Grid Protection for the SB-230

[Allen Jones]

I would like to keep my old SB-230 working as long as
possible and would like to add some type of protection
if grid current exceeds a fixed value.  The grid of
the 8873 is spec 5W so I am told.  The cost of a new
tube would make replacement out of the question should
it become necessary.

Most of the circuits I have seen use a single
transistor to drive a latching relay that will disable
the amp until reset.  The problem is trying to get the
necessary base bias (.6 to .7 V) to drive the
transistor into conduction as current in the grid
circuit is lower than you find in most GG high power
amps.

I was advised to look at the circuit used in the
Ameritron AL-800 which uses a LM393 op amp to drive
the switch transistor.  I haven't been able to look at
the schematic (Ameritron leaves them out of the
manuals available on their site).  I was told to just
order a board from Ameritron but would prefer to build
something myself.  If anyone can assist with design
info or a schematic it would be much appreciated.

Thanks.

Allen, W9DZ

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