On Jul 12, 6:14*pm, "Antonio Vernucci" wrote:
Q1) 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.

R1) If this problem only occurs on 80 meters it means that, although the tubes
are correctly neutralized, some particular effect occurs just on 80 meters which
alters the zeroing of the neutralization bridge on that band.

The most likely cause could be a bypass capacitor having too low a capacity (the
reactance of a capacitor decreases with frequency). For instance, your screen
bypass capacitor may result to have enough capaciity for the 10-40 meters needs,
but not enough capacity for 80 meters. If so, neutralization on that band would
be upset. The same could occur due to a too low capacity cathode bypass
capacitor.

A possibility could be that one bypass capacitor has got "open" (at those times
it was not uncommon to put several bypass capacitors in parallel, so that, if
one capacitor gets open, the total bypass capacity decreases).

So, the simplest thing to do is to try putting a bypass capacitor (say 0.01 uF
with appropriate voltage rating) in parallel with the existing one, on both the
final tubes cathode and screen, and see what happens.

Q2) 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.

R2) Having no grid current is usually due to the fact that the driver tube does
not deliver enough drive power into the final tubes grid. Although this could
possibly be due to a weak driver tube, the most likely cause is that the driver
stage (or the stages before that) are not well adjusted. You could then try
performing the adjustment procedure described in the manual.

In any case, having no grid current does not cause any problem. Probably you now
get a somewhat lower RF output power than if the final tubes grid would be
driven harder.

73

Tony I0JX
Rome, Italy

Tony,

Thank you! I'll try your suggestion of placing additional caps in
parallel with the existing bypass caps.

Incidentally, I found a spare set of 6146 tubs, installed them in the
SB-401 and tuned-up on 80 meters. Even though I didn't neutralize the
tubes, the '401 exhibited the same behavior I described above. This
suggests the problem is not with the tubes, but with some other
component.

Yeah, come to think of it, little or no grid current isn't anything
much to worry about. The 6146s are pentodes. Their grids shouldn't
draw much power, in any. I think the narrative in the manual is
misleading.

-Dave, K3WQ