Fisher & the 12AX7LPS (watch for this)

Fisher amps use some of the 12AX7s in in the power tube cathode circuit to develop power tube cathode bias plus apply DC (direct current) to the filaments to eliminate AC (alternating current) hum. The 12AX7 tubes are set up to use 12 volts each in the Fisher amplifiers & receivers at the phono, input and tone sections. Do note the 12AX7 can be wired for 6.3 volt or 12.6 volt operation depending upon the wiring at the socket.

Some Sovtek 12AX7LPS do not have equal current draw in each filament. A wide variance will starve one side of the 12AX7 filament. These unbalanced tubes shows up on tube testers with unbalanced emissions that use 12.6 volts filament on 12AX7 tubes. The unbalanced 12AX7 filament draw can cause one channel to sound mildly or even extensively distorted.

Thus, if you purchase a load of these 12AX7LPS due to the low cost of re-tubing a Fisher, the phase splitters operate at 6.3 volts and in the case of the receivers, the phase splitters and multiplexer operates off of 6.3 volts. Use the unbalanced 12AX7LPS filaments in those positions. The unbalanced filaments will not matter due to the filaments are wired in parallel.

Perhaps the Russian Genelex Gold Lion 12AX7 long plate gold pin tube that sounds the same to me as the 12AX7LPS are factory selected for equal filament draw. However, the Genelex Gold Lion costs 2.5 times of the 12AX7LPS!!! (Ya, how about that 10 cent coffee, Starbucks is killing me!)

Long Live the King- FISHER

Sony, your a genius, still looking around for my 12a's for my 202B! I enjoy and learn a lot from your posts..

Thats honestly not something I'd have ever considered checking. Thanks for the heads up. Makes me wonder if the ones in my PAS2 are behaving properly since it runs them on the 12v wiring.

It has always been fairly common knowledge that the current draw of one 12AX7 versus that of another could be slightly different in series connected circuits -- but it was always generally understood that within a given tube, the current draw was fairly even between sections, when used in series connected circuits. Thanks for pointing out that that understanding is not always true with tubes of modern manufacture.

Dave

dcgillespie said:

It has always been fairly common knowledge that the current draw of one 12AX7 versus that of another could be slightly different in series connected circuits -- but it was always generally understood that within a given tube, the current draw was fairly even between sections, when used in series connected circuits. Thanks for pointing out that that understanding is not always true with tubes of modern manufacture.

Dave

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I was a little surprised myself the 12AX7LPS had enough variance to read poor on one section and perfect on other section on my 12 volt tube tester. It sounded like one bad channel in a Fisher X-103C. Never seen this on any NOS 12AX7s.