On 7/7/2011 5:33 PM, David wrote:
I have a Heathkit SB-401 transmitter I refurbished a few years back
that exhibits a couple of peculiarities during tune-up:

1) First, on 80 meters (and only 80 meters) when I tune the final
amp’s plate circuit, the plate current dip point does not correspond
to maximum output.

2) Second, I never read any grid current during tune-up. However, the
manual states that I could see as much as 0.5 mA during tune-up.

I’ve check the components in the final amp, to the extent I can
without un-soldering anything, and found no problems. So I’m
wondering if I have flaky tubes in the final that can’t be neutralized
(problem 1) and draw negligible grid current (problem 2). The tubes
are 6146s and one slightly glows blue during key-down in CW.

This really isn’t problem--just a nag. I monitor my on-air signal with
an SB-610 monitor scope and the output looks fine.

-Dave, K3WQ

The blue glow on the glass is from electrons striking the glass. That is
not a problem. If the glow is inside the plate (between the plate and
grid) that is gas.

In my experience, the tuning problem is from not completely neutralizing
the amplifier.

Bill K7NOM