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Thermionic diode multiplier

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Adam Munich

Sat Jan 29 2011, 06:00PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I as thinking about future projects, and I still have those 16 homemade caps from that million volt marx I never finished...

So... I was thinking of turning them into a Cockroft Walton multiplier. Now I don't want to spend 70 something dollars on a whole bunch of UF4007s, so I had the idea of using thermionic rectifiers. They are cheap, sometimes even selling for only $2 each. I also imagine they are very hard to kill, unlike Si diodes. They are super fast too since they don't have the recovery time that Si diodes have. Also, I imagine they can handle very high peak currents without damage, and that means very loud sparks.

My only concerns are:
Corona loss from the homemade caps at 40kV (oil could fix that I'm sure).
X-rays from the thermionic diodes.

Thoughts/opinions?

Proud Mary

Sat Jan 29 2011, 07:03PM

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

You will have to provide a floating supply for each valve's filament or heater, which in practice is likely to mean one, two, or three NiMH cells per valve. Clearly, this isn't the only possible means of providing a floating supply for each valve, but is probably the simplest.

The voltage drop across TV EHT rectifier valves is usually between 200V and 600V in normal operation, so there will be no X-ray production unless a heater or filament fails, when cold cathode X-ray emission could occur.

The indirectly heated GY501 was one of the last EHT rectifiers of the Thermionic Age, a robust diode that could handle 100mA peak current, but you should study the duty cycle caveats in the data sheet.

I would expect the filament in directly heated EHT rectifiers to fail quite quickly outside of the data sheet design values.

Adam Munich

Sat Jan 29 2011, 07:47PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Some diodes only need 1.5V for the heaters, so a D battery for each could suffice.

Do you really think that a millisecond, or even microsecond pulse from the discharge would damage the tubes? This isn't really continuous current, and at 5-10 sparks a second that's not much power.

Proud Mary

Sat Jan 29 2011, 08:03PM

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

The filaments in directly heated valves are more prone to thermal stress failure - expansion and contraction - than indirectly heated types, which is one of the reasons why they fell from favour in the 1960s.

How long would a particular EHT rectifier valve last when abused? I have no idea. You would have to try it and see.

Nah

Sat Jan 29 2011, 08:20PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

And don't use a 6x5.

Adam Munich

Sat Jan 29 2011, 09:04PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hmm, so indirectly heated diodes then?

It'd be neat to make it all tubes. I have no clue about tube circuit design, but...

Proud Mary

Sat Jan 29 2011, 09:53PM

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

In the normal course of events, I would have thought that a push-pull valve power oscillator

such as these

would have served your purpose well, but the resemblance of your circuit to the Mazzilli driver circuit makes me think that you'd do better to consult one of those that have made this type of inverter their special interest.

James

Sat Jan 29 2011, 10:24PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Sure, it can be done. You shouldn't have to spend that much on diodes though. Have you checked out OT Electronics? They have some cheap HV diodes, I've ordered a few different things from them in the past and have been satisfied. They have 8kV 10mA diodes for 26c each, and 12kV for 36c.

radiotech

Sun Jan 30 2011, 02:48AM

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

Sure you can use termionic rectifiers easily in a RF flyback.

The bottom end of the coil had little 1 turn rings on the rectifier
sockets lights the filament, and the tubes were arranged around
the bottom of the coil. In some TV with 15750 Hz flybacks, 3 tubes
in a multiplier loops .around the core, You must consider the power
needed to light the tubes draws the Q down of the main coil. If using
60 Hz iron, then no problem except insulation. You can not cast a
strong 60 Hz field far from core easily.

Adam Munich

Sun Jan 30 2011, 02:56AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

@mary

It's pretty much the ZVS with the fets replaced with triodes and the diodes replaced with thermionic ones. The resistors and turns were adjusted for the increased voltage.

It'll probably blow up.

@James

The point of this is to use thermionic diodes, not silicon ones.

@radiotech

Wut?

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