[Amps] Filament monitoring - LOOKING FOR ELMER

[Steve]

Don't rule out an intermittent in the heater wiring, socket connections 
or even inside one of the tubes. A bad joint or dodgy connection will 
show up as low current and cause a trip.

Your guess at the operation of the current monitor is correct. 3A 
through 0.3ohms gives 0.9V of AC across the resistor. The two 1N5711s 
rectify and double the AC to give a bit over 2V DC, then the opamp adds 
x1.5 gain so normal heater current gives something over 3V at the 
monitor output.

Steve

> I am taking every opportunity to learn more and working on my Alpha 87a
> always provides a good lesson.  I have been getting intermittent faults on
> the filament monitor for the 3CX800A7's showing the filament current is low
> - suggesting that one of the tubes has an open filament.  I know this is not
> the case as it is intermittent fault and when not faulted I can get full
> legal power.  I figure it's an issue with the filament monitoring circuitry.
> I understand the basic concept of current monitoring using a parallel
> resistive shunt load and feeding that resistive voltage drop into an op amp
> to provide a corresponding proportional voltage that could be read by a
> microprocessor analog input.  The 87a filament monitor per manual is looking
> for 3 states, less than 2 amps or more than 4 A and anything in between so
> it seems the current sensing method would not need to be too precise.