[AMPS] SS750 20 meter test and (maybe) explanation

[Bob & Linda McGraw K4TAX]

For what it's worth. 

I've looked at the figures regarding input vs. output.  The amp seems to
be providing some degree of compression in the form of non-linearity in
as much as the input to output gain appears to be changing with power. 
This would lead one to believe that the Vcc voltage is sagging under
load, perhaps at the collector of the device(s), and not the power
supply.

Here's what I see as to gain vs in/out power.  This is shown in Vic's
chart of numbers.


Vic Rosenthal wrote:
> 
> Here are the results of testing the SS750 (8 x 2SC2879) on 14 MHz. including
> collector currents.  Someone also asked what the bias voltage is; it is +0.66
> volts on all the modules.  Testing was into a dummy load whose SWR was < 1.1:1.
> 
> In    Out    Ic at 13.8v	Gain Db
> 10    142    30		11.5
> 20    262    40		11.2
> 30    368    48		10.9
> 40    420    52		10.2
> 50    460    55		9.6
> 60    500    58		9.2
> 70    540    62		8.9
> 80    560    64		8.5
> 90    590    65		8.2
> 100   611    66		7.9
> ...
> 170   721    72
> 
What this seems to say is that compression is not taking place but most
aptly the Vcc is droping at the device as current increases.

If the amp had a true 10dB gain figure then 10 in would produce 100 out,
50 in 500 out and 170 in 1700 out.  As they say "I don't think so!"

73
Bob K4TAX

> Oh yes, one more datum:  My 20 meter quad has an swr of 1.54:1.  At 100 watts
> into the amp with the antenna connected, I get 800 watts out with Ic = 75 amps!
> 
> Mu conclusion is that the impedance match provided by the output filter in the
> amp to 50 ohms is not optimal, but whatever impedance it is seeing in the
> antenna is transformed to something more comfortable for the transistors.
> 
> It has been suggested that the wattmeter (RF Applications VFD) is inaccurate
> when used with the antenna because the impedance it sees is not exactly 50
> ohms.  Perhaps this is true to some extent, but the 75 amp Ic reading is not
> changed by the SWR!
> 
> If this is true it's not a problem on 20 meters; but on 15 the antenna impedance
> is not so propitious, and I only get 530 watts into the antenna.  It looks like
> a solution would be to use a tuner (which would NOT be adjusted for exactly 1:1
> SWR).  Too bad; I went solid state to be rid of the need to tune up when
> changing bands.
> 
> In a sense, the 20-meter filter is 'customized' for my antenna, not 50 ohms!  I
> guess I could also 'customize' the 18-28 MHz filter for a better match to my
> antenna on 21 MHz.
> 
> 73,
> Vic, K2VCO
> Fresno CA
> 
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