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Author Topic: Need advice for a blocking capacitor  (Read 946 times)
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W8ACR
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« on: September 04, 2023, 12:36:57 AM »

I have a T-368 that I have converted to a standard shunt fed pi network tank circuit. I used a 500pf CRL 859S doorknob for my blocking capacitor. Somehow, I managed to destroy this very hefty capacitor with arcing from the nearby plate choke. Soooo.... I need a new blocking capacitor, but I don't have any more large doorknobs in my parts stash. Several questions:

1. What is the smallest capacitance that could be used as a blocking capacitor if I want to use it on 160-20 meters?
     500pf? 200pf?

2. For a margin of safety, I'm wanting to have a capacitor rating of 10KV and maybe 4 or 5 RF amps. Is that being too cautious?

3. I have four 1000pf, 5KV 858S doorknobs, I could use these in a series parallel design. This would give me 1000pf @ 10kv, but I'm not sure if two parallel 858's can handle the current at full power of about 450 watts on 160 meters.

4. I have an 857 rated at 10KV and 200pf. This would be good for voltage and current ratings, but not sure if that is enough capacitance.

5. I have lots of those molded Sprague capacitors rated at 500pf and 20kv, but I have never seen them used as a blocking capacitor. I usually see them as bypass capacitors in HV lines such as at the base of a plate choke. I suspect that they do not have very high current ratings. Are these suitable for use as a blocking capacitor? maybe 2 or 3 in parallel?

I have placed an ad looking for a 500pf 857 or 859 unit, but if I can't find one, I need a workaround.

Thanks, Ron W8ACR
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n8fvj
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« Reply #1 on: September 04, 2023, 12:51:06 AM »

I would use two 858 caps in parallel at 10KV. 500 or 1000pF is ok. The four you have in parallel  and series is good. These caps are expensive now.
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w9jsw
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« Reply #2 on: September 04, 2023, 08:05:43 AM »

Those molded ones usually can't handle the current even if the voltage rating is adequate, in my opinion. I use the big Russian caps on my rigs. They are harder to get now but still available.

An example - https://www.ebay.com/itm/275646985829

John
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WQ9E
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« Reply #3 on: September 07, 2023, 05:57:33 PM »

Like John, I rely on the Russian "hockey puck" capacitors.  I have had zero issues with them in over a decade of use.

Rodger WQ9E
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Rodger WQ9E
K8DI
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« Reply #4 on: September 07, 2023, 08:35:58 PM »

Another option would be parts from a broadcast rig. Most of them use fixed capacitors and tapped inductors, so when one converts one to ham bands, they are left with a box of fixed capacitors. My RCA leftovers included 630 and 1300pF values, at 10kV, and obviously capable of multiple amps of RF. You also see these on eBay, Sangamo mica capacitors, although what's listed there right now seem a little pricey.

Ed

https://www.ebay.com/itm/166309624531
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
n8fvj
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« Reply #5 on: September 12, 2023, 01:27:26 AM »

Your four 858 caps are fine in parallel series and will handle over 1.5KW. No need to spend money for another capacitor.
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