations gives a V^(3/2) characteristic for tubes, or a exp(V) characteristic for transistors.  (Depending on large/small signal regime.  Tubes, FETs and BJTs are all exponential at low current densities, driven by the tail of Maxwell-Boltzmann statistics.  At higher current densities, tubes go towards V^(3/2), FETs go towards V^2, and BJTs remain ~exponential.)

Of course if we put in a plasma, we get all kinds of weirdness, typically negative resistance and bistability.  This happens because emission is proportional to current flow times a multiplier (an ion impact begets multiple free electrons, on average).  The plasma is also not easily controlled by fields: due to its being conductive, it's an effective [electric] shield.  (The same is of course true of semiconductors, but we can harness very large surface areas in that case, making FETs practical.)

Tim
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Bringing a project to life?  Send me a message!
 
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Offline ZeroResistance

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #13 on: August 13, 2019, 05:07:11 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.

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>
 

Offline ejeffrey

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #14 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
 
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Offline TimFox

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #15 on: August 13, 2019, 05:23:00 pm »
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.
 

Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #16 on: August 13, 2019, 05:38:15 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.

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


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>

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.
« Last Edit: August 13, 2019, 05:52:08 pm by Gyro »
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Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #17 on: August 13, 2019, 05:57:13 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

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

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #18 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.



 

Offline ZeroResistance

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #19 on: August 13, 2019, 07:41:03 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.

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?
« Last Edit: August 13, 2019, 07:49:40 pm by ZeroResistance »
 

Offline ZeroResistance

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #20 on: August 13, 2019, 07:45:41 pm »

Once you bias the anode positive and the tube starts conducting, these electrons flow towards the anode. If you pull too much current, the space charge is exhausted and you start physically 'ripping' electrons from the Cathode surface, irreversibly damaging it (particularly if it is an oxide coated one for higher emission). The space charge also helps protect the cathode surface from positive ion bombardment.

Wouldn't the electrons be returned back to the Cathode after hitting the Anode and therefore replenishing it?
In that case why would the electrons in the Cathode get exhausted?

I didn't get the "positive ion bombardment" part too...
« Last Edit: August 13, 2019, 07:47:20 pm by ZeroResistance »
 

Offline hfleming

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #21 on: August 13, 2019, 07:52:01 pm »
Here are 2 datasheets, one is for a 5MW magnetron, the other is for a 66kW broadcast tube
 
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Offline Gyro

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #22 on: August 13, 2019, 08:03:03 pm »

Once you bias the anode positive and the tube starts conducting, these electrons flow towards the anode. If you pull too much current, the space charge is exhausted and you start physically 'ripping' electrons from the Cathode surface, irreversibly damaging it (particularly if it is an oxide coated one for higher emission). The space charge also helps protect the cathode surface from positive ion bombardment.

Wouldn't the electrons be returned back to the Cathode after hitting the Anode and therefore replenishing it?
In that case why would the electrons in the Cathode get exhausted?

I didn't get the "positive ion bombardment" part too...

Don't forget there's an external circuit - electrons reaching the anode will pass into the external bias 'supply' ie. circuit. In fact some electrons can be knocked off the anode (secondary emission) which is responsible for the characteristic kink in a Tetrode's characteristics. This is resolved in the Pentode by adding a coarse negatively charged Suppressor grid between the anode and other electrodes to repel secondary emission electrons back to the anode.

The secondary emission kink is also suppressed in the Beam Tetrode (aka Kinkless Tetrode) which creates a 'virtual' screen grid in the voltage gradient to the anode (it's complicated). It's worth googling Tetrodes and Pentodes at this stage [Edit: and taking a browse through some of the books in Peter Millett's archive].

The Positive Ions are formed by collisions with trace gas atoms remaining in the tube. Being positively charged, they are strongly attracted to the cathode, and being relatively massive, they can severely damage the emissive coating on the cathode. This is why Tungsten is used for very high voltage tubes, even though the work function is lower, the surface is immune from high energy ion bombardment.
« Last Edit: August 13, 2019, 08:19:31 pm by Gyro »
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Offline chris_leyson

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #23 on: August 13, 2019, 09:19:59 pm »
Beam tetrodes were designed to get around the Mullard pentode patent and eliminated the tetrode kink hence valves like the KT66 or KT88, KT standing for kinkless tetrode, allegly.

One thing you don't see with vacuum tubes is the non-linear voltage controlled junction capacitance that you get with semiconductor devices. Less harmonic distortion to some extent.
 

Offline ArthurDent

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Re: Why weren't Vacuum Tubes designed for higher currents
« Reply #24 on: August 14, 2019, 12:16:22 am »
Here is a photo and close-up of a Tung-Sol CTL-6336A dual triode with an octal base. It was typically used as the series pass tube in higher current power supplies than the 6AS7 dual triode mentioned earlier and could handle about 4 times the current or about 450 ma per triode section..

The problem with vacuum is that it sucks as a conductor, both for electricity and for temperature transfer. Conduct may not be the technically correct explanation but that is the effect and easier to visualize. The 6336A didn’t use steel plates like the 6AS7 and other lower current (and less expensive) tubes, but had machined graphite plates that wouldn’t warp if they got hot enough to glow red. The heater was 6.3 volt @ 5 amps or over 30 watt and the plate dissipation was 30 watts for each of the triode sections. The electrons had to be boiled out of the cathode and were literally fired at the plate and their flow was controlled by the potential on the control grid. The spacing between elements of this triode was quite small to increase the efficiency by decreasing the distance the electrons had to travel through the vacuum.

To get higher power, some transmitting tubes didn’t have their plates inside a glass envelope but made the plate the outside of the tube so the heat could be more easily dissipated. This also meant the outside of the tube would have a high lethal voltage on it. The plates could be water cooled and here is the datasheet on a high power water cooled tube. The filament current of this tube is 640 amps and the anode current is 125 amp max. If you’re wasting tens of kilowatts just to boil electrons out of the filament, you can see why much higher efficiency solid state devices are so popular for transmitters today.

http://www.tubecollectors.org/eimac/archives/8974.pdf

« Last Edit: August 14, 2019, 12:30:05 am by ArthurDent »
 


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