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Vacuum tube life in tesla coil use

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Dr. Dark Current

Tue Jan 19 2010, 03:30PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

From what I've seen in tube datasheets, to me it seems that the maximum cathode current rating is way too low for VTTC use, especially when supplied with halfwave rectified/doubled voltage. To me it looks like the peak cathode current must be exceeded long before the plate starts even getting red.

How big of a problem is to exceed the maximum cathode current, even when 'mean' current is acceptable? Do vacuum tubes tolerate better overcurrent or overvoltage? Has anyone actually measured these parameters in a running VTTC?

Steve Conner

Tue Jan 19 2010, 03:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This is a very interesting question that could use some practical research.

First of all, the plates on TV sweep tubes (PL509 etc) aren't supposed to get red! The plate is still dark at the maximum rated dissipation, and running it red will shorten tube life by outgassing. But according to the "VTTC rating", as long as the plate isn't actually melting before your eyes, it's OK...

Second, it's not clear from many tube datasheets whether the "maximum" cathode current rating is peak, or average. For instance, the EL34 datasheet states a max cathode current of 150mA. Two tubes in push-pull will measure 300mA when producing their rated 50 watts out on a sine wave test, so the actual peak cathode current in each tube is nearer 500mA, and therefore the datasheet rating must be an average.

Third, the maximum current might depend on the pulse width. Some tube books suggest that a cloud of electrons builds up inside the control grid, acting as a reservoir. Short pulses draw on this store of electrons and can safely exceed the cathode's own emission rating. This might explain the datasheet confusion.

Fourth, sweep tubes have huge cathodes and their actual emission rating is probably several amps. Directly heated tubes have less emission headroom, because the filament has less active surface area, and the coating isn't as efficient as the oxide coating on an indirectly heated cathode.

Short answer: Try varying the heater voltage. If this affects cathode current on an indirectly heated tube, then you're pushing the cathode too hard for sure! In 4hv style, you could of course overdrive the heater voltage up to get more emission

Dr. Dark Current

Tue Jan 19 2010, 04:06PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well I know that the plate usually isn't supposed to get red, but many people seem to take this as an indication that the tube is owerpovered, not taking other things into consideration such as cathode current.

I also know that horizontal output tubes have quite high current rating, but I was talking more in general, mainly about transmitting tubes which are the most powerful, but to me have suspiciously low cathode currents.

Steve Conner

Tue Jan 19 2010, 04:20PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, those transmitting tubes have low cathode current ratings because they're directly heated, like I explained.

As far as I know, directly heated tubes, at least the plain and thoriated types, aren't damaged by excessive cathode current. They just refuse to emit more than the rated current, unless you turn the heater voltage up above spec, and that seriously reduces the life.

I could be wrong about this, though. Directly heated tubes come in at least three different filament types: plain tungsten, thoriated, and oxide-coated, and the oxide-coated ones may be damaged by too much cathode current. Others on the forum know more about it than I do.

Edit:

Proud Mary

Wed Jan 20 2010, 05:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

As a rule of thumb, the small valves found in consumer electronics such as television receivers are dependent on convection and radiation for cooling, and so are not designed to run with their anodes at red heat. It is not uncommon for medium and high power valves dependent on forced air or liquid cooling to have anodes glowing at dull red heat in normal use.

The small tetrodes used as Line output valves ("sweep tubes") in domestic TVs were designed to mimic the action of a mechanical switch as much as possible, as one sees from the graphs in their data sheets.

There are plenty of large, heavy, power valves designed for pulse operation, the power supplies for which can be very major projects in their own right.

4CW10,000 has been used as a driver-oscillator for a 600kV 10mA C&W here:

Data sheet here:

The description and circuit diagram of a much more recent design using a pair of BW1121J12 triodes as push-pull 120kHz oscillator for a 3 MeV 30kW supply is here:

Broadcast transmitters in the tens to hundreds of kilowatt range are one of the last refuges of new thermionic technology.

The splendid Siemens RS1043CV is one of these, and needs 260A for the heater alone!

Dr. Dark Current

Wed Jan 20 2010, 09:01PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So for uses where the peak cathode current could be exceeded, is it better to use a tube with thoriated tungsten cathode as it will limit the current and won't get damaged?

Proud Mary

Wed Jan 20 2010, 10:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dr. Kilovolt wrote ...

So for uses where the peak cathode current could be exceeded, is it better to use a tube with thoriated tungsten cathode as it will limit the current and won't get damaged?

The purpose of alloying W with 1-2% of Th is to lower the work function of the kathode. In the final stages of manufacture, the filament is heated to a very high temperature such that all the Th migrates to the surface, where it is carburized to improve its stability. This Th film so coating the tungsten has the effect of reducing the work function from 4.52 ev for plain W to 2.6 ev with the carburized Th layer, but it has no role in limiting kathode current, peak or otherwise.

It is possible to partly 'rejuvenate' thoriated tungsten filaments that have been idle for decades by briefly heating the filament to a much higher temperature without applying anode voltage - a process you will find well described elsewhere.

radiotech

Thu Jan 21 2010, 01:35AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Plates or anodes of tubes achieve 'red' heat, not from current flowing through them, because that would take brute amperes. They are heated by the impact of electrons, the velocity is voltage dependant. (In a 3NP4 projection kinescope (Philips Protelgram System) circa 1956, we had holes melted in the face when the sweep failed, Ultor was 30kV) The plate resistance of a tube has nothing to do with the metalic resistance of the plate. One failure mechanism is when the space charge is depleted, and electrons are pulled directly off the cathode, there is no defense against ions of opposite sign from slamming into the cathode . Tubes such as the 0Z4 (so called cold cathode) or ionically heated cathode
depend on this effect when they fire
and become hot cathode. The perveance of the tube is controlled by the ratio of the anode area to the area of the cathode. If a special tube was made for TC use attention would focus on the duty cycle of the peak pulse , the same way it is done for TV line output tubes . The best ones of these came out just as tubes vanished as sets were made solid state.
On a tetrode if the anode voltage fails the screen can burn up since all the electrons accelerating through it , now impact it.

It isn't unknown for tubes such as the 3V4, to keep their filaments alight, even when the A battery is cut off. Leakage of the grid coupling capacitor causes this especially with old tubes.

Dr. Dark Current

Thu Jan 21 2010, 10:33AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

What happens to an overvoltaged tube with no plate current, such as too high anode peaks in a Class-C VTTC oscillator?

Proud Mary

Thu Jan 21 2010, 01:34PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dr. Kilovolt wrote ...

What happens to an overvoltaged tube with no plate current, such as too high anode peaks in a Class-C VTTC oscillator?

A Class C valve amplifier with a purely resistive load would turn a sinewave at the control grid into a series of pulses at the anode, as you suggest.

But where the load is a tuned circuit, the output voltage becomes clamped at 50% of the supply voltage, so the grid bias is also varied in proportion, and
the original waveform is reconstituted. The circuit's distortion is related to the bandwidth of the tuned circuit, with the least distortion at the band's centre frequency,, and greater distortion as one moves towards the band margins.

The higher the Q of the tuned load, the more selective the circuit will be, and the purer the sinewave at the output.

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