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high voltage rectifiers with low power filaments?

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klugesmith

Sat Jan 12 2019, 07:06AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Who's familiar with popular vacuum tubes/valves?

I'm making a 0 to 3000 volt DC power supply for biasing radiation detector tubes. Adjustment by variac.

It would be fun to make it semiconductor-free, by using a vacuum diode instead of microwave oven diode.

Looks like many television EHT rectifiers have 1.25-volt filaments at about 0.2 A -- for example 1J3, 1K3, 1X2A. Might be battery powered.
Then there are 5V and 6V rectifier tubes, some in miniature 7 or 9 pin packages, but not many with multi-kV ratings. Not to mention filament transformers with multi kV of insulation.

What determines the reverse voltage rating of a vacuum rectifier? Suppose we tried using a modern vacuum fluorescent display as a HV rectifer (with anode current much less than 1 mA, and filament power measured in tens of mW)? Suppose we tried using a magnetron from microwave oven, without magnets, as a HV rectifier?

Plasma

Sun Jan 13 2019, 04:15AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Would connect the tip of the magtron to one lead work, depending on direction the feedback would be positive or negative.

Maybe...

klugesmith

Sun Jan 13 2019, 04:56AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

The magnetron cap (antenna) is hardwired to the anode, and is at the same voltage as anode except at very high frequencies.

I bought a couple of 1X2 rectifiers (with miniature 9-pin base) this evening, at a local electronics surplus store on its last day of business. Cost me 99 cents each. Will probably power one with a battery for circuit breadboarding, then rewind some little control transformer to get 1.25 volts.
There would be no point in trying to squeeze an extra winding onto the high voltage transformer, because that will be on a variac. It's the first of two NST's pictured in my Neon Sign Transformer Power thread -- page 1, September 1.

hen918

Sun Jan 13 2019, 01:45PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

A magnetron does not have a low power filament. They take 3 V at 10 A.

Plasma

Sun Jan 13 2019, 05:19PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

I don't think it has a filament, just two wires one to a rod at the center and another to straight slots, if the antenna wasn't connected to the center rod, it would pick up a certain charge that you could have used to subtract or add to one of the wires.
But that's just nitpicking :(

I suppose you could send 10 amp through the anode and antenna to raise its temperature.

radiotech

Tue Jan 15 2019, 11:13PM

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

You could thread a few turns through the core of an open frame small NST
to get enough induction to light the filaments of the TV high voltage rectifiers.

Perhaps make several windings, so you can make a voltage doubler or even
a tripler, with 2 or 3 tubes.

Common practice on old TV sets. Windings looped through core of flyback.

klugesmith

Fri Feb 01 2019, 01:50AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Thanks for the hints, Radiotech.

My 0-3000V power supply is done, using a 30 kV 30 mA solid state rectifier diode. Story telling has been on hold, because it's hard to include illustrations. HV adjustment is made with a small variac on the primary side of an extra-small NST (4 kV 8 mA).

I'm eager to change over to the 1X2B vacuum rectifier, for two reasons:
1. A completely semiconductor-free power supply would be cool.
2. As measured on the bench, the 1X2B has less forward voltage drop than the HV silicon diode, at currents up to 1 mA.

As a first step toward battery-free filament power, I picked a tiny split-bobbin appliance control transformer like the one pictured here:

The gap between winding and core was wider on the primary side, enough to squeeze in six turns of some thin stranded twisted-pair wire. The resulting 12-turn test winding developed 0.6 volts. That's 0.05 volts per turn, about what I'd guessed from the core size.

To power the 1x2B filament, including allowance for IR drop in the winding, I think this wants about 28 turns. A small enough number that the wire can be fished through without taking apart the core laminations. To make room for easy winding and reasonable HV insulation, I think the existing secondary winding should be cut up and removed.

radiotech

Fri Feb 01 2019, 10:26PM

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

Consider a 1V2 rectifier 0.625 Volt 0.3 Amps.

My tests:

G. E. Co Cat No. 54G153 60 Cy. 110 VA 115 primary 5000 V secondary.

Wound 25 turns through core. Open Circuit Voltage 2.5

Loaded with resistor, 4 ohms to 1 ohm~~ still 2.5 volts.

Ran another test: Primary voltage lowered to 75 vols, measure 5000 volt
secondary voltage with 75 volts primary.

Adjusted load on new winding from 4 down to 1 ohm No change in secondary.

I have more than a few of these 5 KV sign transformers.

The reason is, I bought a whole lot of university student spectrum lamp
boxes.

Not all the transformers are the same. The one shown has ample space.

Another in the shop is made differently. I can see light between
the outer layer of winding insulation and the core. I am sure I can thread
an appropriate winding through it.

The 28 gauge wire wire has ample ampacity for a tube filament. Iy space is an
issue I might try Kynar wire. Smaller gauge bit i think the circular mil per amp would
work fine.

By accident I shorted new secondary while the power was on. The transformer
made an angry sound. I have a feeling when you rapidly shift the magnetic angle
it induces possible arcing in an old winding.

Sulaiman

Sat Feb 02 2019, 01:34AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

From memory, 1X2 rectifiers have a hard vacuum and produce x-rays.
could be a hazard
or a project.

Patrick

Sat Feb 02 2019, 01:56AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

emphasize the "project" part minimize the issue of "radiation hazard" here on the forum.

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