Need substitute for old Siliconix FET to replace Thyratron

Maximum Source Gate leakage current

Maximum Source Drain voltage.

Minimum/Maximum Source Drain current.

Source Drain leakage current when off

Source Drain is likely negative in most case, since they are talking about electrons. Or look at Drain Source.

Although it's mounted in a stud package and has that weird gate voltage rating, it's only rated at 20Vds

Big one is 500-900mA Idss .

U244 similar at 25Vds.

Old soviet surplus part KP903 (2P903) (~ ebay or amazon) might fit.

I picked up two last year when they were offered by multiple vendors @ US$5, if that's any use to you.

RL

Vp is the pinchoff voltage; how negative the gate voltage has to be for drain-source to be non-conducting - which could be Id(off) or some other specified "off" current.

Gfs is forward transconductance; how drain-source current changes per change in gate voltage.

Idss is the saturation drain-source current at zero gate volts.

High voltage JFETs are now very hard to get but you should be easily able to use a modern depletion mosfet like LND150/250

piglet

That schematic you linked depicting the FET+SCR replacement for a thyratron tube shows the fet drain supply coming from rectified heater filament 6.3V supply. That is neat as it prevents triggering without the heater supply and C31 has charged up.

I think they are assuming that 6.3V has either one side or its center-tap (if present) near cathode 0V ground which means the drain voltage is going to be well below 20V. Which makes the JFET requirements look very undemanding, I can't see why they specified the U266 - I reckon you can probably use just about any garden variety n-channel JFET (2N5458, J113, 2N3819 etc etc)

I haven't figured out how shield grid 2 influences the triggering, do you have a schematic showing how the 2D21 is used in the jukebox?

piglet

From the solid state schematic, it looks like you could protect the substitute fet's gate with a suitable diode/zener combination.

RL

There may be something seriously wrong with your solidc state sub pinout.

2D21 pins 7 and 5 are normally connected together, internally. 2D21 pin 3 is a heater terminal, with no cathose or anode contact. Your solid state drawing seems to anticipate an anode function. 2D21 pins 2 and 7 will normally never see useful low voltage juice for rectification. I suspect that the sub wants a heater supply on these terminals. 2D21 isolated heater is on pins 3 and 4.

Recheck it. maybe you're numbering from the top view rather than the bottom.

RL

2D21 doesn't have a pin #8 - It's a 7 pin tube.

RL

Pin numbers are for 2050

RL

Actually the schematic drawing is from an old application note (I assume Siliconix made it) for replacing a 2050 with an SCR and JFET - as the original title said.

I'm figuring (hoping) that the 2D21 is close enough in response to the

2050 that the circuit can be used with just putting the 2050 pins to where the 2D21 is.

I have updated the schematic by adding the 2D21 pins (in brackets) - I was fully aware of this, but perhaps this wasn't clearly presented - my apologies!

John :-#)#

G'day!

Here is a link to the section of the manual covering that:

The schematics for the 2D21s as used is on page 5128.

I drew a nasty sketch layout out the external and internal connections for the Read-Out and Write-In 2D21s. The 'scope' sketch is showing the Detent Sw. pulse at the junction of R14 and R12 AIR.

Thanks for your efforts!

John :-#)#

<snip>

Must be a misprint in Towers Intl Fet Selector. - Vdsmax=20V U244, with the same package listed as sub with Vds<=25V.

Ok, good to know that grid 2 is nothing to emulate. I can't figure out from the jukebox schematics how the thyratrons are commutated off after firing. Are they unlatched by disconnect switches in series or are they fed from unsmoothed HV rails so get zero voltage at 120Hz rate?

If a low parts count solid state replacement is wanted then I think a high voltage depletion mosfet makes the job very simple. Otherwise if using the heater supply to delay operation a few hundred milliseconds during power up is wanted then any jelly bean jfet could be used. I put series caps in both heater supply lines in the hope it solves floating heater supply issues.

My sketches here:

piglet

Using the isolated heater supply for local DC biasing in a circuit where the heater supply actually serves multiple tubes could be a right mess, with a resulting direct connection between sections.

RL

Completely agree! I think the old schematic the OP posted used the heater supply to avoid using (at the time) non-existant HV fets. Now we have 500-600V fets it becomes easy.

piglet

For anyone curious that old 2050 solid state emulator schematic came straight out of patent US3770989

piglet

Datasheets specify BVDSS and BVGSS but these are "forward" breakdown voltages - ie the absolute maximum Vds before avalanche breakdown of the drain body pn junction, and the maximum gate drive voltage before the electric field breaks down the gate oxide.

BVDSS - Drain to source voltage breakdown BVGSS - Gate to source voltage breakdown

Datasheets specify BVDSS and BVGSS but these are "forward" breakdown voltages - ie the absolute maximum Vds before avalanche breakdown of the drain body pn junction, and the maximum gate drive voltage before the electric field breaks down the gate oxide.

BVDSS - Drain to source voltage breakdown - third terminal (G) short to source BVGSS - Gate to source voltage breakdown - third terminal (D) short to source.

You can't Specify BVDSS of a Jfet, because a shorted gate results in a conducting device (an impractical test with too much device stress muddying up the water) - but you can specify BVDS becausae it assumes non-conductive biasing of the control terminal.

RL

I think 7 parts (3 res, diode, zener, mosfet, scr) is quite a big simplification over 15 parts (3 res, 1 cap, 7 diodes, 2 zener, jfet, scr) of the Davis patent circuit.

But if you want real simple get rid of the scr series diode, all three resistors, and the zener and it becomes just a mosfet and scr - relies on idss to limit scr gate current and inbuilt mosfet gate zener. I bet such an ultra simple circuit will work just fine.

piglet

In the TSR3-L6, using 2050 types, things are fairly simple with V2 and V6 6V3AC heater lines being grounded on one end and their cathodes being set also at ground potential. Grid bias is developed independently at ~ -28V w/r to ground.

The TSR1-L6 using 2D21s isn't so straight forward. The 6V1AC heater line for V2 and V6 is floating and is also used off-schematic (J7). Cathodes remain grounded and grids are still biased to ~ -28V. However; The V7 and V3 parts are cathode biased to a low-power (zener-regulated ) -120V and their grids are biased separately to a similarly low- power -150V zener. Their 6V3AC heaters are also isolated and exported off-circuit (J17). If the circuits will function with these 'cathodes' shorted together, then the rectified heater source for the subtitute circuit could work without much interference.

Perhaps this is a capacitive discharge arrangement, to energize the solenoids? Could provide quenching, but only on low rep-rates.

RL