Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on June 02, 2011, 03:46:32 pm
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I have one of these awful amps on the bench.It runs for about a minute and then suddenly the bias runs wild and one pair of tubes redplates.I've replaced the bias supply components and the driveline coupling caps and tubes.
This one is baffling me.
Any other reason for this to act this way?Any insight would be appreciated! :BangHead:
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Is it a PCB? If so maybe even in that short time a trace is heating up and goes open?
Brad :icon_biggrin:
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anything is possible is this awful PCB nightmare amp!I'll check for that.
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I have one of these awful amps on the bench.It runs for about a minute and then suddenly the bias runs wild and one pair of tubes redplates.I've replaced the bias supply components and the driveline coupling caps and tubes.
This one is baffling me.
Any other reason for this to act this way?Any insight would be appreciated! :BangHead:
I love those Amps!
does it do it on stand-by too?
check out the screen resistors, I had a weird problem with a X100B and it ended up being a screen resistor (can't remember what happened to the bias, but the tube would red plate).
monitor the PT AC bias supply winding when it hits that failure point at 60sec, see if its a problem with the PT. If you monitor the AC on the transformer side of the diode, if the problem is on the DC side of the diode, it shouldn't affect the transformer side.
If one of the two coupling caps from the PI is leaking, it'll might cause the problem as well.
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I replaced the screen resistors with 1k/5 watt ones so that's not it.I replaced the coupling caps on the drivelines.I replace all the bias supply components including the diode.still does it.
I'll monitor the voltage at pin 5 and see if it's the bias supply or some kind of oscillation.
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I replaced the screen resistors with 1k/5 watt ones so that's not it.I replaced the coupling caps on the drivelines.I replace all the bias supply components including the diode.still does it.
I'll monitor the voltage at pin 5 and see if it's the bias supply or some kind of oscillation.
I'm talking about monitoring the side of the diode that goes to the power transformer, the bias supply tap on the transformer (since you replaced all the caps, resistors, and the diode in the bias supply circuit.,, check the transformer to see if it runs wild...)
maybe ellaborate on 'runs wild'.. goes up and down, or goes down.? or up?
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It runs for about a minute and then suddenly the bias runs wild and one pair of tubes redplates.
When you say "bias runs wild" do you mean the negative voltage applied to the output tube grid is changing wildly?
Or do you mean the current flowing thru the tube is changing wildly and you haven't measured the actual negative grid voltage at the grid pin?
Pull the output tubes and make sure the negative bias voltage at the grid pin is stable before putting the tubes back in.
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It seems like the bias voltage with no tubes stays steady at -46v but the current runs wild when put back in.Just on one side.
Strange.
Haven't measured the OT secondary yet.Hope the OT isn't the issue.This one is going to eat up lots of valuable time.
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Found the problem! It was one of the tubes! a matched set from The tube store and one was drawing too much current.It testes good and I didn't think to try another set as these were right out of the box.
It stays rock steady now and sounds half decent for a crappy,crappy looking and poorly built amp.
Never 'assume' that new tubes are good.that's the lesson for today. :cussing:
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I had the same issue with a tube drawing to much current. They test good on a emissions tester but the one thats drawing too much will have a higher plate voltage. Thats what i found anyway. A real pain if your running low on tubes too