[Boatanchors] Split stator cap voltage

[Barrie Smith]

Mac:

Thank you for a much better explaination than the handboods give!

73, Barrie, W7ALW
----- Original Message ----- 
From: "mac" <w7qho at aol.com>
To: "Barrie Smith" <barrie at centric.net>
Cc: "Boatanchors Mail List" <boatanchors at mailman.qth.net>
Sent: Monday, July 26, 2010 9:30 PM
Subject: Re: [Boatanchors] Split stator cap voltage


> 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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