[Elecraft] OT antenna question

[Al Gulseth]

I should clarify that I was thinking in terms of ~ 100W. There's always the 
old standby of Jennings vacuum variables for QRO use....

73, Al

On Thu January 14 2016 9:44:04 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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