is there a way to find out what a tube is that doesnt have any markings apart from “NEC 48” on the collar at the bottom
Hello mikecasey, would you be able to provide any photos of your part to help us identify it?
Hello @mikecasey,
May I recommend these webpages as they provide very good information about tubes:
As @Rob_Johnson pointed out, a picture of the tube and the associated radio may help us select the tube.
Tech Tip: The tube identifiers (markings) were often included on the glass bulb. With age and aggressive cleaning, these marking are rubbed away. An old technician’s trick to blow on the tube. The condensate in your breath may reveal the markings. It will be faint but, if you are lucky, clear enough to be read.
Best wishes,
APDahlen
Hello @mikecasey,
We are looking at a dual triode. Likely candidates include 6SL7, 6SN7, 12SL7, or 12SN7.
CAUTION: I can’t see the other side of the tube, but I assume both sides are identical.
I’m inclined to say this is a 6SN7 based on popularity and internal structures appear excessive for a 6SL7. The tube is similar to the one in this webpage:
Next steps:
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Verify the pinout against the datasheet.
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Use a DC power supply to verify the filament. For example, both the 6SL7 and 6SN7 draw 0.3A at 6.3 VDC.
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Make a new friend that has a tube tester. Start by searching for radio armatures (HAMs) in your local area.
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Speculate based on the tubes position in the amplifier. A high gain tube such as the 6SL7 would be early in the amplifier chain. A low gain 6SN7 may have been used as a phase splitter to drive a pair of 6L6 output tubes.
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Lacking a friend and a tube tester, the best we can do is guess and install the new tube. Having verified the pinout and filament voltage, you are unlikely to damage the equipment given the typical circuit from the era.
Let us know what you find.
Sincerely,
Aaron
Thank you Aaron,that helps me a lot……
No worries,
Let us know if you are able to restore the amplifier.
I feel I must explain. I am just an old guy trying to keep busy. I have no formal training in amp repair, I’ts just a hobby. I have been playing around with amps for about a year and I am learning all the time. So if it is alright with you I will be asking a lot more questions. You might even call me a nuisance, but if you don’t ask you don’t get to know……thanks again Mike
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Fire away, @mikecasey,
These forum posts live a long time, and others may find the information useful.
Besides, I love talking about tubes.
Sincerely,
Aaron
Sorry to bother you again, but I would like your opinion on this tranny. First of all why all the connections on the primary side and what is the purpose of the heavy wire across the tranny going to ground.
Hello @mikecasey,
This appears to be a conventional tube power supply transformer.
- 6.3 V filaments
- 5 V filament for the rectifier tube
- center tap high voltage to provide full-bridge rectification through the rectifier
- AC inputs
The uninsulated bus wire appears to be ground. Note how it connects to the transformer case (red circle) and presumably to the amplifier’s chassis.
Sincerely,
Aaron
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