[Elecraft] External Amp for the K2

[Ron D' Eau Claire]

 > I am working on a project to see if I can design a "valve" amp for the
> K2.  I am following two tracks and it is really testing me.  I
> haven't done
> anything like this in 20 years so it has been a challenge.
>
> First design is a single 6146 tube - grid driven - Pi-L output.  About 90
> watts with 2-3 watts for drive.  I am planning on making it selective for
> Class A, AB1, AB2, and C service.

> The other track is a little more power using a single inexpensive 811A  -
> grid driven - in the same configuration as the 6146.  The only difference
> would be the plate voltage.  The 811A will give about 200 watts output in
> class AB1.  I am looking at designing different bias schemes as
> well.  Again, I want to make sure the amp can be built easily with simple
> hand tools.

> Lee - K0WA

Another fellow and I are collecting parts for an outboard amp using a pair
of 807's. It will run class C or AB1 by changing the bias. It is based on a
QST design from the '50's.

This is NOT the easy or cost-effective way to add power to the K2. We are
doing it strictly for the challenge and the fun of having something that
"glows in the dark" on the operating desk. One must be a bit of a nut and a
masochist to homebrew a tube amp from scratch. I qualify on both counts.

I gave some thought to the power level too.

Obviously the K2 can drive an amplifier to several hundred watts. The 10 dB
gain that commercial amps are limited to does not apply to homebrew designs.
It may be possible to get nearly the legal limit out of a single stage with
the proper design. However I decided to limit my amp to about 100 watts
output for cost reasons.

Reviewing the cost of power supply parts alone showed that I would have to
spend 3 or 4 times the money for a 2000 volt power supply for a higher
powered amp than I'd have to spend on the parts for an 800 volt power supply
to run the 807's.

Add to that the cost of more robust output circuit parts in the amplifier,
etc., and it didn't make sense to me. Especially considering the added
danger of RFI, etc. I like to be able to QRO when needed, but I haven't run
over 100 watts in 40 years.

Also, to be legal today, the amplifier has to provide better harmonic
suppression that a simple "pi-net" may provide. Those old designs were from
a day of much looser FCC requirements on harmonic suppression. The Pi-L is a
good option for the added harmonic suppression. I will be running a pi-net
based on a roller inductor to a 50 ohm output. That goes into an ATU to
provide additional filtering to be sure it is "legal" on the bands.

Ron AC7AC
K2 # 1289