600V bench variable supply to test transmitting valves

[jackinnj]

This is a Jim Williams design:



The new morgan jones shows how to implement depletion mosfets for protection -- so does the SuperTex website. (SuperTex also has an ap note for using the smaller depletion mosfets to protect instrumentation from inadvertent disasters.)

[Wavebourn]

You can use here Gu-50 in "Right handed" mode, connecting all 3 grids together:

http://wavebourn.com/forum/download.php?id=491&f=7

[Triodethom]

Originally Posted by tubelab.com

I believed this too......until I stumbled across these mosfets on the clearance page at Allied Electronics a couple of years ago. With a BIG (250 watt) 15 ohm resistor in the drain lead they will not blow into a direct short with 600 volts of supply. Of course they will be over dissipated in short order so a foldback circuit or a fuse is needed. For those who don't want to look through the data sheet, the important numbers are 600 volts, 600 WATTS, 43 AMPS of continuous drain current and 172 amps pulsed. They were clearance priced at $5 each when I got some. I don't know if they are still available. I haven't blown one yet.

I made a simple power supply using a large variac feeding a reverse wired 480VAC (2 X 240) to 120 VAC industrial control transformer rated at 1 KVA. There was a solid state FWB on each 240 VAC secondary making two 0-310 VDC variable supplies that could be wired in series or parallel. A smaller 120 VAC to 24 VAC industrial transformer with a small variac provided variable heater power. I used this for years, but there was no current limiting and several experiments ended in smoking or exploded parts.

I tried several post regulators that usually blew up to protect my experiment. That includes sweep tube pass devices. A huge current spike created by a momentary short takes out the metal link inside the tube that connects the cathode to the base pin. The Fuji fet survivied. A dead short across the output at maximum voltage would trip the bench breaker before blowing the fet (about 2 seconds). 60 amp peak currents will stil fry parts. That power supply was large and heavy, nearly 100 pounds. It has been dismantled.

I got a HP 0 to 600 volt 0 to 1.5 amp supply for big amp experiments. I am using that today, but it's current limiter is too slow to save parts. In fact this power supply has created some serious parts explosions.

Allied has them at $ 9.32 still. the 2SK3527-01 with 21 amps is only about $2.35 no stock but the 2SK3528-01R is in stock with 160 watt dist. at $3.60 each .

[tubelab.com]

I thought they eliminated all the old Fuji mosfets since they were all on their "excess inventory" list.

It seems that another of my old favorites is still stocked. The 2SK3675-01. It's not good for a big power supply, but makes a good source follower. Pair it with a triode tube in a darlington like configuration for the mother of all vacuum tubes. You get a triode with 195 watt and 7 amp capability. The key spec here is Crss = 8 pF.

The 2SK3524-01 is a smaller version, but no longer stocked. Maybe I bought them all. Good for direct coupled grid drive for A2 or AB2 operation (PowerDrive).

Note: None of the Fuji mosfets have integral zener gate protection. Wire a small (the smaller the better for low capacitance) 12 volt zener from gate to source to avoid blowing the gate in transient situations.....like a guitar amp cranked to 11.

[tomchr]

Originally Posted by Wavebourn

You can use here Gu-50 in "Right handed" mode, connecting all 3 grids together:

http://wavebourn.com/forum/download.php?id=491&f=7

Actually, the title of the plot says that the screen and control grids (G1, G2) are wired together with the suppressor grid (G3) grounded.

~Tom

[Wavebourn]

Originally Posted by tomchr

Actually, the title of the plot says that the screen and control grids (G1, G2) are wired together with the suppressor grid (G3) grounded.

Actually, in triode mode it does not matter where to put the suppressor grid. Try and compare. You can get some difference if add positive bias on it.

[mogliaa]

Originally Posted by Wavebourn

Actually, in triode mode it does not matter where to put the suppressor grid. Try and compare. You can get some difference if add positive bias on it.

Hi Anatolyi,
Thanks for the suggestion, didn't think about the right handed option. It will be better to obtain more voltage span with my current design constraints I think.
Here is the circuit and looking at the loadline based on your curves, it looks like I won't need more than 15V of positive grid bias to sustain 70mA load at low output voltage.
So if replacing protecting diode by a 15-18V zener should ensure positive grid voltage is limited, but wouldn't this zener blow up if output is accidentally shorted?

[Wavebourn]

Sure, when shorted current will go also through 10K resistor and Zener, if voltage control pot is on maximum. 600V/10K = 60 mA. 36W on 10K resistor, 1 W on Zener. Total current will be the sum of this current and current through the tube. Current through the Zener will be less: part of it will go through grid-cathode. So you should worry now about 10k resistor and the pot.

[mogliaa]

So is worth using the 10k resistor as a protecting mechanism or is there a way of improving this any further?

[Wavebourn]

Originally Posted by mogliaa

So is worth using the 10k resistor as a protecting mechanism or is there a way of improving this any further?

I would add one more current limiting resistor, from pot to grid/Zener. Probably one more cathode follower (or source follower?) may be needed using smaller tube or small high voltage MOSFET if voltage drop on this resistor will be too high in normal operations' mode due to grid current.