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Old 29th May 2017, 02:05 PM   #1
spendorite is offline spendorite  Canada
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Default Trying to use Pete Millet's screen voltage regulator in the SSE

I've only now got around to trying this but I'm not getting the expected screen
voltage, far from it actually.

Last January in this thread I aked a question about lowering the screen
voltage in my low voltage SSE. George suggested using Pete Millet's screen
voltage regulator from his Engineer's Amplifier.

It's a simple circuit based on a mosfet, some resistors and zener diodes.
I used the same mosfet as specified in Pete's circuit FQPF8N60C.
I got all the parts from Mouser.

Assembled it on a piece of perf board and wired it to the SSE as per George's
suggestion.
Powered up with a 6087 rectifier,12AT7, 6Y6G power tubes and 680 ohm cathode resistor.

Checking the screen voltage all I'm getting is 78mV, yes that's millivolts,
B+ and Vp are 296V, just about where they should be, normally Vp is several volts
below B+

I am able to play music but with quite obvious distortion.

I measured the voltage going to the input resistor in the circuit (R21 4.7k) and
it is 296V but at the other end it is only 620 millivolts, I'm assuming this is due
to the effect of the rest of the circuit but I wonder if the problem is actually the
resistor.
This resistor gets scorching hot after a few minutes, I can still feel the heat from
it with my hand more than six inches away !!
At the output end of the circuit (R42 330k) going to screen, voltage is just
78 millivolts.

So I'm thinking maybe the mosfet is bad or damaged, I was careful not to overheat
when soldering. I also reflowed all the solder joints to ensure I did'nt have a cold
joint.
I believe I have the circuit wired correctly. I've attached some photos of the assembly
and a copy of the schematic, maybe George or others can offer comments and suggestions.
Attached Images
File Type: jpg P-Millet SVR schematic.JPG (53.6 KB, 344 views)
File Type: jpg P-Millet SVR assembly.JPG (87.9 KB, 345 views)
File Type: jpg P-Millet SVR wiring.JPG (110.5 KB, 342 views)
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Old 29th May 2017, 03:00 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Sounds to me like you have a direct short between R21 (colder end) and ground. If the FET was short I would expect a little higher than 150V.
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Old 29th May 2017, 05:07 PM   #3
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
Sounds to me like you have a direct short between R21 (colder end) and ground. If the FET was short I would expect a little higher than 150V.
The nearest ground for R21 is the ground point for C22. While I can't test continuity between those two points
I'm getting resistance of about 69.6k ohm and I am getting continuity from R33 to ground of C22.
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Old 29th May 2017, 05:57 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The Drain of your FET must be touching a ground somewhere. What have you got between Drain and -ve C22/21 resistance wise? It should be pretty much Open Circuit.
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Old 29th May 2017, 05:59 PM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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There should be no continuity between R33 and ground.
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Old 29th May 2017, 06:56 PM   #6
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
The Drain of your FET must be touching a ground somewhere. What have you got between Drain and -ve C22/21 resistance wise? It should be pretty much Open Circuit.
Yes, you're correct, there is a short between drain and ground but no wires are in contact. However, here's what I've found, R42 has no resistance, I'm measuring only 1.6 ohm whereas it should be 330k, also, I'm getting continuity across that resistor. I always check resistors with my multimeter before installing them and it was okay then. Don't know what happened there.
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Old 29th May 2017, 07:20 PM   #7
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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At least you have found the problem. Good.
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Old 29th May 2017, 08:21 PM   #8
kevinkr is offline kevinkr  United States
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Trying to use Pete Millet's screen voltage regulator in the SSE
I find with that sort of perf board that shorts are almost inevitable. I recommend carefully removing a few of the unused pads around various HV circuit points using an x-acto knife. With care you can make this work.

I purchase perf board from vector that does not have any copper on it for these sorts of uses.
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Old 30th May 2017, 03:22 AM   #9
w5jag is offline w5jag  United States
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I also find that type board problematic to work with. I use it for stuff that requires high mechanical rigidity - like RF oscillators - and that's it. Rework - which I always have to do a lot of - is maddening. Regular perf is much easier to build with.

If it makes you feel any better, I just got through letting the smoke out of a transformer that I built around, physically and electrically. Part hasn't been in production for decades, so I'm .....

Win W5JAG
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Old 30th May 2017, 08:39 PM   #10
spendorite is offline spendorite  Canada
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Disappointed to report that the resistor R42/330k is not the cause of the short. When I checked it in circuit I was only getting about 1.6 ohm but
when removed it tested at 327k so it's okay.
It looks like the culprits are those zeners. I isolated them from the rest of the circuit and got continuity between D5 and D8. Also did a diode test on each
of them with my multimeter diode checker and got 0v in both directions. Not sure how or why this happened.

I checked for solder bridges and found none I also checked for shorts between the solder pads close to the HV rail and those close to ground, none found.

Didn't expect to have these problems with such a simple circuit.

I think later I'll assemble a new board, I have spares and I can salvage the caps and resistors from this board.
I'll get perf board without solder pads. How do you guys install mosfets on perf board without solder pads ?
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