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I have a MPX-65 in my Fisher 500c. I believe Fisher used that same multiplexer for the 400, 500b, 800b & c.
I have heard that some of the 400s used 12AT7 in two posistions on the multiplexer.
Does anyone know if this is true, and which 2 of the 3 positions could I substitute a 12AT7 for a 12AX7?
I ask because I have a lot of Amprex 12AT7 Tubes - not many Telefunken 12AX7 smooth plates.
I was a vegetarian for 15 minutes, until the main course.......Meat; It's the right thing to do. Romans 14:2
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I seen other Fishers use two 12AT7 (forget model- TA-600?). Some early MX-65s used a 12AT7 on the output (tube next to blue color balance control). Believe it or not, when Fisher changed that 12AT7 audio out tube to a 12AX7 no changes were made to the circuit itself.
I rebuild Fishers for best possible audio performance and change the 12AX7 audio out tube to a RCA 12AT7.
Thanks for the reply.
My service manual shows those tube marked as v100, v101, and v102. V102 is the tube adjacent to the balance control; Which one is the other tube I can swap out with a 12AT7?
I read somewhere that early Fisher 400's had two 12AT7 tubes rather than three 12AX7's.
If I understand your reply; Your saying that the 12AT7'S are a upgrade over the 12AX7?
I just changed out the 330k grid resistors with 200k, as well the .047uf coupling caps with Russian K40 .1uf PIO caps.
I was a vegetarian for 15 minutes, until the main course.......Meat; It's the right thing to do. Romans 14:2
The unit clearly says 12AX7 and that's what you should use. There are several good varieties being produced currently. The Sovtek 12AX7LPS is clean and quiet. The "reissue" long plate Mullard has some of the famous midrange accent. The "reissue" TungSol will give you a dose of NJ "chime". Select a variety based on the "tone" you want.
All 3 of the above are made by New Sensor at their Saratov, Russia, plant.
Eli D.
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