Can be used as a microphone

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Can be used as a microphone

Postby dhuebert » Sun Dec 06, 2009 6:21 pm

I brought one of my combos to the recording studio this past week to give an artist some choice for a variety of sounds. Maddeningly, the amp seemed to have a microphonic tube and rattled and rang as a result. After spending all day swapping tubes and rewiring the pre-amp circuit and not fixing anything, I took out the 12AT7 I was using as a preamp tube and replaced it with a 12AX7. Problem solved. It is working but I will have to finalize component values and do some more soldering. All of the ATs that I installed, including Marconi NOS, old pulls of various brands, new EH and Sovtek all had the same problem with being too microphonic to use as an input tube. Lesson learned; 12AT7 cannot be used in a high gain situation. The amp would actually feedback through the tube and speaker, even with the chassis out of the cabinet but placed nearby!

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TRy old Tung-Sol 12AT7 WAs

Postby EWBrown » Mon Dec 07, 2009 8:23 am

I found with the Botlehead Paramounts and Paramour IIs, the supplied (NOS Sylvanias and Electroharmonix) 12AT7s all tended to be microphonic, in varying degrees, from mild to ridiculously so..

I also went through the re-iterative tube-swapping / rolling process, and finally setteled on some old, dusty, used Tung-Sol 12AT7WAs of which I have a plentiful supply, and not only were they not microphonic, they also sounded by far the best of all the varieties that I tried.

Even some vintage Mullard 12AT7s extracted from an old (but lightly used) Heathkit color TV dot / bar generator were not as nice sounding as the old Tung-Sols.

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