Topband: Gamma match success

[David Olean]

I would echo Tim's remarks. I bought a bunch of Russian caps on EPAY for 
a 160 meter amplifier and they overheated and changed value.  One blew 
up. They were horrible as a plate coupling cap. The seller advised me 
that they were only good for uncritical bypassing chores.  Now I have a 
pile of caps that are only good for target practice.

Dave K1WHS


On 10/16/2018 5:55 PM, Tim Shoppa wrote:
> I would caution against using Russian military surplus doorknobs, or
> Chinese doorknobs, in a tuned circuit without understanding their
> dielectrics.  I tried a couple and was very disappointed.
>
> With HEC 7.5kV doorknobs, you would want to make it out of values of 170pF
> or below. Below 170pF they have NPO characteristics.
>
> Tim N3QE
>
> On Tue, Oct 16, 2018 at 1:46 PM Herbert Schoenbohm <
> herbert.schoenbohm at gmail.com> wrote:
>
>> Unless you use LMR 600 or Andrew heliax a cap made from standard RG-8 or
>> 213 has its limitations. eBay has a ton of high current fixed caps very
>> close to the value you need.  Some are from Ukraine (Russian military
>> surplus) and are very inexpensive.  You might also consider a home-brewed
>> sandwich cap made from aluminum plates and Teflon insulation between them.
>> This method of heavy duty fixed caps seems to be the vogue of many present
>> amplifiers and tuners.  Another alternative is to get a handful of 5KV
>> doorknob caps at 100pf and add them in parallel or series until you come up
>> with the value you need.
>>
>> Good luck
>>
>> Herb Schoenbohm, KV4FZ
>>
>> On Mon, Oct 15, 2018 at 5:18 PM Richard (Rick) Karlquist <
>> richard at karlquist.com> wrote:
>>
>>>
>>> On 10/15/2018 10:43 AM, Tree wrote:
>>>
>>>> I replaced the variable cap with a home made cap using RG8.  I had one
>>> Sometimes, capacitors made from coax are lossy.  I modeled your
>>> coaxial capacitor using Simsmith (very easy to do) and the Q turns out
>>> to be 340, if I did it correctly.  So the coaxial capacitor gets a clean
>>> bill of health and probably doesn't have anything to do with
>>> your broad bandwidth.
>>>
>>> Rick N6RK
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