If I recall correctly the 12AT7 is not recommended for the BHK Pre.
That’s also my firm recollection.
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It wasn’t quite a circuit that would be plugged into a tube socket it was a completely different design solid state front end. Either way Bascom convinced Paul that the tube provided the best performance.
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Just going by memory a 12AX7 has 5X more gain than a 12AU7. Not a good combination for a circuit not designed for it.
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You got that on the nose. The mu (amplification) factor for a 12AX7 is 100 and it’s 20 for a 12AU7.
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a little advise as a user of bhk preamp, in the newversion at certain time later, possible to come with 2 pairs of XLR output that would more benefit in some cases
This exchange reminds me of one I had years ago. I had a Berning TF10, and had a question about it, so I called Berning. When they answered the phone, I started rattling off whatever question I had, and asked to talk to the tech department. The guy on the other end said: “this is David Berning.” He was very nice, and open, and wiling to chat about my preamp. He told me it was one of his favorites. He recommended to me to try NOS RCA 12at7s, as they were his favorite tube in that circuit. I did, and he was right. Tube rolling used to be more fun, and much cheaper that it is today. I used to drive to small towns that had an older TV repair shop, looking for NOS tubes, that they were usually happy to sell cheap.
I hope to go to Boulder soon. It would be a real hoot to meet you.
Reverently,
Mongo
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