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 16Øm doorknob padder should have a higher ampacity."
From what I see of the schematic, C4Ø: 5ØØpF/5KV is type 85Øs.
:: "The two 8Øm doorknob padders are prone to failure."
They are listed as C41,42: 1ØØpF/5KV, type 858s.
:: "The Plate Blockers are too light."
They are listed as C2Ø,21: 1,ØØØpF/5KV, type 858s.
:: "The Plate Choke RF bypass capacitor is prone to failure."
It is listed as C35, 1,ØØØpF/5KV, type 858s.
Observed: The additional bypass caps on the plate circuit are
2,ØØØpF/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.Ø plate amperes on the 'Sx
version with two 3CX15ØØA7 finals, then is not L9, at 1.Ø 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 16Øm and 8Øm 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:3,9ØØVDC)
(Vac.Var. changing to 7~1,ØØØpF/7.5KV)
Thank you for your opinions.
Hal Mandel
KA1XO
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