• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fisher X100B Iron


2007-06-03 8:54 pm
The transformers are good for about 30 to 35 watts of output (per channel). The original amp would have used 7868 or 7591A valves. These types are electrically the same but use a different base. As for an amp to build, I recommend a 6L6 based amp. They are economical and in current production, durable, easy to drive, sound good and will work with your transformers.

Best regards,

I agree with Brett. Use 7591 O/P tubes, as their drive voltage requirements are the same as EL84s and 6V6s.

The X-100-_ power trafo does not have a combined bias/preamp DC heater supply. The X-100-A schematic, here and here, explains much.

If you can be satisfied by the approx. 15 WPC triode mode yields, I suggest you go that route. The O/P trafos don't have UL taps. So, triode and pentode modes are the only options. Pentode mode done right requires a regulated screen grid B+ supply.

I'm linking the "El Cheapo" schematic, as the 12AT7 splitter/driver used in EC is fine combined with 7591s. Frankly, I'd like to see the X-100 power trafo wired up with the 6.3 VAC winding used in series with the "115" VAC winding to boost the rectifier winding's voltage. Things "simplify" when the B+ rail comes in at "460" V., as both combination bias and ease of 12AT7 loading are "in play". The alternative is to use "fixed" bias on the 7591s and buffer the 'T7 sections with DC coupled ZVN0545A source followers, as described in MOSFET Follies.