[Amps] ALPHA 77 RF POWER CAPACITORS

[R.Measures]

**  Tnx, Frank.  What OS are you using?
--  According to ITT-Jennings, their small 500pF and 1000pF doorknob 
capacitors are for bypassing and coupling applications only because they 
use a temp-unstable dielectric.  For tuned-circuit padders, one needs to 
use N-750  100pF or 200 pF, 7500v units, paralled if necessary for more 
C.   In commercial MF amplifiers, I have seen two square aluminum plates 
with up to 16, N-750 doorknob caps in parallel.  
--  In the Alpha 77, not only are the wrong type dielectric capacitors 
used for tank padders, the tank circulating current they are subjected to 
is above ITT-Jenning's current ratings.

cheers

>Replace each "strange" char with a "0". Then you get "500 pF". 
>But I'm sure you knew already ;-)
>
>Another one of that character encoding sh.. !
>
>73, Frank
>
>
>On Thu, 2004-01-08 at 13:33, R.Measures wrote:
>> **  How much C is:  5ÿÿpF?
>> 
>> >
>> >Gentlemen,
>> >
>> >Not too long ago in this reflector a discussion was had about certain RF 
>> >power capacitors being prone to failure in the ALPHA 77Sx amplifier, more 
>> >specifically thus:
>> >
>> >  :: "The 16ÿm doorknob padder should have a higher ampacity."
>> >       From what I see of the schematic, C4ÿ: 5ÿÿpF/5KV is type 85ÿs.
>> >
>> >  :: "The two 8ÿm doorknob padders are prone to failure."
>> >       They are listed as C41,42: 1ÿÿpF/5KV, type 858s.
>> >
>> >  :: "The Plate Blockers are too light."
>> >       They are listed as C2ÿ,21: 1,ÿÿÿpF/5KV, type 858s.
>> >
>> >  :: "The Plate Choke RF bypass capacitor is prone to failure."
>> >       It is listed as C35, 1,ÿÿÿpF/5KV, type 858s.
>> >
>> >  Observed: The additional bypass caps on the plate circuit are
>> >            2,ÿÿÿpF/6KV disk ceramics. Two of these parallel a
>> >            15uH, 1 amp RF choke, L9.
>> >- - - - - - - - - - - - - - - - - - - - - - - - - 
>> >Questions:
>> >    1. What are your recommendations for upgrading the reliability
>> >       of the capacitors above? Several issues arise: E and I ratings,
>> >       and the best type: HT50, HT58, HT57...?
>> >
>> >    2. If the amplifier is capable of 2.ÿ plate amperes on the 'Sx 
>> >       version with two 3CX15ÿÿA7 finals, then is not L9, at 1.ÿ amp
>> >       prone to failure from over heating?
>> >
>> >    3. The capacitors paralleling L9, the disk ceramics, are rated
>> >       at 6KV. However, should their value of capacitance increase?
>> >
>> >    4. The 16ÿm and 8ÿm padders seem kind of light in capacitance for 
>> >       the new output impedance of two 8877's. Should not their value
>> >       increase dramatically to affect the tank's Q?
>> >(Reference: HV:3,9ÿÿVDC)
>> >(Vac.Var. changing to 7~1,ÿÿÿpF/7.5KV)
>> >Thank you for your opinions.
>> >
>> >Hal Mandel
>> >KA1XO
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>
>