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Driving triode from signal generator

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LAN_403

darkspeed

Sat Jul 03 2010, 10:41PM

Registered Member #2958 Joined: Sat Jul 03 2010, 02:36AM
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Posts: 8

I have a need to drive a GU5B triode from an hp signal generator.

Can I use something like a mosfet driver to isolate the sig gen? I can get 10v out of the sig gen.

Anyone know of a good way to do this?

Thanks

Myke

Sat Jul 03 2010, 10:55PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The grid that you would be driving would pretty much be isolated by the vacuum tube anyways. Just make sure you bias the grid negatively some amount so that you can get the tube to turn off fully instead of just turning into a diode (almost).

radiotech

Sun Jul 04 2010, 08:04AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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You didnt say what frequency. Is the isolation needed to prevent the test circuit voltages from damaging the hp generator?

For RF use a 1:1 transformer with a ferrite core. For audio use a repeating coil (telephone jargon for a 1:1 isolating iron core transformer) they do come in other ratios too.

Proud Mary

Sun Jul 04 2010, 09:25AM

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

I see from the data sheet that GU5B will draw from 450mA to 750mA anode current with zero volts on the control grid, and is intended to operate with a control grid range across -100V to +400V, so it won't be very impressed by your 10V signal, and is likely to feel under-used.

Erlend^SE

Mon Jul 05 2010, 09:34PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Maybe something like the electron-gun driver from a monitor will do?

60-80 V p-p, possibly integrated IC, most likely takes below 5V p-p input and good for quite a bit of bandwidth.

darkspeed

Tue Jul 06 2010, 04:15AM

Registered Member #2958 Joined: Sat Jul 03 2010, 02:36AM
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Posts: 8

So I need to use something like a mosfet driver > 2 mosfet > one for -100v and the other for up to 400v+ to drive it from 5v logic from the sig gen.

HV Enthusiast

Tue Jul 06 2010, 12:48PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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I'm not familiar with the tube, but i doubt your signal generator would be able to drive it.

Grid drive can require LARGE amounts of current to drive, especially at higher frequencies and voltages.

As an example, I have a small tube i'm driving with a half-bridge (Grid ON = 200V, Grid OFF = -250V) that requires about 50W to drive at 100kHz.

Modeling your grid in PSPICE or similar program is very simple.
Basically, the datasheet will provide (3) values - Grid Capacitance, Grid ON Current, and Grid OFF Current.

Model the grid by have a capacitor in parallel with two opposing diode/resistor strings. One diode/resistor string is your grid current in the ON state (usually mA), and the other opposing diode/resistor string is your grid current in the OFF state (usually uA)

Electroholic

Tue Jul 06 2010, 07:49PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

For lower frequency experiments, maybe some high voltage high power opamps would do?

radiotech

Wed Jul 07 2010, 12:29AM

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

For the high frequency models what would the Miller effect do the valves input admittance?

Proud Mary

Wed Jul 07 2010, 08:26AM

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

EastVoltResearch wrote ...

Modeling your grid in PSPICE or similar program is very simple.
Basically, the datasheet will provide (3) values - Grid Capacitance, Grid ON Current, and Grid OFF Current.

This sounds very odd, and not something I recall seeing in the data blurb of this or any other valve except the gas discharge devices, like thyratrons, which consciously model themselves on the idea of being an on-off switch.

A triode valve is very much a dy/dx beastie, and it's most vital statistics are its transconductance, which we measure in mA change in anode current for voltage change in grid voltage, which we write gm = mA/V, and its amplification factor, mu, which is the ratio of the change in grid voltage to the corresponding change in anode voltage

For GU-5B, gm = 12-18 mA/V when the anode current is between 0,4A and 0,7A as stipulated in the (abbreviated) datasheet here:

What this means is that if you set your anode voltage so that the anode current is 0,4A, then a change of one volt on the control grid will cause a 12mA swing in anode current, and at the other end of the safe operating area where the anode current is 0,7A, then a one volt change in grid voltage will cause an 18mA change in anode current.

The reason no absolute value can be assigned to cut-off, is because the cut-off value of grid bias depends upon the anode voltage.

The operating parameters of thermionic valves are presented as a series of non-linear curves in their datasheets, since all the values are variables dependent for their own value on the values of others - excepting, that is, the heater voltage and current in indirectly heated valves.

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