[Elecraft] OT antenna question

[Bob McGraw K4TAX]

First I'd say 680 pf , two parallel, is not at all adequate for a 160M 
KW amp.  More like 4 would be expected for about 2800 pf total required 
for 160M  depending on plate Z.      Also with 3200 volts or so, again 
depending on the plate Z, 7.5 KV again is not much margin of safety.    
Ep x 4 is about right.

Just my comments and data from a Pi L calculator.

73
Bob, K4TAX



On 1/14/2016 9:44 AM, Dave Olean wrote:
> 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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