[ARC5] Utility Amp Driving Advice.

[David Stinson]

Many of my light-air radios output under five watts- some less than 2.
They need a little boost on 75 mtr AM.
I used a Motorola Triton 40S marine SSB rig as the heart 
of a utility linear amplifier.  Works well on the air
on 160, 80, 40 and 30 mtrs (the rig is speced to 12 MC).

Here's a couple of photos:

http://home.netcom.com/~arc5/amp1.JPG

http://home.netcom.com/~arc5/amp2.JPG

I tapped into the driver input and brought-out 
the receiver antenna lead.  The rig is rated at 125 W PEP SSB.
I cranked it back so that the unmodulated carrier is 40 watts, 
which is serviceable on good days and 
can drive a bigger amp if needed.
RF envelope at the output while modulating 
looks excellent and reports are good.

The drive level needed for full output 
is just 10 milliwatts into 50-ohms.  
I terminated the RF Input with a 50-ohm, 3 dB pad.
I'm building a step-attenuator so I can drive it with
anything from 7 watts (my SCR-288) down to 
sig-gen levels.  

I want to protect the input so I don't blow-up the driver.
If I calculated correctly, 10 milliwatts in 50 ohms should 
be 1.414 Volts PTP.  If so, a simple set of four
"up" and "down" ( or "left-n-right"  ;-)  silicon diodes-
each leg with two in series to  to ground, which 
will forward bias at 1.4 volts and limit the input
to 20 milliwatts, should do the trick.
At lower drive levels, they should be "invisible."
At high levels, they'll clip and distort.  
And when I screw-up and pop 40 watts into it,
they should short and save my amp.

First- did I get the math right?
Second- what blatantly obvious thing am I missing?

Thanks, Dave S.  AB5s