[AMPS] Question on reforming caps NCL2000

[km1h@juno.com]

On Sat, 12 Sep 98 18:44:33 -0800 Rich Measures <measures@vc.net> writes:
>
>>
>>
>>On Fri, 11 Sep 98 13:23:18 -0800 Rich Measures <measures@vc.net> 
>writes:
>>>
>>>>
>>>>
>>>>On Fri, 11 Sep 98 08:20:52 -0800 Rich Measures <measures@vc.net> 
>>>writes:
>>>>>
>>>>>
>>>>>The designers rather obviously did not bother to read the RCA 
>>>>>Transmitting Tube Manual.  In stock config., the amp. is 
>potentially 
>>>
>>>>>an 
>>>>>8122-screen-assassinator.
>>>>
>>>>
>>>>No argument...what was designed was not what Mike Ferber ( National
>>>>Marketing VP) forced on the market.
>>>>
>>>>
>>>>
>>>> > If you are interested, I can mail you a copy 
>>>>> of a schematic with needed changes.
>>>>
>>>>I tried them...they dont work. 
>>>
>>>You tried the RCA Transmitting Tube Manual recommended cathode 
>>>feedback 
>>>resistors and they don't work?
>>
>>
>>That is correct....they do not work in the NCL2000's I tried and the 
>amp
>>goes wild on 10/15M. The problem can be traced to the marginal screen
>>bypassing that is used, only 500pf per pin coupled with the 2W 
>cathode
>>resistor lead lengths. 
>
>Are you saying that was with resistors supplied by us?    


Who is "us" ?  I used 2W Allen Bradley carbon composition. This was 10-12
years ago remember.


>
>>Those tests were done about 10 years ago.
>>The amp can be made completely stable by using Johnson, Eimac, etc., 
>>4CX250 screen bypass rings but those items are becoming very hard to
>>find. If you note...RCA specifies those rings in their literature and 
>the
>>early NCL prototypes used them. Marketing nixed the idea as too
>>expensive.
>
>A mistake.  So is using a too-inductive resistor.

Allen Bradley MIL-Spec are not inductive at HF.

  So is using too 
>much 
>lead length.  


You have to use some lead length, I had them as short as possible.
I suspect that even with proper screen bypassing using the resistor in
the cathode method would lead to instability at VHF. I prefer the low
inductance copper strap to ground myself.

>.So is not using at least one resistor per filament pin. 


Why filament pins? The 8122 is indirectly heated and has 3 cathode pins


73  Carl  KM1H



>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  
>
>
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