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5751 operating points

Posted by DanK tubes (A) on March 15, 2007 at 21:26:05

I finally got that hum problem worked out on my phono preamp. Thanks to everyone who helped out with my previous questions on that. After much filtering, shielding, and DC on the filaments, the last part of hum was coming from the 12ax7 input stage. I tried a telefunken long plate, bugle boy d-getter, Tung-sol black plate, and the quietest I had was an RCA black plate. I’ve got some 5751s and I started to try these, but I noticed that operating points change. When I plug in a 12ax7 I get 167V on the plate and 1.35V bias. If I switch to the 5751 I get 150V on the plate and 1.65V bias. I thought that the 5751 is a drop in replacement. It sounds good with the 5751s but what operating points do people recommend for the best sound?

Best regards,
Dan

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