[QRO] Doorknob Capacitors
Deacon Dave
W1MCE at comcast.net
Mon Jan 12 19:48:11 EST 2009
Generally ... It depends on the application.
Several reasons. First, the tolerance on the capacitances are different. The
door knob type has a closer tolerance and a much better temperature
coefficient.
Second, The door knob has a lower dielectric loss, aka higher Q, and hence
has less temperature rise at higher levels of rf current.
Third, the self resonant frequencies may be different. Each capacitor has
stray inductance that creates a self resonance with the capacitor.
So, the door knob type is not suitable as part of the tuning [PI] network in
a tube amplifier .... say for example, to extend the range of a PI network
loading capacitor to 80 and or 160 meters.
BUT! It is possible to replace a 0.001 [1000 pf] with a higher voltage
rating capacitor in certain applications where the capacitor tolerances are
negligible compared to circuit values.
If you are talking about the DC blocking capacitor in a vacuum tube
amplifier, the variation in capacitance and temperature tolerances would be
negligible. However, heating of the capacitor should be verified before the
cover is closed. An RF current of 1/2 ampere [ ~1.5 KW] can cause a lot of
heat if the capacitor is LOSSY!!
----- Original Message -----
From: "Deon Erwin ZS1ZL" <zs1zl at telkomsa.net>
To: <qro at qth.net>
Sent: Monday, January 12, 2009 2:57 PM
Subject: [QRO] Doorknob Capacitors
> When the circuit diagram of a valve amplifier specifies a doorknob
> capacitor
> of 1000pf at 5kV, can one substitute it for the same value in a ceramic
> disc
> capacitor?
>
> If not, what is the reason?
>
> Thanks
> Deon ZS1ZL
>
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