Sandbagger
Administrator/The Boss
Posts: 6,247
|
Post by Sandbagger on Jan 4, 2018 22:44:33 GMT -5
While in the process of restoring a Heathkit SB-220 amplifier and upon the second power-up, one of the 3-500z tube's 1mh 100ma grid chokes immediately melted. It let off some magic fan blown smoke with a lovely burnt varnish aroma. I'm in the process of trouble shooting it and I'm not sure what caused it yet... View AttachmentYou said the original owner complained of a bad tube. Maybe the other one finally went bad too......
|
|
|
Post by Night Ranger on Jan 5, 2018 0:02:08 GMT -5
While in the process of restoring a Heathkit SB-220 amplifier and upon the second power-up, one of the 3-500z tube's 1mh 100ma grid chokes immediately melted. It let off some magic fan blown smoke with a lovely burnt varnish aroma. I'm in the process of trouble shooting it and I'm not sure what caused it yet... View AttachmentMy guess is you have a bad tube. Take the tube out and look for a short between the tube filament pins and the grid pins. If that checks out ok then I'd look to make sure the bias supply is working. Night Ranger
|
|
|
Post by BBB on Jan 5, 2018 11:13:49 GMT -5
While in the process of restoring a Heathkit SB-220 amplifier and upon the second power-up, one of the 3-500z tube's 1mh 100ma grid chokes immediately melted. It let off some magic fan blown smoke with a lovely burnt varnish aroma. I'm in the process of trouble shooting it and I'm not sure what caused it yet... View AttachmentYou said the original owner complained of a bad tube. Maybe the other one finally went bad too...... Seller at the HAM fest said it was good to go. As they always do Could be, I will check it out with the ohm meter.
|
|
|
Post by 2600 on Jan 5, 2018 23:51:03 GMT -5
I would suggest that you put the meter into "continuity" test mode, with the meter's noise maker turned on.
The grid is a cylindrical "cage" structure, with parallel vertical wires. They're held in place by spot-welding each end to a wire circle, and a third one in the center. The ends of these wires will break loose from the weld, and cause no problem until heat causes the now-unfastened end to "curl" and touch the filament. The gap between grid and filament is really close.
We use a "component tester", or octopus connected grid-to-cathode. Gently slap the side of the tube, rotating it to add the pull of gravity while shocking the body of the tube. A loose grid-wire end may touch only briefly. If your continuity tester's noisemaker has a delay, you won't hear anything. The octopus has no such shortcoming and will show you even the briefest moment of contact.
But what you're looking for is an intermittent short from grid to cathode.
Then again, that burned grid choke found it for you.
Already.
73
|
|
|
Post by BBB on Jan 6, 2018 19:36:39 GMT -5
Yep. Sure enough. Pulled it, ohm'd it and then tossed it. Indeed... it was a filament to grid short.
|
|
Sandbagger
Administrator/The Boss
Posts: 6,247
|
Post by Sandbagger on Jan 6, 2018 20:41:32 GMT -5
Yep. Sure enough. Pulled it, ohm'd it and then tossed it. Indeed... it was a filament to grid short. Amplifiers are pretty simple. There are only so many things it could be. Grid to cathode short made the most sense.
|
|
|
Post by BBB on Jan 14, 2018 17:00:45 GMT -5
Since it was an old Eimac tube, I could look inside the top of the plate slits and see the spot where the filament touched the grid. I read (QRZ ?) there is a guy that tries to repair the short by spinning the tube via centrifuge to get the filament/grid to separate enough to work again.
|
|
Sandbagger
Administrator/The Boss
Posts: 6,247
|
Post by Sandbagger on Jan 14, 2018 17:22:31 GMT -5
Since it was an old Eimac tube, I could look inside the top of the plate slits and see the spot where the filament touched the grid. I read (QRZ ?) there is a guy that tries to repair the short by spinning the tube via centrifuge to get the filament/grid to separate enough to work again. I don't know if I would trust a tube that shorted once. It'll likely short again....
|
|
|
Post by BBB on Jan 15, 2018 19:30:45 GMT -5
Yeah, not much clearance between the helical filament and the grid screen in there. Probably less than a millimeter.
|
|
|
Post by cbrown on Jan 18, 2018 16:18:44 GMT -5
I don't know if I would trust a tube that shorted once. It'll likely short again.... Agreed, best to replace it and be done with the worry.
|
|