220/221

[John Farrington]

At 07:22 AM 05/04/2003 Martin Tippin wrote:

 > Will a 500pf door knob take the place of C-56 500pf mica? Thanks guy's.
 > Martin L Tippin WØAKG


Depends on what kind of doorknob and what the particular use is - simple
DC filter or critical high power RF. If it's just a HV DC bypass that
doesn't handle RF current then it may work, but if it's part of a tuned
circuit or has to handle a lot of RF current then the materials it's made
of become critical.

TV-type doorknob capacitors, for example, aren't designed for RF current.
And, depending on where it's used in your 220/221, dialectric temperature
stability may be critical, as incorrect capacitance drift will cause a
frequency shift in tuned circuits. An NPO type is supposed to have zero
drift, but even they are stable only over a limited temperature range;
an N500, on the other hand, will shift value by minus 500 ppm per degree
Centigrade. There will be some heating with RF current, and if the cap is
used in a tuned circuit where other components also drift with heat, then
the cap temperature coefficient should be chosen to compensate.

Transmitting micas of appropriate ratings are better but can be expensive
unless you find some cheap at a hamfest.

Surplus sales for one has a variety of transmitting micas and doorknobs,
but some will cause sticker shock:

<http://www.surplussales.com/Capacitors/Trans_Coup%20Caps/cap_trans.html>

Examples:

Doorknobs:
(CFT)715C0500-30 500 pF, 30 kv doorknob. 1" diameter, 1-1/4" high. $55 each.
(CFC)SPRAG500-15 500pF, 15kvdc doorknob.  Sprague "Fabmica" high stability
       doorknob. 10 amps RF current. 1-3/8" diameter, 1-1/4" high. 1/4"
       thread on top, bottom.  $25 each

Micas:
(CFT)A2000500AOG 500 pF 2.5 wkvdc mica (5kv test) $10
(CFT)F2B00500ARA  500 pF 5 kv 3 amps $95
(CFT)F3000500AUA 500 pF 8 kv 3 amps $75

73

John    KE5ZB

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