[Collins] New historical information on 6146 vs 6146A

[Dr. Gerald N. Johnson]

At the Des Moines hamfest this morning, I met with W0WL, Warren Amfahr.
He was on the design team for the KWM-1.

Before there was a KWM-1, Warren was running a home brew crystal
controlled 75 meter rig with a pair of 6146 in the PA. One morning he
and Art Collins arrived at the same time while Warren was working a
contact across the country. Art asked him what rig he was running. In
time Warren was bragging about how good the 6146 was as a linear tube
and was put on the KWM-1 design team to make a low distortion PA using
the 6146. With measuring tools at hand he found the best he could do was
-25 dB 3rd order intermod and Collins wanted -35 dB. He worked and he
worked, and after some time his boss demanded he contact RCA about the
tube. Eventually he did and RCA told him flat out that it wasn't a
linear tube, it was a class C tube. RCA was reluctant to modify the
tube, but he pointed out Collins expected to sell some 10,000 radios
each with two tubes, and each ham would have at least one spare set
leading to significant tube sales. In that half hour long distance
conference call with several tube designers, they worked out revisions
to make it linear which they called the 6146A. And with 12 Db of RF feed
back Warren achieved the desired -35 dB third order intermod.

That would say we should not use 6146 but use 6146A for best linearity
in Collins rigs. We didn't get into the 6146B, that was probably a tube
Warren didn't have the opportunity to test.

Its a shame that modern radios miss that -35 dB intermod specification
by so much.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer