In the early broadcast days, when broadcast power
could only be achieved by running many tubes in
parallel, it was common to have "reasonably" separated
feed systems for the tubes.
The power supply had to be stiff enough to feed all
(sometimes 16) of the anodes in parallel. An arc or
other fault in one tube, therefore could suffer the
entire current capability of the large power supply.
This huge current surge not only could destroy the
tube that arced, but if that tube's parasitic
suppressor could not handle the entire power supply
current, the suppressor would be vaporized as well.
The separation method involved using a separate
current-limiting resistor per tube, the far side of
which got its own bypass capacitor, then feeding the
anode choke for one tube. Each tube had to have its
own DC blocking capacitor, else all 16 feed networks
ended up in DC parallel anyway. The benefits all
arose from considerations of DC and arc current, and
there were articles written about how to do the
multiple feed network without causing instability. In
other words, in the days when it was commonly done, it
was known to have drawbacks.
In current designs, there's a separate suppressor per
tube already. One could design the amp with only one
suppressor, but I've not seen it done for a long time.
In a two-tube ham amp, it's hard to imagine that
there's a benefit to be gained. You're basically just
adding another anode choke and bypass capacitor per
tube. But there is a potential drawback. If the two
anode chokes couple to each other, it is possible to
create a tube-to-tube feedback mechanism, the results
of which are not likely to be desireable.
73,
Dave W8NF
>kenw2dtc wrote:
>> I want to build a grounded grid amp with two
triodes in parallel.
>> Would there be a problem if there were two separate
sets of plate
>> chokes and suppressors and glitch resistors and
combined the RF
>> output of the tubes by connecting both plate
coupling caps to a
>> common point at the entrance to the pi-network?
To which Vic K2VCO replied:
>Probably not, but the really interesting question
>is "why?"
>Vic K2VCO
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