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 Post subject: National NCX-3 Final Amplifier
PostPosted: Feb Tue 27, 2018 10:11 pm 
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Joined: Mar Thu 19, 2015 11:01 pm
Posts: 30
I replaced the 6GJ5 PA tubes in my NCX-3 with General Electric 6GJ5A's. When I fired it up, it put out plenty of power but the meter didn't move. The meter worked fine on its receive functions but would not indicate cathode current. The voltage that drives the meter when it's measuring cathode current is derived across R5, 3.2 ohms. When I measured the resistance across R5 to ground I got 0.3 ohms. R5 is connected to the cathode of V2, pin 3. When I removed all of the components connected to pin 3, I still got 0.3 ohms from pin 3 to ground. When I pulled the tube, I got the expected open circuit from pin 3 to ground. Taking the ohmmeter to the bare tube, I discovered that pin 3 of the tube seems to be internally connected to pin 7. This makes no sense, because pin 7 is supposed to be internally connected to pin 1 and both are internally connected to the screen grid. I am not sure what is causing the low resistance to ground at pin 3, but I am sure it is on the pin 7 end of the internal connection because I have removed everything external from pin 3.

The last time I worked on the NCX-3, I have some memory of finding out that some versions of the 6GJ5 have internal connections that others do not. But I can't find my notes on that. Can anyone out there help me understand what is going on?

Thanks and 73
Charles Bostian W4KEP bostian@vt.edu


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 Post subject: Re: National NCX-3 Final Amplifier
PostPosted: Feb Tue 27, 2018 10:50 pm 
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Joined: Aug Fri 01, 2014 11:18 pm
Posts: 30
Hi,

Since you are getting power out, I don't think that you could have a short (internal or otherwise) between pins 3 and 7 (Cathode and Screen Grid). The metering circuit goes through the transmit/receive relay and if the only problem you see is in the metering, I would check the relay contacts.

Bob, AB1MN


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 Post subject: Re: National NCX-3 Final Amplifier
PostPosted: Feb Tue 27, 2018 10:56 pm 
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Joined: Jan Fri 06, 2012 8:47 pm
Posts: 6402
W4KEP wrote:
When I removed all of the components connected to pin 3, I still got 0.3 ohms from pin 3 to ground. When I pulled the tube, I got the expected open circuit from pin 3 to ground. Taking the ohmmeter to the bare tube, I discovered that pin 3 of the tube seems to be internally connected to pin 7. This makes no sense, because pin 7 is supposed to be internally connected to pin 1 and both are internally connected to the screen grid.
Well something certainly does not make sense. If pin 3 was shorted to 7 that still could not cause 0.3 ohms to ground unless pin 1 or 7 was grounded. They could not have been grounded or you would not be generating plenty of output power with a grounded screen on one output tube which theoretically would kill your screen supply for both of them.

I would suggest re-inserting one output tube at a time and measuring the resistance of each pin to ground while it is installed. Write these down. Do the same with the other output tube. Then figure out what is really grounded that is not supposed to be.

If you are still measuring 3, 1 and 7 at the tube as being connected together something is seriously wrong. Check what you are doing with one of the tubes you pulled out. Or better yet re-install the old tubes and see what happens.

To the best of my knowledge there are no versions of the 6GJ5 with "extra" internal connections.

Curtis Eickerman

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http://curtiseickerman.weebly.com


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 Post subject: Re: National NCX-3 Final Amplifier
PostPosted: Feb Wed 28, 2018 4:09 am 
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Joined: Mar Thu 19, 2015 11:01 pm
Posts: 30
Good points. Thanks. I'll check with other tubes. With the tube I checked, I certainly am getting a DC short between tubes 3 and 7 on the bare tube.
When I apply the ohmmeter to the end of R6 that goes to the meter (the other ohmmeter lead is grounded) the meter deflects full scale. This is what made me think that R5 is somehow bypassed to ground and no voltage is being developed across it. That would ground the cathode of the tube (which is only 3.2 ohms DC above ground anyway) and I think the amplifier would still produce output. That-s consistent with my measuring 0.3 ohms from the hot end of R5 when the tube is in the socket73 Charles W4KEP


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 Post subject: Re: National NCX-3 Final Amplifier
PostPosted: Feb Wed 28, 2018 4:26 am 
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Joined: Mar Thu 19, 2015 11:01 pm
Posts: 30
Only this tube shows an internal short between pins 3 and 7. I checked two other tubes and they showed the expected open circuit between 3 and 7. Apparently I have some kind of a freak bad tube. Why this didn't blow the screen supply I don't know. Maybe it was because I only powered up the PA for a few seconds.

Thanks again to the people who responded.

73 Charles W4KEP


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