15-07-2015, 01:19 PM
What it does is to set the currant by monitoring across the fixed resistors so that the two LEDs switch over when the voltage across the resistor is in my case 5 volts so that there is no need for a second meter as these were expensive when I made it. The base currant is then read off and the gain calculated. Now I have a bench PSU I can set it to constant currant and feed it between collector and ground in order to get more currant ranges.
A valve or FET version could be created by fitting the range selector resistors between cathode and ground and with FETs take the gate from a potential divider across the source resistor and HT supply. The gate voltage on FETS is positive. With valves the grid needs to be negative so it would most likely need an isolated grid bias supply and pot as your circuit has. With a 3 position switch you could do valves FETs and transistors with ease.
If I built a new one there would be an analogue self adjusting circuit and no pots.
A valve or FET version could be created by fitting the range selector resistors between cathode and ground and with FETs take the gate from a potential divider across the source resistor and HT supply. The gate voltage on FETS is positive. With valves the grid needs to be negative so it would most likely need an isolated grid bias supply and pot as your circuit has. With a 3 position switch you could do valves FETs and transistors with ease.
If I built a new one there would be an analogue self adjusting circuit and no pots.
