Many amps use Tune caps rated at 500-1500V higher than their unloaded
voltage. The Heath SB-220 is about 600V difference and is known to arc
under some tuning conditions. Some of this arcing voltage can be snubbed
resulting in a very stable amp.
Other amps using a bit more overhead ratings such as the 8874 Alpha
series, MLA-2500 and similar dont arc under normal use.
Most USA built amps since around 1972 use variable capacitors that have
removable plates and all come from a single manufacturer so there is
pretty much complete interchangability. Individual plates or complete
caps can be ordered. Ten Tec is an exception.
Carl
KM1H
----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: "Ulf Tjerneld" <star@frizon.org>
Cc: <amps@contesting.com>
Sent: Sunday, May 17, 2009 12:39 AM
Subject: Re: [Amps] first time questions
> ORIGINAL MESSAGE:
>
> On Sat, 16 May 2009 17:38:44 +0200, Ulf Tjerneld <star@frizon.org>
> wrote:
>
>>
>>2, I have a perfect capacitor for the Pi-tank in the above example.
>>But is rated exactly 2500 volts (Johnson something...). Would that be
>>to little for an unloaded plate voltage of 2500 volts?
>
> REPLY:
>
> The short answer is no, that is not enough voltage rating and here's
> why: When everything is tuned up correctly and not overdriven, the
> peak RF voltage will be almost exactly equal to the DC voltage. The
> problem is, there will be times when it is not quite tuned right, i.e.
> there is too much drive for the amount of loading, or you might QSY
> and forget to retune, or you accidentally select the wrong antenna,
> etc, etc. If any of those things happen, the capacitor will arc and
> very likely will be permanently damaged. You didn't say whether the
> cap is an air or vacuum variable but it doesn't really matter. Once a
> capacitor arcs, the surfaces become pitted and become even more prone
> to arc the next time. I would never use a cap rated less than twice
> the DC voltage, and more is better. In my current amp with 2900 VDC
> under load, I have a 15 kV cap. Some would say that is overkill, but
> it has never arced and probably never will, no matter what dumb thing
> I do.
>
> 73, Bill W6WRT
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