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In Reply to: RE: Fisher Phase Inverters posted by sony6060 on August 15, 2014 at 19:15:20
Watch the heater current draw! The 6U8, etc., draw 450 mA. while the OEM 'X7 draws 300 mA. Don't "fry" the power trafo.
You may be better off appropriating the circuitry in (IIRC) Fisher's X-100. That setup uses the 'U7 section of a 12DW7/7247 as the phase splitter. The 'X7 triode is a fine voltage amplifier, but it SUCKS at other tasks. Using a 'U7 triode as the phase splitter disposes of the drive capability issue, without increasing the stress on the power trafo.
Eli D.
Follow Ups:
The 12DW7 is a good alternate. As Michael stated the 2x 6U8 family of tubes with 1.9 watts extra load on a 300+ VA (watt) transformer and 15 tubes plus two dial lights using 6.3 volts AC has no effect on the Fisher power transformer. The dial lamps could be replaced with LED type lights to recover the difference in current too.
I still prefer the Amperex 6BL8 with a socket converter as installed in an upgraded Fisher TA-800 for best sound. After extensive upgrades, that TA-800 outperformed the other Fisher 400, 500 & 800 receivers with same Vit Q & K40Y-9 coupling caps and same brands of tubes in power, preamp & phono sections.
So, my Fisher 400 will get the same TA-800 phase inverter design wired for 6BL8. I also like the Mullard E80CF.
Eli
The 500C has a huge power trafo as does the Scott 340B and Paul Gryzbek has converted many 500c and 400 Fisher amps to EL34s using the same power trafo. Even running the four EL34s which run almost 2.5 amps more total current,the power trafo stayed right at 6.37vac so there will be no issues running the 6U8.
Honest amplification is better than excessive 2nd order distortion anytime.
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