hitman47
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- Sep 27, 2009
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Hey everyone,
I've recently completed my first Mosfet-Max, and as I turned it on for the first time (all trim pots except RB12 set to max. resistance, RB12 set to min. resistance on both sides, I hope that is correct?), I noticed the voltage the power supply produced started out at 15V, slowly climbed up to 20V, stayed there for a few seconds and then went back down. I also noticed that the left QB9 was turning very hot and so, after turning it off and on again (everything repeated itself as I wrote above, I tried a couple of times) measured the Mosfet bias. It was waaaaay too high at >2000mV and I quickly turned it off again. The right QB9 doesn't have any of those problems. I haven't dared to turn it on again since then, does anyone have an idea what the problem could be? Should I get some new FETs for that position?
EDIT: OK, so I figured that one out myself. The trim pots were turned in the wrong direction, I'm still kinda confused about which direction increases and which decreases the resistance on which trim pot, but that's not much of an issue. However, I think I might have broken something (I was trying to measure the bias voltage on the right tube with the DMM still in high-current-measuring mode and accidentally shorted the measurement point with ground, incredibly dumb mistake I know), because it's become impossible to bias the right tube at a sensible voltage; It's constantly at 20+V and the right DB bias stays at 0, no matter what I do. Also, the right QB9 stays completely cold, whereas the left one warms up a bit. Might it be broken? I'll try to replace it tomorrow (still have some left for a second build I'm planning, and I'll have to order a ton of stuff for that anyway as I broke some other stuff and didn't order enough of yet some other things.), but if anyone could confirm that suspicion or maybe hint at something else that's likely the problem, I'd be very grateful.
EDIT: Forgot to say this, the left side can be biased without any problems, I have the tube running at a perfect 13.5 and the DB at 250mV. It's just the right side that's making trouble.
Edit: I replaced QB9R, everything's still the same. I'd really appreciate any help as I'm at a loss right now.
I've recently completed my first Mosfet-Max, and as I turned it on for the first time (all trim pots except RB12 set to max. resistance, RB12 set to min. resistance on both sides, I hope that is correct?), I noticed the voltage the power supply produced started out at 15V, slowly climbed up to 20V, stayed there for a few seconds and then went back down. I also noticed that the left QB9 was turning very hot and so, after turning it off and on again (everything repeated itself as I wrote above, I tried a couple of times) measured the Mosfet bias. It was waaaaay too high at >2000mV and I quickly turned it off again. The right QB9 doesn't have any of those problems. I haven't dared to turn it on again since then, does anyone have an idea what the problem could be? Should I get some new FETs for that position?
EDIT: OK, so I figured that one out myself. The trim pots were turned in the wrong direction, I'm still kinda confused about which direction increases and which decreases the resistance on which trim pot, but that's not much of an issue. However, I think I might have broken something (I was trying to measure the bias voltage on the right tube with the DMM still in high-current-measuring mode and accidentally shorted the measurement point with ground, incredibly dumb mistake I know), because it's become impossible to bias the right tube at a sensible voltage; It's constantly at 20+V and the right DB bias stays at 0, no matter what I do. Also, the right QB9 stays completely cold, whereas the left one warms up a bit. Might it be broken? I'll try to replace it tomorrow (still have some left for a second build I'm planning, and I'll have to order a ton of stuff for that anyway as I broke some other stuff and didn't order enough of yet some other things.), but if anyone could confirm that suspicion or maybe hint at something else that's likely the problem, I'd be very grateful.
EDIT: Forgot to say this, the left side can be biased without any problems, I have the tube running at a perfect 13.5 and the DB at 250mV. It's just the right side that's making trouble.
Edit: I replaced QB9R, everything's still the same. I'd really appreciate any help as I'm at a loss right now.