[AMRadio] Ham mods (was Re:SP600 Question)]

Todd, KA1KAQ ka1kaq at gmail.com
Wed Sep 16 07:54:35 EDT 2009

My recollection is that the Valiant was also limited by the design of the
iron used in the audio driver stage. But as mentioned, back in the day
'punch' was the name of the game, not smooth fidelity. This can clearly be
seen also in the venerable Collins KW-1 which, while perhaps the overall
highest quality transmitter of the day has several weaknesses in areas like
the audio driver circuit as well as restricted audio overall. Interestingly,
the models in roughly the first half of the run had nicer audio, but
apparently Art wanted the transmitter to cut through the QRM (heterodynes)
better so some simple changes were made, like tightening up the values of
coupling caps. Just going through and fattening them up helps out
tremendously, as does swapping out the grid input resistor in the mic amp
(1M) for a 4.7M or higher value. Fatter electrolyitics will help out with
the lows as well, but at the end of the day it will take a rebuild of the
audio driver (6B4G x 2) to 4 tubes and some other tweaks to really drive
those 810s. Not worth the bother IMO. Easier to find or build something else
and enjoy the old rig as it is with just the simple changes to smooth things
out, including bypassing the clipper.

~ Todd,  KA1KAQ/4

On Wed, Sep 16, 2009 at 7:34 AM, Bry Carling <bcarling at cfl.rr.com> wrote:

> Charles - that would probably be due to the nasty 6AL5 clipper circuit
> used in the Valiant.
> It would affect the audio badly even when turned all the way down.

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