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.
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.
73,
Mauri I4JMY
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
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