[Boatanchors] Split stator cap voltage

[mac]

Barrie,

The voltage rating of your  split stator cap is fixed and is  
determined by the spacing between the rotor and stator plates.   
Different plate tank circuit configurations will result in different  
voltages being APPLIED to the capacitor but the voltage RATING of the  
capacitor itself will not change.  Presume you will be using a push- 
pull circuit configuration in your VT-127A PA.

(1) One possible configuration here is to feed the B+directly into the  
center tap of the tank inductor, bypass this point to ground and use a  
non-split stator tuning cap across the inductor.  This capacitor would  
have to be insulated from ground, of course but no HV DC would be  
applied across it, only the peak RF voltage which under modulation can  
be taken as being twice the DC supply voltage.

(2) Another popular configuration had the B+ being applied to the  
center tap of the tank inductor through an RF choke and a split stator  
tuning capacitor connected across the inductor with the rotor  
grounded.  In this configuration the capacitor must have spacing  
sufficient to withstand the peak RF voltages in the tank circuit PLUS  
the peak voltage coming in from the power supply and the modulator,  
i.e., 4 X the plate supply's DC output voltage.

(3)By connecting the rotor to the incoming B+ (plus modulation) line  
instead and bypassing the rotor to ground through a fixed cap, the  
voltage stress across the tuning cap plates is reduced back to only  
the peak RF values.  Now, the tuning cap must now be insulated from  
ground, of course, with an insulated shaft coupling included between  
the cap and the tuning  knob on the front panel  of the rig.  (B+ and  
modulation also feeding CT of tank coil as in (2) above and the bypass  
cap must have a voltage rating of at least  2 X the DC supply voltage.)

All this covered very well in the older handbooks, my 1948 ARRL  
edition for ex.

Hope this helps.

Dennis D. W7QHO
Glendale, CA



On Jul 26, 2010, at 4:56 PM, Barrie Smith wrote:

> Hello:
>
> I'm getting very close to completing a push-pull pair of VT-127As  
> for 10 and 15M, only.
>
> The split capacitor I'm using is a Millen right-angle, center-drive  
> rated at 6000 volts.
>
> This deck will be AM modulated, so I can likely use only 1500 volts B 
> +.
>
> Unless!
>
> According to the older handbooks, if I bypass the rotor to ground  
> for RF, the voltage rating for the cap will increase.
>
> None of the books I have say how much it will increase.
>
> Also, the handbooks show a method of increasing the voltage rating  
> by directly attaching the B+ to the rotor, or attaching the B+ to  
> the rotor through (any) RF choke, or through a 47K resistor.
>
> Again, no specific number is mentioned.
>
> Does anyone know how much voltage increase I could expect by using  
> one, or the other, or both of these methods?
>
> TNX, 73, Barrie, W7ALW
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