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

[Fred Jensen]

On 12/20/2010 2:09 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.

Grid-driven vacuum tube amplifiers can exhibit much more gain than the 
grounded-grid triodes we are all very familiar with.  The problem is 
that, while grounded-grid amplifiers are "sort of" inherently stable 
[except for parasitics], grounded-cathode amplifiers are not.  Their 
input impedance is also variable and can be nowhere near 50 ohms.  The 
input pad corrected that and help stabilize the amplifier.

The [in]famous B&W "all band folded dipole" capitalized on this -- big 
50 ohm load resistor = 50 ohms everywhere.  I still see them around 
National Guard Armories.  At an AFB club station, we had a tribander 
that was 50 ohms everywhere.  Coax was full of water.  Needless to say, 
it was a lousy antenna.
>
> 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?  :-)

I'd be stunned if that was the case.
>
> 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.

The current FCC gain limit in the US for commercially manufactured and 
marketed linear amplifiers is 15 dB.  We can all do the math.
>
> 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.

See above if they are to be legally marketed.  And, that sounds pretty 
scary.  Many QRP radios have adequate spur suppression and phase noise 
... at their normal low power outputs.  However, their spurs and phase 
noise get amplified along with the desired signal.  Raise the phase 
noise by 15 dB [or more if the amp is illegal], and you're probably 
putting a lot of trash into your neighbor ham's receiver.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2011 Cal QSO Party 1-2 Oct 2011
- www.cqp.org