[AMPS] SS750 output -- more tests

[Steve Thompson]

-----Original Message-----
From: Vic Rosenthal <rakefet@rakefet.com>
To: Amps reflector <amps@contesting.com>
Date: 08 May 2000 03:33
Subject: [AMPS] SS750 output -- more tests


>
>I'm still trying to figure out why I can only get about 600 watts output on
14
>MHz from my Henry SS750 amplifier (8 x 2SC2879).
>
>I measured the output into the dummy load with various levels of drive,
from 10
>to 100w in 10 w steps.  I found the following relationship:
>
>In     Out
>---    ---
>10     113
>20     216
>30     305
>40     384
>50     427
>60     482
>70     520
>80     540
>90     570
>100    601
>....
>150    687
>
>Obviously, the gain drops as the power increases.  Is this normal?
>
>I did some research, and the 2SC2879 is rated at 13 db gain (minimum) at 28
MHz.
>with only 12.5v on the collector.  The attenuator that's in the Henry
circuit on
>14-28 MHz is only 2 db.  That should leave 11 db gain.  Allowing for 2 db
of
>loss in the splitter, combiner, and filter, that leaves 9 db, which should
>produce 800 watts with 100 watts drive.  One would expect even more gain
with
>13.8v.
>
>Something isn't right...I think.  I am not a solid-state guy  -- help!


A couple of thoughts;

Firstly, the gain spec. is usually checked using a single frequency tuned
amplifier circuit - the tuning might well be tweaked for each individual
transistor to ensure maximum yield in production. It's unusual to get the
same gain in a broadband circuit, where there are compromises.

It's entirely normal for the gain to drop as the output increases. This is
affected by both the device and the circuit. For neighbour friendly SSB
operation, don't go beyond where the output is 10% below what you would
expect out if the gain didn't drop off.

In a SS amp. like this, there's no tuning adjustment available - the
collector load is fixed by the matching circuits and whatever you hang on
the output connector, and your output power figures could easily arise from
the load line being wrong (knowing the supply current figures would help
check this). Batches of transistors vary and a circuit designed around one
set won't always suit another batch, especially if the design dates back
many years. Manufacturers sometimes switch the die they use in transistors -
as long as is passes test in their tunable circuit, it meets their spec. but
it can mess up fixed tuned amplifiers a treat.

Try running it through an ATU and see if you can coax more power out, then
look again at the linearity.

Steve


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