[AMPS] tuning capacitor & insulator

Rich Measures measures@vc.net
Fri, 18 Dec 1998 08:56:03 -0800

>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! 

However, what one sees on the S-meter is  6db better than Eimac's 2-tone 
imd test indicates.  Another factor is how the amplifier is loaded.  On 
my 2, 7580 2m amp., I load it for c.5mA of total screen current.  The net 
result is that, according to neighbors, the amplifier is not noticably 
more filthy than the Kenwood SSB transceiver that drives it.  
>Very extensive practical on-air testing has shown that 250W per tube is
>a pretty good limit for acceptable linearity. 

At what screen and anode potentials?  To realize the full SSB-mode 
ability of a Tetrode or Pentode, one can not be stingy with these 
potentials.    For example, if the screen potential of a 7580 were 
reduced from 400v to a more conservative 250v, instead of 1.30 peak 
amperes of anode current (@ 0 grid v), 0.6a would be realized.   However, 
there is a "gotcha": . . .  If a screen potential of 400v were used in 
FM.mode or 
A0.tuneup.mode, the max. continuous plate/anode current rating of 0.25a 
could easily be exceeded --  and the cathode's emissive barium/strontium 
oxide coating brought to tearful ruin.   

>At this power level, the
>tubes will produce a much cleaner signal than at those greedy Eimac ratings.
I see no significant difference in the linearity of the 7580 
constant-current curves for 250 screen V compared to the curves for 400 
screen V.  

>>>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.
The realization of its higher SSB power ability is tied to taking 
advantage of the higher screen potential capability of the 7580.    

-  later, Ian


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

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