Microphonic Polyester Capacitors

[PRR]

> Says 5f2a in the first post

Also says 18 Volts supply. Tubes are flexible, but 1/17th of usual voltage is not a "comparable" condition.

[mozz]

Could be a valvecaster takeoff then. Low voltage and 12au7 does make sense.

[amptramp]

The only time I had a problem with microphonic plastic film was in an infrared detector array used to detect the exhaust plume from an approaching aircraft.  This used photovoltaic InSb detectors operating in the 2 - 5 µmetre range of the infrared spectrum.  Since linearity is dependent on keeping the voltage change small with a photovoltaic cell, we used transresistance amplifiers with a 25 megohm gain.  These were low-noise op amps where the input was connected to the inverting input of the op amp and the current in was transformed into voltage out by the 25 megohm feedback resistor.  The detector array, which operated at the 77K liquid nitrogen temperature, was connected to the electronics by a Kapton polyimide flexprint.  The vibration of the refrigerant unit created some noise in the Kapton film.  We figured that this was due to capacitance changes in the film.  A capacitor has a voltage across it equal to;

I = d(CV)/dT

where V = voltage
      C = capacitance
      I = current
      T = time

Normally, only the I varies but in our case, we had evidence that C was varying as well.  We isolated the noise and tuned it out of the system but it was a problem we had to solve.  Polyimide is ideal for flexprints since it has the same thermal coefficient of expansion as copper, so we couldn't change the plastic formula.

[printer2]

> Says 5f2a in the first post

Also says 18 Volts supply. Tubes are flexible, but 1/17th of usual voltage is not a "comparable" condition.

Second that. Could be a Valvecaster but the resistors on the cathode and anode says higher voltage. Just dropping the voltage will not have it operate as on high voltage.