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VTTC Flashover Questions

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LAN_403

megabyte

Thu May 31 2012, 02:04AM

Registered Member #4953 Joined: Sat May 19 2012, 11:56PM
Location: Australia
Posts: 12

Hi guys,

What happens if you flashover your vacuum tubes by cranking up the voltage too high?
Does the coil still run, or does it stop?
What does it look like in the tubes?
Is it easy to tell over the brightness of the filaments?
Are the tubes instantly destroyed, or how quickly are they damaged?
Are flashovers a common occurrence when tuning, or is it something to be avoided at all cost?

Thanks.

Mads Barnkob

Thu May 31 2012, 05:57AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

megabyte wrote ...

Hi guys,

What happens if you flashover your vacuum tubes by cranking up the voltage too high?
Does the coil still run, or does it stop?
What does it look like in the tubes?
Is it easy to tell over the brightness of the filaments?
Are the tubes instantly destroyed, or how quickly are they damaged?
Are flashovers a common occurrence when tuning, or is it something to be avoided at all cost?

Thanks.

A flashover will most like occur between plate and grids, grids are thin and not made to take a beating like a plate can do for short. You risc doing great damage to your grids from a flash over.

If the flash over is sewer enough, the coil will stop working as the grids are destroyed or only work at lowered effectiveness.

You will see flash over as arcing inside the tube and its easily to hear that something is wrong.

The brightness of the filament does not tell you anything other than the filament voltage.

If you are lucky and the flash over is very short, the tube have a chance of surviwal.

Avoid flash overs at any cost! Tuning does not have anyhting to do with going over the voltage rating of the tube!

Dr. Dark Current

Thu May 31 2012, 09:13AM

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

The flash over in directly heated tubes usually happens at a very high voltage, when you are exceeding the maximum rating several times.
As the usual power supply for a VTTC is a MOT with a doubler and just a little rf blocking cap, the flash over duration would be very short and the tube would probably survive. I have seen a tube flash over in a VTTC, most likely because of "squegging" or parasitic oscillations, and the coil continued to work normally.
That said, you should still try to avoid flash overs as you can.

However, the flash can happen also between the grid and the cathode from excessive grid drive. This is usually not damaging, but can possibly weaken the tube after prolonged sparking.

In indirectly heated tubes, the cathodes are less rugged and spark overs are more damaging, often they rip off pieces of emissive material from the cathode. Also these tubes can usually take less grid drive and spark from grid to cathode sooner, this sparking damages the cathode.

megabyte

Thu May 31 2012, 10:54AM

Registered Member #4953 Joined: Sat May 19 2012, 11:56PM
Location: Australia
Posts: 12

Thanks for the wisdom guys.

This leave me wondering though, how can you tell when you've reached your maximum voltage without letting a flash over occurr? E.g. Mads, you ran 3200v on your coil which is way above the tube's rating, but when you're using staccato, how do you know you can't go even further?

Dr. Dark Current

Thu May 31 2012, 06:58PM

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

The maximum voltage rating on transmitting tubes usually means a steady-state voltage, with no modulation. The tubes are often designed to take short-duration input of up to 2 times the DC voltage, for the purpose of plate voltage modulation, however if the datasheet doesn't state this, I don't know of any other way how to tell this.
That said, the ratings for the tubes usually apply into the several MHz or tens of MHz range, for VTTC frequencies they will take more voltage, partially also because of the decreased capacitive currents inside the system.
The tubes are usually designed with a relatively high headroom, probably because of manufacturing tolerances and such to make sure every tube will stand the rated voltage. I have heard of 3kV DC tubes being run on voltage doubled 4.4kV AC, which means up to ~12kV momentary input and 24kV peak voltage (because of the doubling action of the class C oscillator). I have also seen a 1.5kV DC tube (the GK-71) being run on voltage doubled MOT with up to 6kV momentary input.

Steve Conner

Thu May 31 2012, 07:31PM

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

As tubes age, conductive material from the cathode gets sputtered over the insulators inside, reducing the creepage distance and increasing the chance of a flashover. So, a new tube will take more voltage, or put another way, the voltage rating is made conservative to guarantee a decent lifespan.

AFAIK.

Dr. Dark Current

Thu May 31 2012, 08:22PM

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

I always thought the life of the tube depends on the cathode losing its emmision, or the filament breaking like in a light bulb. But Steve makes a point too.

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