On 07/07/2011 08:33 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
A pale blue glow can be a normal ionization glow, doesn't mean gas.
The tank circuit on most rigs for 80 meters tends to be low Q which
means it will tune broadly. If the the plate current dip and max power
output doesn't match it could mean a miss-match between your antenna and
the rig, IE: high SWR. The tank circuit on the '401 is designed to
match between a 50 ohm load, stray too far from that and the tank
doesn't tune correctly, 80 meters is the worst in this regard due to the
limited range of the variable caps on that band.

No grid current? check your bias setting, driver tube OK?