Elam 251 quick test with 12AX7?

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mkiijam

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Aug 25, 2017
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70
I have a 251 that has low output. After it warms up a bit it starts a 60/120Hz hum. I don't have a sub 6072A tube to swap in real quick, but do have 12ax7's laying around? Would that help as a quick test? The 6072A tests just under "OKAY" on my BK 667 tube tester, but though subbing would be the best test.

Power supply seems to measure fine. 120Vdc and 6Vdc heaters.
 

TillM

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May 21, 2016
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Berlin - Germany
You have to change the plate resistor.
Maybe it’s a better idea to buy a 6072a tube from Electro Harmonix to test, if it’s the tube.
 

Gus

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Jun 3, 2004
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You need to give more information

Is it a real AKG 251?

Is it a DIY 251 like circuit?

Is it a bought "clone"?

Did it work before or is it new to you?

Do you have test gear like a signal generator etc.?

Is the hum 60Hz (grounding, connections) or 120Hz (power supply)?

Was the microphone dropped?

Also read Tim's post
 

mkiijam

Well-known member
Joined
Aug 25, 2017
Messages
70
You have to change the plate resistor.
Maybe it’s a better idea to buy a 6072a tube from Electro Harmonix to test, if it’s the tube.
Silly me. I was listening to the Mic with "LINE" input on my DAW. Meaning, it was fine level when going into a proper micpre!

The hum is now gone as well, of course I did take it all apart so everything got "reseated."

Thanks guys!
 
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