Hi Jan:
In a cathode driven amplifier, there are significant reasons to use a tuned
input. First, less drive is required. Secondly, there is a slight decrease
in distortion products (3 to 5 db typical). Third, the swr will generally
be lower. In some rigs, the power foldback may decrease the drive even
further.
On the other hand, if you have sufficient drive and the tube has excellent
intermodulation distortion products, perhaps you can avoid switching
additional circuits.
The 8875 actually has pretty good specs - unlike sweep tubes that need all
the help they can get. So, you will probably be ok imd wise without the
tuned input. Like Phil says, folllow the guidelines and the tube will
perform fine.
A tuned input can be added. In one amp, I added a deck on the switch -
which switched relays under the chassis - which operated the tuned input
circuit. Additionally, you can generally add an external pi-network to the
input and switch it using the control outputs from the transceiver.
Key to long life is watching the grid current.
73, Colin K7FM
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