> In some amplifiers, a component failure can result in a large voltage
between a
> grounded heater and cathode. I've seen suggestions that to protect the
tube in
> this case it might help to isolate the heater from ground.
In the case of the 77Dx (but not the Sx -- see below), it appears quite easy
to fully isolate the heater.
According to the schematic, the heater transformer secondary is grounded at
a chassis point in the PS cabinet. The circuit is completed at pin 1 of the
tube socket where it too is grounded to the chassis.
A heater feedthrough cap pass-through exists for use when upgrading the 77Dx
to Sx at C13. Provided than one is willing to abandon the two-hole Sx
concept, one simply lifts the chassis ground at the heater transformer and
again at the tube socket.
Then run a suitable AWG size wire from the heater transformer secondary
terminal strip to feedthrough cap C13 (again, the C13 position exists for
the second tube in the case of the Sx upgrade). Another wire is then run
from C13 under the RF deck to pin 1 of the tube socket. Matching the bypass
cap value already existing on the other side of the heater, a 0.02 uF bypass
cap is then installed between the tube socket pin 1 and ground.
Hal Mandel:
"I modified my Alpha 77Sx for floating DC filament supply
and also installed two (2) bifilar wound filament chokes
with appropriate bypass capacitors."
So then, it appears that optionally, one could add the heater common-mode
choke to the now floating heater, taking Vic's input tuning warning into
consideration. Between (i) floating the heater circuit above ground; and
(ii) adding the CM choke, this would make for the best heater distribution
design in the 77Dx.
-Paul, W9AC
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