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Mosfet for Bias control in ARC VT200
Mosfet for Bias control in ARC VT200
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Old 16th June 2019, 08:44 PM   #11
BluesTM is offline BluesTM  United States
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If all around the FET it's -150 then I'm guessing that's making the driver tube draw too much current and dragging its plate voltage down because now the driver cathode voltage is too low. Normally the driver cathode is -50 and the driver grid is -60. Now the cathode voltage is probably below the grid voltage and so it's loading down the supply.

If there is a problem with the coupling caps that would screw the whole thing up because you wouldn't be able to set the bias with the adjustment pots correctly. If the grid voltage at V7/8/9/10 is good then you're probably ok.
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Old 18th June 2019, 01:54 AM   #12
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Quote:
Originally Posted by BluesTM View Post
If all around the FET it's -150 then I'm guessing that's making the driver tube draw too much current and dragging its plate voltage down because now the driver cathode voltage is too low. Normally the driver cathode is -50 and the driver grid is -60. Now the cathode voltage is probably below the grid voltage and so it's loading down the supply.

If there is a problem with the coupling caps that would screw the whole thing up because you wouldn't be able to set the bias with the adjustment pots correctly. If the grid voltage at V7/8/9/10 is good then you're probably ok.
That's the kind of info I've been looking for.Parts should be in tomorrow.I will surely let you know how it goes.
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Old 3rd July 2019, 11:10 PM   #13
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I put in the DN2530 you recommended and the voltage now around the mosfet output is -100vdc.( not the -80 vdc I was looking for).Also at the plates of V10 and V9-the voltage should be +260vdc after the 6.19k resistor coming off B+320 vdc PS.(which has gotten hot and turned brown)I'm measuring anywhere from 160 to 200 vdc.I don't know why.I get the 165vdc on front side of .33mf cap and the -60vdc on the other side.Still too unsure on putting in 6550's to blow up again.All resistors check out as marked on schematic.I'll also check my pcb traces to make sure solder isn't laying across and traces.
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Old 4th July 2019, 06:30 AM   #14
BluesTM is offline BluesTM  United States
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When you use the bias adjustment does it change the voltage at the cathodes of V7/V9? The goal was to get that around -50 +/- (with room for adjustment) which means that you would now have control of the grid bias on the output tubes.

The FETs from ARC are probably picked for specific Id and Vgs which is what the color coding is probably all about so the match may not be perfect. Swap V7 and V9 (or 10 or whatever) and see if it makes a difference. The low plate voltage is a little disconcerting but maybe the bias adjustment or tube roll will make a difference.

On the driver tubes the grid voltage should be lower than the cathode voltage. The exact value depends on the tube (the schematic shows 10v). If the driver draws too much current it lowers the driver plate voltage and raises the driver cathode voltage.

Good luck. Don't put the output tubes back in until you're confident you can control the grid voltage on each of the output tubes (pin 5).
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Old 4th July 2019, 02:28 PM   #15
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Is there any way to closely match the mosfets with a voltmeter using ebc(emitter-base-collector) metering setting?
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Old 4th July 2019, 08:13 PM   #16
BluesTM is offline BluesTM  United States
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I used this thing to characterize the devices-

Peak Atlas DCA Pro model DCA75 | Peak Electronic Design Limited

Once I had the device parameters I started hunting down candidates. I'm sure you could do it with a power supply, a couple of resistors and a volt meter but you have to know what you're starting with. With this little guy you don't even have to know what the leads are, it tells you.

So you pull an original one, test it with the DCA 75 and then find a match. It's not the same as matching it like the factory does but it's a start.
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Old 4th July 2019, 09:15 PM   #17
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Just got it ordered.Thanks for the heads up.
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Old 9th July 2019, 06:00 AM   #18
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Default Mosfet Peak Measurements

I got the mosfets measured.Now which parameters do I need to be looking for?
Some of the data at Mouser doesn't match too well.
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File Type: pdf green gray white diode.pdf (163.0 KB, 10 views)
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Old 9th July 2019, 06:09 AM   #19
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Default Mosfet Peak Measurements

I got the mosfets measured.Now which parameters do I need to be looking for?
Some of the data at Mouser doesn't match too well.I checked the diodes I got from Mouser you recommended and they were shorted out.New measure fine-of course.
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Old 10th July 2019, 03:58 AM   #20
BluesTM is offline BluesTM  United States
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I looked at Vgs on and Vgs off. If the 2530 is too low the 2535 was higher (well more negative). They spec them by range. I bought more than I needed and picked the best match. The Id values given by the tester are fixed parameters for the test, like 5ma. For parallel devices Rds might be useful.
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