R/E/P > Klaus Heyne's Mic Lab Stickies



I am working on a 48 with a vf-14 tube the mic is noisy  and no output. checked the tube and capsule voltages and the capsule is missing the 50+ voltage to the BP it is at 5 volts. checked the schematic and found that there is a 29 ohm resistor off the tube heater leg p11 and the schematic  for the 48 what looks like a 290k "can't be right".I know the 47 uses a 29 ohm. I did order a new tube and am waiting on it .The v rail the junction  of R8 ,9 is 105v. should a bad tube cause the back plate voltage to drop that much?

The voltage you'll read at the terminal for the backplate depends on the resistance of the meter. Most contemporary DVMs have an internal resistance of 10 MΩ. With the 100 MΩ resistance of R1 between 1/2 of 105 V (split by R9 and R10) and the backplate) your meter with 10 MΩ resistance will read 10 MΩ/(100 MΩ+10 MΩ) or 1/11 of the 52.5 V or 4.8 V, which closely matches what you reported. Your measurement of 105 V at the junction between R8 and R9 leads me to conclude that the current drawn by the tube and its filament are correct, leading me to conclude that the VF14 is probably perfectly fine, and I suspect the capsule as the most likely cause for low sensitivity and excessive noise!
Due to the low sensitivity performing the 'breath test' may be incloncusive. But if you have access to two good quality 82 pF capacitors (mica, polystyrene, ceramic), substitute them in place of the capsule and listen to the microphone's self noise. If it is relatively quiet, you have confirmation for a defective capsule. While you are already working on this microphone, you may as well check the performance of the electronics by supplying an audio signal from an AF-generator and measure the output.
Without the capsule or 82 pF substitute capacitors connected, through a capacitor (value not critical, preferably between 82 and 1000 pF) supply a 1 kHz sine wave signal with a level of 100 mVrms to the tube's grid (= capsule's front terminal) and measure the output. With the output impedance set to 200 Ohm, the gain should be within 1 dB of unity.

One noise problem I have found is dirt on the plastic area below the capsule, making a high impedance track on the plastic. Make sure everything on the capsule connection path is very clean.


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