[Elecraft] OT antenna question

[Dave Olean]

I bought a bunch of those Russian TX caps. Mine were 680 pf and looked like 
the 7.5 KV Centralab ceramic TX caps with threaded holes on each end. I put 
two in parallel and used them for the plate blocking capacitor in a 160 
meter KW triode amplifier. Big mistake. They cannot handle much current at 
all. When I keyed the amp, I saw the power output drop very fast. Those caps 
got very hot and failed miserably. I complained to the eBay seller and he 
advised that they were only for use as bypass caps and should not be used 
directly in the TX line. (Silly me!) Their heating cycle reminded me of what 
happened with Vic's antenna system of late.
    I still have them. They might make good target practice subjects. Buyer 
beware.

K1WHS

----- Original Message ----- 
From: "Bob McGraw K4TAX" <rmcgraw at blomand.net>
To: <wb5jnc at centurytel.net>; <elecraft at mailman.qth.net>
Sent: Thursday, January 14, 2016 2:47 PM
Subject: Re: [Elecraft] OT antenna question


> When I updated my 2 tuners, I used some doorknob style capacitors, of 
> Russian source I presume,  which I purchased via E-Bay. Something like 5 
> for $20.    These were 470pf units rated at 16KV.    Don't be mislead with 
> the voltage rating, as it the current handling ability being the key for 
> RF usage.
>
> In most all cases one will need to provide a somewhat different mounting 
> as they have screw terminals on either side and are larger physically.  I 
> just happen to have one in the desk drawer. Typically 1.0" dia x 0.75" 
> tall with, guessing, #6 or #8 screw terminals on either side.  Could be 
> Metric screws as I don't recall and can't tell without my thread gauge.
>
> Another approach is to divide the total current  by using 3 or 4 caps in 
> parallel,  if you wish to use disk ceramic caps.  Thus if your circuit 
> requires 330 pf then use 3 pcs of 100 pf or 3 pcs of 120 pf in parallel 
> and keep the voltage rating the same as the originals.  This way the total 
> current is divided between 3 devices as opposed to one device.  This also 
> works.
>
> In various transmitter updates, I have used dipped silver mica caps. 
> These work well.  Just keep the voltage rating the same as the originals. 
> And it won't hurt to use 2 or 3 units parallel by taking the original 
> capacity value and dividing it  by 2 or 3 for the pf value.    This 
> approach was used in restoration of some of my old boat anchor AM 
> transmitters.  These caps are more stable, meaning less tuning drift, thus 
> work much better than the original disk ceramics units.
>
> 73
> Bob, K4TAX
>
>
>
> On 1/14/2016 8:27 AM, Al Gulseth wrote:
>> Bob,
>>
>> Are silver(ed)/dipped micas (CDE etc.) the preferred type for a tuner, or 
>> is
>> there a superior newer technology available?
>>
>> TNX/73, Al
>>
>> On Wed January 13 2016 2:52:52 pm Bob McGraw K4TAX wrote:
>>> If your tuner uses disk ceramic caps, as many do, these can be heating 
>>> thus
>>> not being able to handle the RF current.  They heat and cool and change
>>> value which in turn changes tuning.
>>>
>>> Replacing them with suitable RF current rated units is the solution.
>>>
>>> Bob, K4TAX
>>>
>>>
>>> Sent from my iPhone
>>>
>
>
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