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Message: [QUOTE="rmburrow, post: 1014457, member: 52955"] The late Oliver Archut liked the 5840 triode connected. A 5703 (triode) selected for noise should also work. I haven't tried any of the other tubes and don't have one of the 6S6B-V to try. Assuming the preamp output traffo is for a AC701, your tube choice should have anode impedance near that of a AC701. Now for the bias: Apparently -1.6 volts on the grid (with respect to the cathode) is optimum. I like the fixed bias version where the cathode and H+ are tied together, and two resistors for a voltage divider off H+ for the bias. No bypass capacitor for the tube cathode is needed inside the mic. Apparently Scheops used 100k and 150k resistors in series with 4 v H+ (in the M221a); at the junction of those resistors -1.6 v was present (with respect to cathode) and the 180M resistor went to the AC701 grid. A 6 volt tube can be fixed biased; a different resistor ratio is needed to get the desired -1.6 v bias. Keeping the 100k resistor off H+, the closest standard value for the bottom divider resistor will be 270 k. With 6 volts across the divider, the current through the divider will be 16 uA, the bias will be approximately -1.6 v with respect to the tube cathode. Note the biasing arrangement for a M221b preamp looks different. There was a M221b preamp on ebay recently but no mention of size or version. The resistive divider is taken off the tube anode voltage rather than the DC heater voltage. Assuming the "operating" anode voltage of the AC701 is 40 vdc, the M221b divider places approximately -2 v bias on the grid. (The 3.9 M resistor should drop 38 vdc.) One comment: Before the tube heats and draws current, the voltage across the divider is almost full HT, and 5% of full HT could be at the tube grid. Look carefully at the parts, they will help identify your "version" of the preamp. PS I am looking for a similar preamp which will fit a 95 mm by 20 mm body. [/QUOTE]

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