>About ten years ago, owning an ham shop with lab. repair, the most common
>problem in TL922 and SB220 was the bandswitches damage (contact fingers) but
>also the opened grid coil that acted like a fuse (counted at lest 50 cases)
>wasn't so rare or impossible like sentenced by some of the claims, here.
Who could possibly believe that the TL-922 oscillates intermittently at
120MHz?. Such oscillations arc the bandswitch as well as cause grid /
fil. shorts. The fix is to lower the Rp of the vhf suppressors and add
grid fusing. (see photo on p.33 of "Parasitics Revisited", Oct, 1990
QST.) See Figure 23 on my Web site.
>Agreable that a grid trip or a better fusing method would be a better choice
>than the very thin wire of a coil, I don't see any other protection in the
>original layout of the named amplifiers.
>
indeed, Mauri. Unprotected tubes are a bad idea.
>
>PS: Hardly the grid circuit introduces negative feedback because operative
>band (HF) is very off the circuit resonance and inductance high. What counts
>at HF is if capacitors reactance is neglactable or not at the frequency in
>use. In such occurrence, neglectable reactance, the coil is there only to
>establish a DC path.
>
>
>> >>Again (this was my point before the
>> >> twist and stretch), unless implementing some grid trip circuit or grid
>> >> fuse, it's potentially dangerous for tubes in SB220 and TL922 the
>> >> replacement of the grid circuit with a direct ground connection.
>> >
>> >Why? Why would it be "dangerous"?
>> >
>> >The choke and capacitors do nothing to protect the grid.
>>
>> True. The choke is too difficult to open. A fuse or a frangible
>> resistor would against grid/filament shorts.
>>
>> >Their
>> >entire function is to add negative feedback, which they do not do
>> >well at all. It is a totally silly idea.
>> >
>> not quite
>
>
>
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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