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In Reply to: RE: low 12AX7 filament voltage in my Fisher X-100-A amp posted by Jim McShane on December 03, 2016 at 12:27:50
I agree with Jim. The "Cheap Charlie" method of DC heaters stinks on dry ice. :>((Install a proper RC bias network in each channel and construct a regulated 12 VDC supply for the 3X 12AX7s. A $10 AnTek AN-0107 toroid is the heart of the new supply. Phase up and series connect the 2 secondaries. FWCT rectify the composite with a MBR20100CT common cathode twin Schottky diode. Filter the "raw" DC with a 25 WVDC/6800 μF. 'lytic. Regulate the rail with a 7812 3 terminal IC.
BTW, Fisher phono sections are notorious for poor bass extension. After getting the unit squared away, should better bass be on your list of things to do, we'll talk about reworking the phono preamp to use tweaked RCA topology.
Eli D.
Edits: 12/03/16Follow Ups:
Thanks for the tips Eli. I would definitely be interested in your mod for the phono section. This particular amp uses those PEC modules for the equalization circuit. Would I have to take these out and replace them with discrete components? Please provide a link if you can. Thanks!
Yes, those blasted PECs go. Replace the source selector switch and hard wire 1 set of RCA I/Ps to the phono preamp. Tape head is passe. Get switches out of mV. level signal lines!
The tweaked RCA setup needs stable 250 V. B+. A TO-92 case LR8 regulator in each channel gets the job done, while improving channel separation.
Experience shows that a protective 12 V. Zener diode should be installed between the tube side of R3 and the ZVN0545A's source electrode.
Eli D.
nt
I had a "Senior Moment". I provided the wrong toroid. The AnTek part needed is AN-0112. Unfortunately, that item is out of stock. The "simplest" work around is to use the larger $11 AN-0212.
Eli D.
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