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. 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.
Another thing to remember is that there is a wide range of 8122 bias
tolerances; typically from about -23V to over -40V for the same idling
current. The "hotter" tubes are more prone to instability.
. . You tried a shunt screen regulator
>to
>protect the delicate 8w screens and it does not work?.
I use G3SEK's regulator circuit in my 6M NCL-2000. It works fine and
improves 3rd and 5th IMD by 5 and 6 dB with no other changes. Higher
order crud is down by an even greater degree. The Brits have really
pushed tetrode IMD performance due to the widespread use of 4CX250 type
tubes on the VHF/UHF bands in Europe.
You tried the
>simplified grid bias circuit that eliminates delicate parts and it
>doesn't work either?.
No need to bother, simply change the delicate germanium bias transistors
to a 2N6246 or similar and the Zener to a modern 1W and you will never
have a failure again....been doing that mod for about 16 years. No need
to re-invent the wheel.
>
>>If you have recent upgrades I would also appreciate a copy.
>>
>>> With some changes and run in AB1 instead if AB2, the NCL-2000 is
>clean
>enough to
>>>avoid being tarred and feathered at the next DX convention.
>
>>
>>It takes nothing except resetting the bias AND watching the grid
>meter to
>>make the NCL an almost clean amp with -28dB 3rd IMD
>
>Agreed, Carl. Another method of assuring that no grid current flows
>would make it easier on the eyes. . The RCA xmtting tube manual
>tells
>how to make AB1 operation with the 8122 even mo' betta.
>
>> even with the
>>unregulated screen supply. Adapting the G3SEK screen regulator
>
>Is this a shunt regulator?
Of course it is and it was discussed here at length last year. See Oct
1997 QEX for a very indepth description and analysis. You can order PC
boards from Ian or a complete kit from DownEast Microwave.
73 Carl KM1H
>- later
>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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>
>
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