>
>>>I read that Carl suggested removing mods to go "original",
>>>but following the input circuit changes per Rich's suggestions
>>>to reduce VSWR (reducing coil windings and changing in/out caps)
>>>is pretty hard to undo.
>>
>>indeed, Jon, and for what?. . The idea that input SWR could be
>>significantly related to intermittent Tune-C arcing is not reasonable.
>>One of the nagging problems with parasites is that they are seemingly
>>female, and thusly can change their minds at any time.
>> . As Murphy said: Some things are more complicated than they look.
>
>Well, first of all, I wasn't sure which input changes Bennet had
>originally done. Yes, you are correct, decreasing input VSWR should not
>correspond to the arcing. What my thought was (and I am grasping for
>something here) is that if something is miswired or hooked in
>incorrectly, there could *possibly* be a problem. I am trying to follow
>what I would do in this case. To paraphrase Sherlock Holmes, "When all
>reasonable explanations have failed, the one of the remaining
>possibilities is the answer however unlikely." It went something like
>that! So I am just trying to eliminate all possible sources of problem
>however unlikely they may be.
>
>I was under the impression that some of the mods he did on the input side
>might have been adding nichrome suppressors to the grid leads like you
>suggest.
The resistance-wire grid-grounding suppressors I recommend for the
4-1000A/8166 do not increase or decrease the inductance in series with
the grid. Such suppressors decrease the Q of the unavoidable VHF grid
resonance.
> If one put too much inductance in that part of the circuit, one could cause
> potential instability.
True
>Same thing goes if there is not
>enough inductance (which is why you suggest adding some).
The 8166 is a special case due to the screen resonance at c. 25MHz. In
some commercial 8166 amplifiers, a variable reactance is used in the
screen current path to gnd in order to bring about stability. .
>Instability is not caused ONLY by the output circuitry.
VHF anode-resonance is seemingly a major part of the parasite problem.
>
>Anyhow, the question of wether or not his arcing is due to parasitics is
>still not settled in my mind. That's why I suggested he go over with a
>fine tooth comb all of the mods he had done and look for errors.
>
positioning a neon bulb near the anodes when the Tune-C is intermittently
arcing might help. A spectrum analyzer would be a bigger help. A
cheaper method is to install lower VHF Q suppressors and see if the
intermittent arcing continues.
- later, Jon
Rich. Measures. Web site: www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|