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" <[email protected]> > To: <[email protected]>; <[email protected]> > 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 > > > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[email protected] > > > > This list hosted by: http://www.qsl.net > > Please help support this email list: http://www.qsl.net/donate.html > > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
