RADIO-----Drake L-4B-----SWR METER-------Dummy load

Now the SWR stays low when I turn on the Drake L-4B for both the 1 KW
dummy load and the external antenna.

The transistor uses MRF455 transistors. Why would the SWR meter go up
significantly when I turn on the 2 transisor amp, but stay low when I
turn on a Drake L-4B.

If I had to guess, I'd guess that your MRF455 amplifier may have some
instability issues, and may have a nasty parasitic oscillation going
on. You might also want to check its grounding, and the RF relay
which I presume switches it into or out of the circuit. I'm guessing
that you've got RF flowing where it shouldn't.

I picked up a few MRF455 transistors last year and have toyed with the
idea of using them as a PA in a homebrew rig, but from what I can see
in the application notes they require a goodly amount of care in the
amplifier design and circuit layout. If they aren't used properly it
looks to me as if they can be quite prone to misbehave, oscillate, etc.

If you can borrow a spectrum analyzer, take a careful look at the
amp's output, and wave a probe around the amp itself (prior to the
low-pass output filter). It would not surprise me if you find that
you're spurring the horse a bit :-)

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!