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Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Okay folks. After a thorough search of the interwebs, and beating myself up over the apparent simplicity of the answer to this question, I've decided to consult 4hv:
I'm using a 3-500Z Class B Triode with +15V on the grid in order to drop a few kilovolts down to the point that the current can be regulated with a MOSFET. I would like to have an idea of on what order that "deamplification" is working (gain < 1).
What is the equation for the gain or a zero bias mode triode (Class B) in this configuration? (I apparently cannot properly interpret the diagrams on the 3-500z datasheet).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think you need to think more carefully about what the tube is actually doing. Is it acting as a cathode follower? A grounded grid amplifier? +15v relative to what?
Your link is broken, so I can't see any schematic you might have posted.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Weird about the link. If you just copy/paste it into the address bar, it certainly works. (I thought I tried it).
Anyway, this tube is not being used in any standard way, and I'm not sure how to describe it so that we can all understand.
The explanation given by one implementer, John Doran, can be found here:
"This is a “cascode†connection of a power field-effect transistor (the actual constant-current source) and a large, high-voltage vacuum triode. The triode drops the excess voltage (that is, any difference between +HV and the voltage across the laser) while passing the laser current on to the drain terminal of the FET. The operational amplifier ensures that the voltage across the resistor R is equal to VC. (John Doran)"
See attached .gif for the relevant schematic snippet.
Is there a technical name for which we're using the 3-500z?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Ok, that's a standard configuration, it's a cascode.
Best way to analyse it is look at the extremes. When the FET is on as hard as it can, there's 0v on the cathode, 15v on the grid. Look at your tube curves, will it pass the amount of current you need with +15 on the grid, and HV supply minus laser tube drop on the plate?
Now look at the other end. How big a negative grid voltage does the triode need to "cut off", that is to reduce its cathode current to a negligible amount, with the full supply voltage on the plate? Your FET voltage rating has to be that plus 15, or it will blow.
It's hard to analyse the circuit. The constant current drive to the cathode makes the plate impedance infinite, and so the voltage gain of the circuit is determined only by the load. You're using a discharge tube, a non-linear load, so you can't use the usual methods. But if you check it at the two extremes like I suggest, it should be ok at all points in between.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Given a classic 3-500Z Triode and IRF830 in a cascode configuration shown below:
With FET full-on and the grid at +15V, a plate-to-grid voltage of 5000V (HV:15000V, dropping 10000V) I estimate, via the 3-500Z datasheet, a max current of 150mA, well above my regulated 80mA maximum. (Incidentally, this implies that the minimum plate-to-grid voltage at 80mA is ~3500V).
With filament-to-grid voltage of 500V (max on IRF830), and plate-to-grid voltage of 20000, well, we're completely off the charts. However, the 1mA line is fairly linear and can be approximately expressed as y = 0.0075x, where x is the plate-to-grid voltage and y is the filament-to-grid voltage. So, if the plate-to-grid voltage was 20000V, the filament-to-grid voltage would be ~150V -- again, at 1mA and easily within our 500V FET limit. So, if we were to approach the operating limit of the IRF830 FET, I think we can assume that we would be in negligible current range.
Does this reasoning pass the insanity-check phase...?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yep, seems OK. If it were me though, I'd still put a 400V TVS across the MOSFET just in case.
And some sort of clamp on the 15V supply too, to save any other electronics connected to it if the tube decides to have a plate-to-grid arc. Which is quite possible if you're running it at plate voltages so far above its rating.
Earlier you said the "deamplification", and I was having a fun time trying to figure out what that meant. I guess it would be how much the cathode voltage changes for a given change in plate voltage, and as far as I can see that is just 1/mu.
(Mu is -dVp/dVg for a common cathode stage with a hypothetical plate load of infinite resistance. But in this circuit, with the grid held fixed and the cathode driven, dVg = -dVk.)
I still recommend my original method though, because you can't assume mu constant over the full range you'll be using.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Let's see: (1) The IRF830 has a built-in MOS diode clamp source-to-drain. Do you think a 400V TVS would improve the safety margin a lot over this? (2) The grid also has a neon lamp voltage spike suppressor for just that eventuality (plate-to-grid arcing). Sorry, that's not on the schematic snippet. Thanks especially for the insight into tube dynamics.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
sngecko wrote ...
Given a classic 3-500Z Triode and IRF830 in a cascode configuration shown below:
With FET full-on and the grid at +15V, a plate-to-grid voltage of 5000V (HV:15000V, dropping 10000V) I estimate, via the 3-500Z datasheet, a max current of 150mA, well above my regulated 80mA maximum. (Incidentally, this implies that the minimum plate-to-grid voltage at 80mA is ~3500V).
With filament-to-grid voltage of 500V (max on IRF830), and plate-to-grid voltage of 20000, well, we're completely off the charts. However, the 1mA line is fairly linear and can be approximately expressed as y = 0.0075x, where x is the plate-to-grid voltage and y is the filament-to-grid voltage. So, if the plate-to-grid voltage was 20000V, the filament-to-grid voltage would be ~150V -- again, at 1mA and easily within our 500V FET limit. So, if we were to approach the operating limit of the IRF830 FET, I think we can assume that we would be in negligible current range.
Does this reasoning pass the insanity-check phase...?
While reading through this thread, I see no mention of plate dissipation. As the number implies, the 3-500Z can handle a maximum plate dissipation of 500 watts. Therefore, if you attempt to operate it at its max plate current (150mA) with a voltage drop across the tube of 10000 Volts, you will greatly exceed the max plate dissipation.
Back in my Ham radio days, I ran a 4-400 in grounded-grid mode with an input power of 1KW. With the pi-network output tank properly tuned for a low vswr, the plate of the tube barely glowed red. But when I changed bands, and the vswr increased, the plate glowed orange very quickly while transmitting, which indicated that I was exceeding its max plate dissipation of 400 watts. So, I suggest that you keep an eye on the plate during operation of your laser. If the plate starts to light up like a lightbulb, then you'd better shut it down real quick!
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Zero Bias Mode Triode Gain Equation
I guess it would be how much the cathode voltage changes for a given change in plate voltage, and as far as I can see that is just 1/mu.
