[Amps] Checking for IMD

[Colin Lamb]

I have been thinking, the last few days, that I should clear off a spot on the desk and put my spectrum analyzer and amp on a clean desk and make some tests to see what the input circuit does.

I have an old Swan Mark II, which did not come with tuned input.  I added it underneath, with small relays and toroids and an additional switch deck.  Since it runs 2 x 3-400 tubes, it would be similar to many of the amps out there.  It would be relatively easy to switch in and out the input circuit.

As I began thinking about that, it occurs to me that any meaningful test would not be that easy.  As soon as I remove the tuned input, the load for the exciter changes substantially, which means impedances and drive changes.  Those things alone could change the imd sufficiently in the exciter to show a few db difference in the imd of the amplifier.  

That would imply that a lot more testing would be required to understand what is being tested.  Perhaps one of those class A transceivers would be useful as a driver.

Maybe I should just wait until evening and open a beer and forget the whole thing.  A test run poorly is not as good as a beer drunk poorly.

73,  Colin  K7FM