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Old 28th December 2006, 02:56 PM   #11
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Originally posted by SY
One caution: air vane caps tend to be very microphonic. As appealing as air-as-dielectric might be, you might do better with a Teflon trimmer.
I recall having a Heathkit VF-1 VFO back in the 1960's -- all you had to do was breathe heavily and the frequency would shift.

In SoulMerchant's application, however, a few PF one way or another isn't going to matter much, as it isn't (hopefully) an oscillator. The capacitance of a variable cap will also change with the relative humidity. Best bet is to hook up the preamp to an Inverse RIAA network, then adjust the 270 pf cap (using a 365pF air-variable) for best response -- the 12 meg resistor will also need adjusting. When you have determined the optimal value swap out the air-variable and substitute in polystyrenes or mica for the exact value. For very small values of capacitance you can twist together a pair of wires -- called a "trick" capacitor.
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Old 28th December 2006, 03:16 PM   #12
SY is offline SY  United States
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I have in front of me my QSL card from 1968 with... a Heathkit VF-1 visible in the photo. It wasn't until I finally got a Collins 32S-1 that I ever experienced stable operating frequency.

Completely agree about the use of a trimmer to determine the right value, I just wouldn't want to leave it in. Microphony means generation of a voltage at the resonant frequency. A small voltage, I grant you (I haven't calculated an order-of-magnitude), but right at the most sensitive point for human hearing and in an environment where there's lots of vibration in the air to excite it.
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Old 28th December 2006, 04:37 PM   #13
radtech is offline radtech  United States
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I agree, use a variable to determine the value, then replace with fixed. Judging from the 365pF caps I've used in radio stuff I can imagine they might be very sensitive to vibration right in the audio range, some of them made a nice little tone if you ran your finger over the plates.
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