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Dr. Dark Current

Wed Apr 11 2012, 05:39PM

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

A quick question, but more than a year of searching and no answer...
Lets say I have an *indirectly heated* tube, running in a VTTC service with pulsed supply. The cathode current at the peak of supply voltage is 1A, but averaged over one mains period is just 0.3A. The maximum cathode current of the tube is 0.5A. The question is, am I exceeding the cathode current rating of the tube or not?

Steve Conner

Wed Apr 11 2012, 06:24PM

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

Ha! Talk about analog black magic.

I've never found a definitive answer on this either, but reading between the lines of the tube datasheets:

The maximum cathode current figure is what would be shown on a moving coil meter in the intended application. So it's an average value. The peak rating can be inferred from the peak-to-average ratio typical of the application. In the usual kinds of amplifying circuits, the peak current would be 1.5x the average for a push-pull pair (both cathodes together) and 3x for a single tube.

Tubes intended for pulsed service will also state the real peak current limit, and the time for which it holds, on the datasheet. 12A for 1us or whatever.

The space charge between cathode and control grid is a reservoir that smooths out the emission demands on the cathode, but nobody knows how much.

I wouldn't be surprised if there were no hard current limit, it is probably just a matter of the higher the current, the shorter the tube life.

The GZ34 indirectly heated rectifier datasheet goes into great detail on peak currents. Maybe you can calculate the peak amps of emission per watt of heater power and extrapolate to other tubes.

Dr. Dark Current

Wed Apr 11 2012, 06:45PM

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

Hi Steve,

regarding the "absolute peak current", after some measurements I came to the conclusion that the cathode indeed averages the current over a HF cycle at typical VTTC frequencies. When I take my example, the tube in question is putting out around 5A of maximum peak current. It doesn't seem to go much over that if I increase the voltage even more, but what I observed is that when I decrease the conduction angle, the current easily shoots over the 5A value but the average current over one HF cycle (which is 1A in my case) doesn't increase.

So I think the "absolute peak" of the current doesn't really matter for indirectly heated tubes, but I have no idea if the same applies for low frequency modulation of the current. I would say the time constant it takes the cathode to rebuilt the space charge is much shorter than one mains cycle, however it is depleted just as quickly. When running such tubes in interrupted mode, I observed a current peak the time the tube is turned on, which lasts several HF cycles and is much larger than the steady-state value. But this may also have something to do with the circuit trying to start the oscillation.

So this maybe comes down to the question, "does it matter if the space charge is depleted if the DC cathode current is kept within specs"?

Steve Conner

Wed Apr 11 2012, 10:49PM

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

Well, surely the whole point of the cathode current spec is that it's a conservative value that prevents the space charge from ever being depleted.

According to the old tube manuals, the space charge is needed to protect oxide coated cathodes from damage by ion bombardment. They should never be run in temperature limited emission, because it means the space charge has run out.

An interesting experiment you could try: does that 1A average/5A peak change with filament temperature? If it does, there's proof that you're running the cathode too hard.

Dr. Dark Current

Thu Apr 12 2012, 08:36AM

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

You say temperature limited emission, I have seen videos of tube coils where the cathode starts glowing more brightly after the power is applied, could this be the sign? I've looked at the cathode of my tube and it doesn't change its color (stays dark red).

I'm pretty sure the cathode voltage does change the current, I may try that, but I think you could observe this with any cathode current...

Uspring

Thu Apr 12 2012, 09:23AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Steve Conner wrote:

The space charge between cathode and control grid is a reservoir that smooths out the emission demands on the cathode, but nobody knows how much.

I'm no expert on tubes, but I doubt this. If you e.g. confine one nanoCoulomb worth of electrons to a volume of a cubic centimeter you'll get electric fields around 1kV/cm near this region. I can't imagine fields near the cathode being able to confine them. So I guess a nC being an upper limit to the space charge. With a current of 1A this charge would be depleted within 1ns.

I hope this makes sense.

Steve Conner

Thu Apr 12 2012, 09:32AM

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

It does make sense, but bear in mind that in high gain, high power tubes the electric field due to the control grid can be much more than 1kV/cm.

The grid-cathode spacings are measured in thousandths of an inch and the grid is driven with hundreds of volts.

Having said that, I wouldn't be surprised if the "reservoir" is really some surface chemistry effect, and the space charge explanation was a lie to protect trade secrets of cathode chemistry.

If the cathode starts glowing more brightly under power, that would be really bad. Marko and I have tried abusing old EZ81 rectifiers, and even with almost 1A of cathode current, the cathode doesn't get brighter. I did see the whole space between cathode and anode filled with a worrying yellow plasma, which was probably made of cathode bits.

It's more likely that the grids are glowing white hot from excessive drive, and that just makes the cathode look brighter.

Dr. Dark Current

Thu Apr 12 2012, 10:07AM

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

I think I'll leave the circuit as is, box it up and we'll see how long the tube lasts. Even if the life of the tube drops from 5000 to 50 hours, the VTTC will last a lifetime...

Steve Conner

Thu Apr 12 2012, 10:23AM

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

Sounds like a good plan :)

Out of interest, what tube is it? A PL509 or similar?

Dr. Dark Current

Thu Apr 12 2012, 10:38AM

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

It's a russian 6P45S tube, like EL509. Pretty amazing what it can do... The sparks are shooting around 25cm long from a little secondary with 1.5kV of peak plate input.

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