So much for the "magic" of nichrome. The anode is loaded with
nichrome, and it is oscillating.
> 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.
Might be a good idea, this doesn't usually change stability much
because the feedback mostly involves the grid and layout more
than anything else. But it sure affects efficiency and even IMD!
The cathode needs to see a low impedance at harmonics of the
operating frequency.
> >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.
Best to find out why the tube has coupling between the input and
output and happens to have two resonances at the same general
spot.
By far the largest worry with the cabinet is ground path length and
ground loops.
Few people have cabinets that are 1/2 wl in the hundred MHz
range, or have three foot stubs sticking up in a four foot cabinet.
While there may be exceptions, it is more likely people are
changing something they don't understand (like the grounding of
the tuning cap or path length back to the socket from that point)
and blaming it on a "cavity resonance".
73, Tom W8JI
W8JI@contesting.com
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