If the problem is ONLY on 80 meters then the issue may be with the
output loading capacitor. On 80 meters the three gang variable loading
capacitor isn't big enough so the band switch adds an additional fixed
cap in parallel. Maybe that cap has gone bad or the switch is not
making contact on that band. Try putting a 500pf 500v cap in parallel
with the output loading capacitor and try it on 80 meters.

On 07/12/2011 06:14 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