Tom Rauch wrote:
>you do need a tuned
>input near the tube that is a low-pass or band-pass type with a low
>VHF impedance at the cathode. That means a C-L-C pi or a
>parallel tuned network.
>
For amps that use a significant run of coax between the input pi-
circuits and the socket, the coax must act as a high-impedance quarter-
wave stub at some VHF frequency, because it's shorted at the other end
by C2 of a pi-network. Would there be an advantage in mounting the 10m
pi-circuit right at the socket, so that its C2 acts as a permanent VHF
bypass? Obviously the networks for the lower bands would need to be
modified to allow for this, but that's easy enough using modern RF
design tools.
>e thing that someone mentioned was the size of the cabinet- It is
>> large. The tank compartment is 22"w x 16"d x 14"h . It was a nice case
>> I picked up at a hamfest. How critical is size? Could I have possibly
>> constructed a big VHF cavity?
>
>No, that's non-sense.
Ummm, not so sure about that... it does happen in big amplifiers, but
you'd also need to be unlucky with feedback paths and other in-circuit
resonances in order to get a VHF oscillation.
I get the feeling that we build lots of stray VHF resonances into our HF
amplifiers, but those resonances generally don't line up unless we're
unlucky two or three times over. And then a perfectly reasonable-looking
amplifier turns into a real pig...
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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