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Old July 30th, 2011, 05:14 PM   #1 (permalink)
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Blown OT?

While I'm waiting on a new tube set for my Ampeg Gemini I to arrive, I've had it open to take measurements and so on. I originally thought one power tube was dead, because compared to the other it was looking pretty dull with the amp on. Just looked weak with no glow. But when I switched sockets, the 'bad' tube suddenly looked nice and glowy, and the 'good' tube looked dead. Hmmm. Both sockets seem clean, and the grid resistors and cathode resistor are both good. One problem though-the plate voltage at the left socket is less than 1V! And this is the case with either tube installed at that socket. I pulled the tube at the right socket and tried to play, but no sound. I reinstalled the right and pulled the tube at the left socket, and I was getting the tones I had been hearing previously(kinda farty, buzzy, volume swells...). So there is something going on at that socket, specifically at the plate

I'm suspecting a bad output transformer, but maybe(and hopefully) there is something else goin on here. What can I check?

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Old July 30th, 2011, 06:50 PM   #2 (permalink)
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No voltage on the anode is not good, but that should not effect how much glow you see, I would also check heater voltage on the bad socket and make sure the contacts on the sockets are tight. Just to be clear, never stick anything other then a tube into a tube socket unless you know for sure all the caps are discharged
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Old July 30th, 2011, 08:06 PM   #3 (permalink)
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Quote:
Originally Posted by EETStudent View Post
While I'm waiting on a new tube set for my Ampeg Gemini I to arrive, I've had it open to take measurements and so on. I originally thought one power tube was dead, because compared to the other it was looking pretty dull with the amp on. Just looked weak with no glow. But when I switched sockets, the 'bad' tube suddenly looked nice and glowy, and the 'good' tube looked dead. Hmmm. Both sockets seem clean, and the grid resistors and cathode resistor are both good. One problem though-the plate voltage at the left socket is less than 1V! And this is the case with either tube installed at that socket. I pulled the tube at the right socket and tried to play, but no sound. I reinstalled the right and pulled the tube at the left socket, and I was getting the tones I had been hearing previously(kinda farty, buzzy, volume swells...). So there is something going on at that socket, specifically at the plate

I'm suspecting a bad output transformer, but maybe(and hopefully) there is something else goin on here. What can I check?
It looks like you might have taken to learning the hard way. If it is your OT the money spent on that replacement tube set could have gone into a new OT. On the bright side now you'll have an extra set of tubes on the way should you ever need them.

You can do what Celeste mentions and test the heater voltage but if you're not seeing any voltage on that plate then your OT is not likely delivering it. I would double check all leads, sockets, lugs, for solid connectivity and then pull the OT for testing.
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Old July 30th, 2011, 08:54 PM   #4 (permalink)
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There might be a little confusion. The tube/socket that I was thought was 'good', seeing as it was glowing nicely, is actually the socket that is showing no voltage on the plate. Whatever tube I stick in that socket glows brighter than the other, as seen in the picture, however it is the socket that is not performing The heater voltages are correct at both sockets.

What tests can I do to check the OT?
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Old July 30th, 2011, 09:28 PM   #5 (permalink)
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If it's the OT it's possible that it has developed an issue on the primaries between the center tap and the lead that goes to the suspect plate. You can test between these leads with an ohmmeter. If testing for and open circuit you will see an infinite reading on that plate lead whereas testing between that center tap and the other plate lead will yield a resistive value of some reading all be it low so set your ohmmeter accordingly. If you are getting zero ohms on that suspect winding you're looking at a short.
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Old July 31st, 2011, 03:53 PM   #6 (permalink)
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Did that test. Looks like its open. Im getting an infinite reading from the B+ lead to that primary, whereas the other reads under 500 ohms. Darn
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Old August 4th, 2011, 09:48 PM   #7 (permalink)
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Bummer but all is not lost a drop in is 69.00$ at fliptops and you'll get to know Bruce.
I also see he's stocking a 4/8 ohm version these days for 75.00 Hummm...http://www.fliptops.net/catalog/prod...roducts_id=251
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