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Microphone of the month - Old Czech tube mic: Tesla?

This is the first 'Microphone of the Month' blog, featuring classic or unusual microphones. Hopefully I'll manage to find time each month for this!

This old Czech tube microphone - a recent ebay find - may well have been made by Tesla. The capsule is connected using a connector that can also be found on old Tesla and Phillips microphones. Some of the capacitors are also made by Tesla, who were a large state owned electronics company in communist Czechoslovakia.

The 'Tesla' looks very much like an imitation of the Neumann / Gefell CMV563 bottle mic. In fact the microphone is smaller in diameter than the CMV, has no output transformer and has an unbalanced output. Like the CMV, the capsule may be swapped, and presumably other polar patterns were available. This one is marked with a red circle, which probably means omnidirectional. (I have yet to test the capsule).

The amplifier is a very simple grounded cathode amplifier, based around a Soviet 6Ж1Л (6Z1P) tube, which is a small signal pentode similar to EF95. These are also found in some Lomo and Oktava microphones, including the Lomo 19a9 and Oktava MKL2500.

Without the original power supply we can only speculate on the operating voltages. However, a B+ supply of 90V would be a good place for experiments to start - this would give a voltage on the capsule of around 60, and a sensible current through the tube circuit.