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I finished my project amp 25w PP Fenderish - running 2 6l6gc's - sort of a mongrel. I still have a lot of digging to do but I thought I would throw it out there... while checking the current on the output tubes (amp works sounds great, all voltages check out) I accidentally hit a wrong pin with dmm. It arced and then the problems started. I had one end of the dmm to B+1/OT and intended to measure current to the other side of the OT at the tube pin 3 plate. I hit either the screen pin 4 or the grid pin 5. Afterwards, it still has sound but I noticed the other tube that I did not touch red-plating. So I swap tubes to rule that out... doesn't matter what tube is in there. I remove one tube(the one that is not red plating) and it still produces sound. Remove the tube that is red plating - no sound. I switched the plate leads at the tubes (blue and brown)from the OT and get a horrible shrill squealing, sqwaking and immediately turn it off. Did I fry the OT? I'll be taking voltages tomorrow...
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Put the plate wires back where they belong.
Pull the power tubes. See if your meter still works, you may well have blown an internal fuse. Set for current, your meter is basically a short circuit, so you touched B+ to something.
With tubes gone, measure voltages around each power tube socket. DO you get B+ at pins 3 AND 4 of both sockets? DO you get whatever your bias voltage is at both socket pin 5? I assume something like 40-50v negative.
MAke sure the cathodes are grounded, pin 8.Education is what you're left with after you have forgotten what you have learned. -
This got me thinking... I have test points for bias current pin 8 to test point then 1r 1W resistor to the other test point/ground. I checked the resistor and bingo - fried. Very hard to see because it is a flame-proof and it was on the underside, but a tiny little black spot. Installed a new one and all is good.MAke sure the cathodes are grounded, pin 8
Thanks for steering my brain in the right direction Enzo...Comment
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wrap some electrical tape around those DMM test leads leaving just the very point exposed. That will keep you from doing that again.Warning! Some Electronics devices contain lethal voltages that can kill you. If you do not feel qualified to work with dangerous voltages, refer your repairs to a qualified technician. By giving you online advice, I am assuming no liability for any injury or damages you might incur through your own actions.Comment
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I'm going to get those probes that have a sleeve that retracts just a bit and then clamp on... I actually placed the probe on the wrong pin by mistake... I'm careful with this stuff but was rushing - always when you make a mistake!Comment
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My shop teacher, many decades ago, use to tell us, "You never have time to do it right, but you always have time to do it twice."
Turn the power off, and use clip wires to connect to the circuit. Then power back up.Education is what you're left with after you have forgotten what you have learned.Comment
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Hi Guys
The transformer shunt method of current measurement is hazardous in many ways, most of which have been outlined here and on other forums for a long time.
One hazard is that in some builds, the amp can become unstable when the ammeter is clipped in. The feedback loop sees the lack of hum balance in the output stage as an error and ultimately the other tube oscillates trying to correct the condition and make the feedback loop happy.
An easy way to keep the amp stable while performing the dangerous nonrecommended bias method is to short the feedback signal to ground with a clip lead. In most amps, the Schmitt splitter sits on top of the feedback loop, so ground the splitter-end of the feedback resistor will eliminate the feedback signal.
Have fun
Kevin O'ConnorComment
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