Fisher 500B Schematic Question

[wrenchone]

A year or two back, I purchased the iron from a Fisher 500B on Ebay, wanting to eventually use it to roll an amp of my own design using 7591A output tubes. Just today, I was looking at a 500B schematic I gleaned from the internet to see what windings were available on the power transformer and noticed that the filaments for the 7591 output tubes and the 12AX7 output driver tubes are run from the same filament winding on the power transformer. This doesn't smell right at all, since the 7591 has a 6.3V filament, and the 12AX7 has a 12V filament. All the filaments in question are run in parallel, no trickery. What gives? This looks like a mistake.

[Sherman]

I don't know if this is how Fisher did it but a 12AX7 can be used with 6.3V. From memory, so check before wiring - tie pins 4&5 together and connect the 6.3V lines to pins 4 (or 5) and 9.

[mrshow4u]

I think Sherman is right. A center tapped heater, making 2X 6.3V

[wrenchone]

I didnt realize the 12AX7 had a center-tapped heater - shows how much I use them.... Mystery solved! Thanks.

[Eli Duttman]

Wrenchone,

If you are going to use Fisher's topology, a 12DW7 is better than a 12AX7. Use the high mu section for voltage gain and the low mu section for the "concertina" phase splitter.

Scan the archives and you'll find a 7591 project being scoped out by dhaen. There's lots of food for thought there.

[wrenchone]

I'm using either a 6CM7 or 5965 front end, different topology fron the original Fisher circuit. I will be using a concertina splitter, though.