> >Does your rig normally shut down at that SWR? That would be the
> >only ill effect.
>
> No, not at all. What I *do* see is below-normal grid and plate current
> compared to the lower bands, and commensurately less output. It's been
> opined that the problem is not the match to the transceiver, but rather
> the match from the input network to the tubes. What say you?
A mismatch, by itself, causes no loss unless a dissipative
resistance is involved in the mismatch. It certainly can't be power
dissipation between the input and the tubes, because the wires are
reasonably short and not made of high loss material.
A mismatch can result in less that optimum power transfer from
the source, in this case your rig, to the load (the tubes).
Measure the power applied to the amplifier by subtracting reflected
power from forward power to find true power. Perhaps your radio is
supplying less power on ten meters, but the power meter reads the
same on forward because of the reflected power.
Remember the forward power indicated is reflected power plus the
true forward power.
Adding the tuner in the rig might not help for two reasons:
1.) The tuner adds loss. That might offset improved true power with
the better match,
2.) Low power can also be caused by harmonics from the cathode
showing up on the cathode.
There could also be a problem that is not SWR related.
The cathode ALWAYS should be "grounded" for harmonics of the
drive frequency by using an input circuit that presents the cathode
with a low impedance at harmonics of the drive frequency. That is
generally solved by using a pi-network or parallel tuned network
located near the cathode of the tube.
The SB220 is marginal in this design area. Also, the grids float on
capacitors. That is a poor idea for band to band gain linearity.
73, Tom W8JI
w8ji@contesting.com
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