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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?
Follow Ups:
The 5751 is not the same as a 12ax7. The gain is different, so is the plate resistance.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Good point Tre! I neglected to mention they were different tubes - I assumed Dan knew, but I might not have been correct in that assumption. Thanks for jumping in.
I know that the 5751 has a lower gain, but I didn't realize the plate resistance was lower. That makes sense with what I saw (lower plate voltage and higher bias voltage). People always talk about dropping in a 5751 for a 12ax7 and that the 5751 sounds better. But nobody ever mentions what operating points the 5751 sounds best at. Any recommendations there?
Thanks,
Dan
Tube operating parameters vary greatly from tube to tube, even if they are the same number. You simply can't compare one 12AX7 to one 5751 and draw any real conclusions. You will find variation within the same type at least as large as you expereienced. In the case of 12AX7s variation of 40% is not unusual.BTW, have you tried using a tube shield? It can be very effective at reducing hum in some circumstances.
Hi Jim,
Thanks for the info. I didn't expect that much variation from tube to tube. I put a tube shield on as an afterthought, but I guess I should role through the others again with the sheild on.
Dan
Hey, if it makes you feel any better, transistors are much worse in sample to sample variation!But yes, 40% is not at all unusual for perfectly healthy 12AX7s - and many other tubes for that matter.
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