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## [AMPS] Another Stupid Question

 To: [AMPS] Another Stupid Question W8JI@contesting.com (Tom Rauch) Mon, 3 Dec 2001 15:17:53 -0500
 ```My point, which is sometime lost when threads are so long, is that the rule of thumb of using a blocking cap 25% of the reactance of the tuning cap is not a good rule of thumb. The blocking cap reactance only need to be low compared to the operating plate impedance of the tube. > > Let's assume the blocking cap, at the lowest frequency, would be set > > a reasonable value of 10% of the impedance looking into the tank, or > > approximately equal to the value of the tuning cap with a Q of ten > > (using the simple but not absolutely correct value of Rp/Xc. > > You have inserted the phrase: "at the lowest frequency" which was > not > a feature of the original question. The "10 % of the impedance" was > formerly at the frequency of operation. Respectfully, no person with any understanding of how this stuff works would ever size the capacitance of a blocking capacitor at a frequency ABOVE the lowest operating frequency. Some things are not said because they are assumed to be universally understood. We are sending quick e-mails, not writing detailed engineering descriptions. > > Using an Xc equal to 10% of the plate operating impedance, and > > assuming a choke somewhat larger than the plate impedance, the > > change in impedance looking into the tank is: > > Wrong! The plate choke may not be ignored as it is typically only > "marginally larger" than the plate impedance in 160 meter ham amps. I > stated in my previous message that it is a worse offender, at this > point, than the coupling cap in the real world. Somewhat larger and "marginally larger" are both subjective terms meaning the same thing. > > 300pF blocking (~300ohms 1.8 MHz) 2878ohms > > > > 1200pF blocking (~75 ohms 1.8MHz) 2964ohms > > > > The tank input impedance, with no other changes except a > > readjustment of the tuning cap by 2 pF to compensate for the > > reactance change, changes less than 100 ohms out of 3000 > > ohms....a totally insignificant change. > > > > And the small effect above is on 160 meters, where the problem is at > > its worse point. On higher bands, the effect would be less because > > Xc would decrease with increasing frequency. > > Today it does, but, look at the way the question was originally > posed, > where for instance 50pf could be used on 20 meters, then the 10pF > "typical" tube output C comes into play and you'll see what's > troubling. You are forgetting or ignoring in this case that the 10pF is a major player in the tube's output impedance. When that is considered, if you follow the rule that the blocking cap only needs to be a fraction of the plate impedance, everything is fine. > Now, at the risk of prolinging this thread, I'll just add the > comment > that the smaller value (<500pF) ceramic doorknobs are typically > fabricated from lower loss material than their larger valued cousins. > If one chooses to use a lower valued part, thus making it part of the > matching network calculations, these small guys are better suited to > the task. Indeed they are. I've read articles where someone claimed, mistakenly, that .01uF coupling caps on a 3CX800 amplifier was used to "insure high efficiency". Try to find a high-current high-voltage .01uF capacitor sometime. 73, Tom W8JI W8JI@contesting.com -- FAQ on WWW: http://www.contesting.com/FAQ/amps Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com ```
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