Ron,
You could separately put an inline fuse in the feed to the filament/bias transformer to protect it BUT if you have the stock Heathkit setup, the grid chokes should let go before the filament transformer primary is hurt. What you would likely lose is the bias secondary which could be fused with a small fuse.
The plate meter is the most likely thing to go with SB-220 issues. Some of the aftermarket boards like the W7RY board I am using have protective devices for it.
Rodger WQ9E
Meter protection isn't rocket science, but it will effect metering accuracy.
Typically a set of back to back diodes (with enough in series to allow for the metering voltage) is applied across the metering terminals.
Problem is this. If you put JUST enough to protect the meter at full scale, it will statt to be Jon linear towards full scale as the diodes start to conduct. Add any heating effects, the problem gets worse.
If you put enough diodes in series for this not to be an effect then you'll still pin the meter during a short condition and negate the use of the protection mechanism.
It's a conundrum that can be fixed by using a variable voltage supply and putting a proper meter scale on the face of the meter to take into a count any non linearity. The changing effect as the diodes heat up, not so easy to work around.
Myself, I out the diodes in. I rarely operate any meters at full scale so it's not a problem.
--Shane
KD6VXI