Tube affecting signal output

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Potato Cakes

Well-known member
Joined
Jul 1, 2014
Messages
2,258
Location
Nashville, TN
Hello, all,

I'm wrapping up an 251 build and after correcting some wiring mistakes, I have very low output, although it is clean and full range (not one-legged). I injected signal into the transformer where it connects to the output capacitor and I getting proper signal (I believe) out the audio + and - pins on the connector. I've tried swapping capsules and I got the same result. Then only thing I can think of now is the tube might be the issue, even though it's new. I'm using an Electro Harmonix 12AY7 and I do not know if there is a quality history with these that I should be made aware. I also don't know if it's possible that a tube could be putting out low signal and not have it be distorted and thin sounding. I've built this particular mic before with the same layout but with a NOS 6072A, so I at least know the layout I am using will work. Does anybody here have some information regarding the EH 12AY7 that I should know about?

Thanks!

Paul
 
There shouldn't be a significant difference. I think you should first swap this one for another that you know works; if there is adifference, the EH may be defective. Several people have mentioned some EH tubes to be noticeably unreliable.
 
Thanks for the info.

I have a feeling I'm down to a defective part. I've emailed the folks at TAB to send me the resistance values to double check the transformer, but I believe it's good.

I'll have to order another tube if that is the case as I don't have a spare one here.

Thanks!

Paul
 
Turns out it was the not so common case of a bad transformer. Dennis at AMI sorted me out very quickly. However when the capsule is hit with a heavy "P" (also my new hip-hop moniker) the mic emits a brief moment of static followed by what sounds like radio modulation then goes away. Granted I was not using a windscreen as I was just doing initial audio checks. At some point I would like to get a quality replacement for the EH tube that was purchased solely for testing. And a really good windscreen....

Thanks!

Paul
 

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