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vacuum triode on resistance?

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alf

Fri Jun 10 2011, 06:13PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
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Posts: 121

hi,

what is the resistance of a triode when it is fully switched on? for example an ecc81.
because i thought i could use an old triode as a switch to control my coilgun, as the capacitors are at 400v, which is suitable for a vacuum tube. i know that there would be about 400A flowing through the triode, and the plates may be damaged, but i have many vacuum tubes which have had their markings rubbed off, so i could just use them for that.

thx. Alf.

Proud Mary

Fri Jun 10 2011, 06:54PM

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

alf wrote ...

hi,

what is the resistance of a triode when it is fully switched on? for example an ecc81.
because i thought i could use an old triode as a switch to control my coilgun, as the capacitors are at 400v, which is suitable for a vacuum tube. i know that there would be about 400A flowing through the triode, and the plates may be damaged, but i have many vacuum tubes which have had their markings rubbed off, so i could just use them for that.

There will not be 400 A flowing through the poor little RF double triode - more like 10 mA !

I think it would help you if you were to read a few basic textbooks on thermionic valves, such as:

Basic Radio - The Essentials of Electron tubes and their Circuits, J. Barton Hoag, 1942, 379 pages

"This college-level electronics text is "designed for the student with only a limited background in physics and mathematics". Covers the basics of AC and DC circuits and radio propagation, then vacuum tubes and circuits that use them. Has some information not found on other texts, like high-frequency and microwave radio, transmission line theory, and DC amplifiers."

You can download it free as a .pdf here:

alf

Fri Jun 10 2011, 07:10PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

haha ohh ok,

why is it that when a triode is fully on, there is such a high internal resistance?

so do you guys know wat i should use to switch the coilgun capacitors to the coil?

i know i posted it in the wrong category.

thanks!. Alf.

Proud Mary

Fri Jun 10 2011, 07:27PM

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

alf wrote ...

why is it that when a triode is fully on, there is such a high internal resistance?

Because thermionic valves are inherently high impedance devices. In the Thermionic Age, gas switches such as thyratrons and ignitrons - which are not high vacuum devices - were used for high voltage high current switching when electro-mechanical relays alone were not enough.

Perhaps you should ask more questions only once you have committed your first thermionic valve textbook to memory, so you have at least a small hope of understanding the replies.

Dr. Dark Current

Fri Jun 10 2011, 08:55PM

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

Vacuum tubes can switch surprisingly high currents, eg. the PL504 - a standard horizontal output pentode for b/w TV (250 mA max. cathode current) was tested by my friend to switch a 5 Amp impulse. However this is not a continuous rating and excessive abuse with such pulses may damage the cathode. Still this is nowhere near your 400 Amps, maybe you would want to look into a Thyratron or a similar device. But ayway a SCR will be your best bet.

Mattski

Fri Jun 10 2011, 09:22PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

For a given amount of power handling roughly speaking a vacuum tube will handle a higher voltage at lower current, while a semiconductor device will handle lower voltage at higher current.

But in a coilgun you generally want low (hundreds of volts) voltages and high currents (hundreds of amps peak), so semiconductor devices fit in this niche. Tubes handle higher voltages up to 10's of kilovolts I think, but at much lower currents.

Proud Mary

Fri Jun 10 2011, 09:54PM

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

Dr. Kilovolt wrote ...

Vacuum tubes can switch surprisingly high currents, eg. the PL504 - a standard horizontal output pentode for b/w TV (250 mA max. cathode current) was tested by my friend to switch a 5 Amp impulse. However this is not a continuous rating and excessive abuse with such pulses may damage the cathode.

I'm sure you intended to add that there was nothing 'standard' about the electrode architecture and performance of line output valves, which were designed to mimic the action of an on/off switch as much as possible - most atypical of thermionic valves, as a glance at their characteristic curves will show.

Another on/off thermionic curiousity is the long extinct 5963 double triode, designed to operate for long periods under cut-off conditions in computer service:

radiotech

Sat Jun 11 2011, 04:52AM

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

The "conductance" of a thermionic valve has two levels. the normal
level is when electrons are being drawn from the space charge.

When this is the case the current will vary as a function of the plate
voltage raised to 3/2 power multiplied by a constant, known as perveance.

The perveance is controlled by the ratio of the anode area compared to
cathode area with a given spacing.

So your resistance would be the voltage divided by the current at the
point where the grid is at a voltage causing electrons to be drawn from the space charge at the maximum rate.

If you tried to move much beyond that point the grid would probably became an anode, and anode current would drop and the cathode would
start sputtering towards the grid.

Sometimes this is done in cathode ray tube rejuvinator.

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