[ARC5] Protecting Battery Tubes in the Learavian P-10A

[Neil]

> This should cover all bases but one-  what if 
> the zener opens?  The buss is probably going to 
> soar too high if plugged into 125 VAC.
> What's you solution- a second zener?
> An SCR with another zener on the gate
> to crowbar the buss to ground should it rail above
> 9 volts or so?  
> 
> 73 DE Dave AB5S

I would use an op-amp that keeps a hefty series
transistor turned on if the voltage is below the
value that you have preset with a pot (or voltage
divider). If the voltage rises above that value,
the op-amp simply stops supplying current to the
transistor's base, turning it off.

Op-amps trigger extremely fast, they are either
on or off, no in-betweens.
You connect one of the inputs to the wiper of a
pot (or centre of a voltage divider) that is
supplied from a well regulated voltage source
that must be higher than the threshhold voltage.
The other input of the op-amp is connected to
your filament supply (8V).
The inputs are polarized and must be chosen
correctly.

This system has the advantage of easy testing:
you can simply rotate the pot down to a lower
and lower voltage threshhold (without disturbing
normal operation in any way) until the normal
circuit voltage is reached, at which point the
pass transistor will abruptly turn off.
Rotate the pot back up again and it will turn on
again.

73 de Neil ZL1ANM