-----Original Message-----
From: Vic Rosenthal <rakefet@rakefet.com>
To: Amps reflector <amps@contesting.com>
To: <amps@contesting.com>
Date: 09 May 2000 03:36
Subject: [AMPS] SS750 20 meter test and (maybe) explanation
>
>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
>10 142 30
>20 262 40
>30 368 48
>40 420 52
>50 460 55
>60 500 58
>70 540 62
>80 560 64
>90 590 65
>100 611 66
>...
>170 721 72
Looking at the gain and efficiency figures, it is behaving normally for an
amp where the collector load impedance is too high.
>
>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.
Yep - although it's not just the filter - it's all the matching between the
transistor terminal and the output connector.
>
>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!
While I have little faith in power and SWR meters sold on the ham market, I
think that level of inaccuracy is unlikely at 1.5:1VSWR.
>
>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.
It appears that there are some versions that work ok, and at least one other
that goes like yours. It might be that there was a production change of
circuit or component type that messed things up. A change of material in the
iron/ferrite cores would be a good candidate. Unfortunately, if the finished
unit meets a minimum cw output spec., many manufacturers won't care about
anything else :-( some don't even worry about the cw spec. :-(((
Steve
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|