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Ok so I was testing the 37-620 cathedral after a full recap and I am getting a real loud base squealing. If i turn the tone control it get higher pitched and then starts sputtering intermittently.
It was not making any sound at first so I checked all the tubes and cleaned the controls but now this is happening..
What the heck did I do?
Kirk
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(This post was last modified: 09-09-2014, 08:25 AM by OldRestorer.)
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Do you have the tube shield on the 75 tube and the grid cap wire inside the shield?
The 75 is right next to the 42 output and it picks up feedback from the output tube unless the shield is installed.
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I was about to ask same: do you have any tube that should have grid cap wire inside the ahield and has it outside?
And for this matter, are the shields installed?
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Shields first. And watch the proximity of that 75 grid cap lead to the 42.
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What 75 tube and what 42 tube?
Here is the schematic
http://www.nostalgiaair.org/PagesByModel...013212.pdf
Kirk
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Quote:What 75 tube and what 42 tube?
He was looking at a schematic for a model 620. Your first post didn't specify that it was a 37-620 
John KK4ZLF
Lexington, KY
"illegitimis non carborundum"
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oopsie!
Update, I re-cleaned all the contacts on the tubes and turned it on. I jiggled the 6Q7G tube and the noise stopped. jiggled more and back came the sound and the vibration. I of course don't have another 6Q7G but it tests good on all settings. The base is loose, as is the top contact so I dont know...
Kirk
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(This post was last modified: 09-09-2014, 08:26 AM by OldRestorer.)
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Maybe you need to clean contacts and not change the tube.
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I cleaned them already. I will do it again but even when the sound stopped there was no reception...
Kirk
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Replace the socket if you can't clean and re-tension. You can also get a 6Q7 (metal version,) and make sure pin 1 is not used as a tie point for anything other than ground (re route if needed,) and then ground pin1.
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Tension is good on the socket and I dont think I even have a 6Q7 metal but I will look.
Thanks
Kirk
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Quote:The base is loose, as is the top contact so I dont know...
You can re-glue the base as well as the top cap with a drop or two of glue. Then you can heat up each tube pin and flow in some fresh solder. That would eliminate a flaky connection at one of the tube pins. Also try lightly tapping the tube when testing, especially when testing for shorts.
John KK4ZLF
Lexington, KY
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What did you use to clean the volume control?
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
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Knowing Kirk....coarse grit sandpaper 
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Contact cleaner spray was used.
Sandpaper.... har har har 
Kirk
Times I have been electrocuted in 2021
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