>
>Tom Rauch wrote:
>>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.
>
>That's surely true!
? Does the amount of feedback-C inside the tube really matter?
>
>>But it sure affects efficiency and even IMD!
>>
>>The cathode needs to see a low impedance at harmonics of the
>>operating frequency.
>>
>
>May be worth trying, then...
>
>>
>>> >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".
>
>Agreed.
>
>Picking up Rich's point, adding or removing the covers is definitely
>going to make some changes in the current distribution inside the
>enclosure, and therefore also affects the feedback paths. In a marginal
>case this could tip the amplifier into instability - but only because
>it's the "last straw".
? I have a TL-922 with resistance-wire suppressors that takes-off
roughly every 500-hours for no obvious reason. My guess is that it is
teetering on the edge. Next time I repair it, I am going to increase
L-supp in order to lower Rp - thereby (hopefully) decreasing 120MHz
amplification.
>For such an amp, it would be wrong to blame a
>"cavity resonance" as the sole cause of the instability. A better
>viewpoint is that such an amplifier is *already* potentially unstable.
>When that real problem is fixed, the covers won't matter.
? Good point. However, I know of amplifiers that had more than a
single instability related problem. My policy is to leave no stone
unturned. A good example is the the three problems in theHenry 3K-A. To
make matters worse, there is no glitch protection on the kingkong HV PS.
.
>
>The exceptions to this are VHF/UHF amps where the covers definitely do
>form part of eg a tri-plate stripline; or physically huge HF amps which
>are big enough to have VHF resonances.
>
? -- If this is the case, why does the SB-220 typically blow the grid
choke only on V2? My guess is that the physical location of V2 and V1
inside is somehow involved in its intermittant oscillation at c. 110MHz.
"Nothing is as simple as it looks." -- Murphy
cheers, Ian.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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