Mike Silva wrote:

Chuck Harris wrote:

Hi Mike,

Neutralization is more than just interelectrode capacitance. It is
also affected by plate to everything outside the tube capacitance, and,
of course, the layout of the transmitter finals cage.

The plate of the 6146B is bigger than the 6146, or 6146A. It has about
33% more plate dissipation as a result. There is more coupling between
this bigger plate, and the outside world.


I don't have an example of both tubes in front of me, but I don't
remember the plate structure of a 6146B being larger than a 6146(A).
Can anybody confirm that the 6146B plate structure is bigger? I
thought the higher plate dissipation was due to some combination of
different plate material and/or thicker plate material.

Is it possible that the Collins neutralizing capacitor range was
marginal to begin with, and that the higher power that the B tubes were
capable of just brought the problem to the surface? Just wondering...

73,
Mike, KK6GM


Hi Mike,

Collins went from a 7-50pf, to a 1.5-8.7pf. They made a few other tweaks
in the neutralization circuitry too. I have worked on both pre, and
post neutralization modification KWM-2's, and I can tell you that when used
with the proper tubes, the caps in either end up in the center of their range.

The KWM2A that is in my station right now has 6146W's, and the 1.5-8.7pf
neutralization cap, and the cap is in the center of its range. The heaters
on my 6146W's barely glow, so they are probably 6146A's. I don't recall,
are the B's dark heater too?

You get the same power out on the KWM-2 with any of the 6146 family, so I
don't think excess power output is the problem.

Collins said that the original neutralization circuitry tended to burn up
with the 6146B tubes. I'm willing to trust Collins to know the why's and
wherefores.

You can view the complete Heathkit SB101 ECO notes somewhere on the net. Heath
said there is something different about the 6146B's that requires changes
in the PA section. [ They also had a tremendous problem with variations in
other tubes made by different manufacturers. They out and out banned certain
manufacturer's tubes from use in their rigs.]

That the 6146B is different isn't just the rumblings of some flaky ham.
Collins and Heath made the necessary changes shortly after the B came out,
and long before I ever owned a rig using a 6146.

-Chuck