I think there may be a couple of errors in the on-line schematics of the
Alpha 8410, and I wonder if somebody with an 8410 could check the schematics
in their manual for me to see if the diagrams are the same?
First, please check the RF deck schematic. The on-line schematic of this
page is crudely drawn and shows the grids of V1 and V2 shunted to ground
with three paralleled 22 ohm resistors. That's surely an error. My guess is
there shouldn't be any resistors there at all.
Also, the schematic shows J9 going to the tube screen grids and J10 going to
the control grids. I believe that is also an error. To be consistent with
the input network schematic ABX-X437, J9 and J10 should be 3-pin connectors
which go separately to V1 and V2, with pin 1 going to the screen grid and
pin 3 to the control grid.
As drawn, if one removes the 22 ohm resistors the 8410 will require very
little drive power. I'd think Alpha would have inserted an attenuator at the
input, say 6dB, to raise the drive power to e.g., 50-75W or so. I'd
appreciate knowing if you see anything like that on the schematic in the
manual.
The input Z of the 8410 is determined by a 50 ohm power resistor (actually
two paralleled 100 ohm resistors) with a relay-selected input LC network
that tunes out the capacitance of V1 and V2 on the higher frequencies. The
50 ohm resistor is DC-blocked on each end with capacitors so as not to load
down the bias circuit. (It's grounded at RF through a .02uF capacitor, with
the bias applied at the cold end of the resistor.). That's an unusual
design. Normally, one would ground the 50 ohm resistor directly to the
chassis, use capacitive coupling to isolate the grids from ground, and an RF
choke to keep the RF away from the bias supply. I'm not sure which approach
is better, or whether it makes any difference.
73,
Jim W8ZR
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