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Re: [Amps] Fabrication & part placement

To: amps@contesting.com
Subject: Re: [Amps] Fabrication & part placement
From: Manfred Mornhinweg <mmornhin@gmx.net>
Date: Fri, 18 Apr 2008 16:26:57 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Colin,

you forgot the DC voltage at the plate in this:

> If you look at the plate tuning capacitor, you will see what the
> minimum spacking needs to be between the top of the tube and the
> cabinet.  If it does not arc in the cap, it will not arc to the
> cabinet.

In truth, at the tuning capacitor you just have the RF voltage, while at 
the plate you have the DC voltage added to the same RF. So you would 
typically need twice the capacitor's plate spacing between the plate 
connector and the cabinet, to avoid arcing.

 >  Remember, however, that you 30 pound cat can jump on top of
> the top and deflect it a bit, so allow that.

Yes, true, unless you have a stack of five more amps sitting on the poor 
lowest one, like some people do! That keeps the cat from jumping onto 
the amps, except perhaps the top one, if there is still space between it 
and the ceiling. However, the five more amps on top of the lowest might 
perhaps eventually also cause some slight deflection of the top of the 
lowest one, so you are still right, a safety factor is needed!

In practice, however, the separation between the cabinet and the high 
voltage RF carrying parts, such as tube plate, tuning cap, tank coil, 
choke, and coupling cap, are usually dictated by the need to minimize 
stray capacitance, rather than by a minimum spacing to avoid arcing. 
When using high impedance circuits at 10 meters or even higher, this can 
be very important. Just a few pF of additional stray capacitance can 
cause the impossibility of tuning to the amp's highest band! This factor 
should be analyzed before moving any of these parts closer to the 
cabinet than they were.

Manfred.

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