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In Reply to: RE: 12AX7A contact-potential biasing posted by Brian C. on May 11, 2008 at 08:48:34
LeBong's design already uses contact bias in the 1st gain block. 47 Kohms is not going to get much of a differential between grid and cathode. However, it seems to be enough for the mV. level I/P signal.
Since your objective is eliminating the 100 muF. 'lytic, consider a different tack. Replace the 3 KOhm/100 muF. bias network with a small, precharged, NiMH or NiCd cell. A fixed bias of 1.2 V. should be just fine. The rationale for using the physically smallest cell you can find is that the low Ib of a 'X7 section is not going to provide much of a charging trickle.
ALL common cathode 12AX7 gain blocks exhibit absolutely wretched drive capability. I strongly suggest you graft the DC coupled ZVN0545A source follower onto this circuit. I too have little affinity for BJTs, AKA 3 legged fuses, in HIFI audio circuitry. However, FETs are, for the most part, heaterless pentodes. The big "gotcha" in using the ZVN0545A is its high sensitivity to static discharge. Wear a grounded wrist strap, as is done when handling digital stuff, when you wire the little MOSFET up. Once all the connections are in place, vulnerability to being "zapped" ends.
Eli D.
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