[AMPS] tuning capacitor & insulator

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Fri, 18 Dec 1998 09:07:20 +0000

Rich Measures wrote:
>>Rich Measures wrote:
>>>>and last ,i'm looking for a high power amp
>>>>for 2 meter ssb ( 500 + out put) any body
>>>>want to sell one?
>>>A pair of 4cx250Rs in parallel, self-contained, designed and built by
>>>yours truly.  20db gain.  Class AB1.  About 1000pep watts out on SSB.
>>>designed for10w pep drive.  The built in 2200v power supply is not
>>>suitable for FM, RTTY or A0.  $100 plus ups.  120v/20a.  Must be tuned up
>>>with a keyer sending 50wpm dits, or with a tuning pulser.
>>The gain is right - and by goodness the price is certainly right -
>>but expect no more than 500W PEP of *linear* SSB. Otherwise expect to
>>lose a lot of friends.
>  The 7580/4CX250Rs  have 400v on the screens.  According to Eimac's
>curves, c.1.3 peak amperes of anode current flows when the grid potential
>is 0V and the screen potential is 400V.  The 1.3a constant-current curve
>appears to be reasonably linear up to 0V on the grid.  By my calculation,
>the pep input is around 1850w.
Certainly you can get the raw power out - but it is impossible to tell
what the IMD levels will be by eyeballing a constant current curve. Even
a 1% deviation from a straight line makes a lot of IMD.

The real test is on the air. Believe me, we have had a LOT of experience
over here with people who think they can get the UK limit of 400W PEP
SSB out of a single 4CX250-something, or 4CX350... and it has all been
BAD experience from the viewpoint of neighbors on the air! 

Eimac must take a lot of the blame for this. Their "typical" operating
data for the 4CX250R claim 470W PEP output - but with 3rd order IMD at
only -23dB, and 5th order at only -27dB. Can you imagine what the
higher-order IMD looks like? By any reasonable standards for coexistence
on the air, that SSB signal is vile! 

Very extensive practical on-air testing has shown that 250W per tube is
a pretty good limit for acceptable linearity. At this power level, the
tubes will produce a much cleaner signal than at those greedy Eimac

>>No criticism of Rich's particular design; the limitation is in the tubes
>  Did you perhaps presume that the 7580 has the same power capability as
>a 4CX250B?

The raw power capability of the 7580/250R is obviously greater than the
250B; but the LINEAR power capability is much the same - see above. The
advantages of the 'R in this context are a little more power gain, and
more comfort when operating at very similar levels to a 'B.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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