[Elecraft] OT: modifying your amp for lower drive requirement

[Scott Ellington]

It's very unlikely any solid-state amplifier uses an input attenuator:  Transistors and FET's just don't have that much gain.

Grid-driven tube amplifiers may indeed have enough gain (without the input pad), but there are a couple potential problems.  Without the input pad, the amplifier may not be stable and/or its input impedance may not be anywhere near 50 Ohms.  If that isn't a problem, spurious outputs may be.  Spurious outputs from a QRP transmitter that cause no trouble barefoot might be way above the FCC limits when amplified by 25 dB.

73,

Scott  K9MA


On Dec 20, 2010, at 4:09 PM, DOUGLAS ZWIEBEL wrote:

> Almost 30 years ago now, before I was 99.9999% a QRPer, I had an Alpha
> 76CA (3 tubes).  It had a giant swamping pad on the input (I still
> have it - the swamping pad, that is).  Just take it out and like
> magic, you could drive the amp with QRP.  My intention was never to
> exceed legal limit, just to be able to use the amp when driven with a
> QRP rig. I don't remember how much out I could get with QRP in, but it
> was nowhere near 1500 watts.
> 
> I have no clue what they have on the inside of the KPA500, but if
> there is a swamping pad, you can probably take it out.  What is
> Elecraft's motto?  Hands-on radio?  :-)
> 
> When I used to use a Ten Tec Titan Amp, it required 36 watts of drive
> for 1500 watts out on 20m.  That was probably 25 years ago.
> 
> Finally, if you want to fly under the radar, there are certain high
> quality amplifier manufacturers overseas (closer to W6 than W2...hint
> hint) who will custom make you an amp for low power drive.  Check the
> FT817 Yahoo group archives.
> 
> de Doug KR2Q
> had my fill of QRO ages ago
> ______________________________________________________________
> 

Scott Ellington
Madison, Wisconsin
USA