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Old 29th April 2015
  #25
Gear Head
 

$2860 for a NOS VF14M....probably money well spent to keep the U47 original.... Used but noisy VF14's...does regeneration of the cathode give long term noise free service?

Remember the AR47 kit and the 13CW4 nuvistor....worked but big impedance mismatch to the U47 output transformer...
WE407A...connect for 40 volt heater, both sections in parallel...worked in a mic I converted from a AR47 several years ago...
WE408A...use two...connect heaters in series for 40 volts, connect one 408A as a triode (G3 to k, G2 to anode) on AR47 base...
Some "fine tuning" on the AR47 board should closely emulate a VF14 for a lot less $$...

The AC701 is going the same route....Oliver (RIP) complained of the AC701's high price and hoarding...he recommended the 5840 (wired as a triode) as a substitute with some mods needed to the power supplies....I used a 7586 nuvistor in place of a AC701, but the characteristics of the triode connected 5840 appear to better match the AC701 output transformer primary....

The Capps CM2250 mic used a 5703 triode...the Altec M30 used a 5840...