17-11-2016, 05:44 PM
As you know, when I tried to be clever last week, and spent a lot of time building a Mutual Conductance Valve Analyser using the old Radio Constructor circuit, it didn’t work! 
This morning, I decided to throw something together using my time-honoured methods of what I think may work, keeping it as simple as possible, and never mind the theory!
I made a simple plywood chassis into which I fitted the two 150 Volt panel meters obtained a few days ago from China. Along the bottom is row of sockets from left to right. HT- Cathode, G1 G2, G3 and anode, followed by two smaller sockets labelled Grid Bias (test points), a grid bias on/off switch and a grid bias adjust control.
The unit is fed from a variable 0 to 300 Volt home-made supply that consists of an isolating transformer feeding into a variac, feeding into a conventional full-wave silicon diode rectifier and smoothing circuit, complete with bleed resistor.
The variable grid bias circuit is via a 5K wire-wound pot. connected across a small 12 Volt battery (from a doorbell). The switch is necessary because the pot. Takes 2.4 mA. This gives a smooth bias voltage of 0 to -12. It is set using an external meter on the test points. I connected it up to my valve-holder test panel and put negative 8.5 volts on the grid of a 6C4 triode. I then turned the anode voltage slowly up to the 250 Volts specified in the data book. I had to try several different valves until I found one that passed the specified 10.5mA with 250 Volts HT and –8.5 grid bias. Then I turned the grid bias up to –9.5 Volts, whilst maintaining the HT at 250 Volts, and the anode current fell from 10.5mA to 8.2mA, indicating a gm of 2.3 mA/V! I feel that is a pretty good result, as the data book specifies the gm is 2.2.
No doubt the emission can be calculated quite easily in % when supplying the correct grid bias and anode voltage. The other 250V meter is for when I add the variable screen voltage. This will be in the form of a coarse control with 12 positions ranging from 0 Ohms up to 120,000 Ohms, in series with a 10,000 ohm wire-wound pot. Giving me a choice of 0 to 130,000 Ohms in the screen grid circuit. When that is fitted, I should be able to test pentodes, although I will no doubt have to balance anode and screen voltages by first the variable input, and then the adjustable screen voltage controls.
Series test points will be fitted in anode and screen wiring, with a shorting switch across each, so I can use external digital meters for the valve currents. The heater is supplied externally.
I expect that someone will tell me that my reasoning is flawed in some way, but at last I feel I am getting somewhere. No photographs available yet, as it is dark!
Bob
This morning, I decided to throw something together using my time-honoured methods of what I think may work, keeping it as simple as possible, and never mind the theory!
I made a simple plywood chassis into which I fitted the two 150 Volt panel meters obtained a few days ago from China. Along the bottom is row of sockets from left to right. HT- Cathode, G1 G2, G3 and anode, followed by two smaller sockets labelled Grid Bias (test points), a grid bias on/off switch and a grid bias adjust control.
The unit is fed from a variable 0 to 300 Volt home-made supply that consists of an isolating transformer feeding into a variac, feeding into a conventional full-wave silicon diode rectifier and smoothing circuit, complete with bleed resistor.
The variable grid bias circuit is via a 5K wire-wound pot. connected across a small 12 Volt battery (from a doorbell). The switch is necessary because the pot. Takes 2.4 mA. This gives a smooth bias voltage of 0 to -12. It is set using an external meter on the test points. I connected it up to my valve-holder test panel and put negative 8.5 volts on the grid of a 6C4 triode. I then turned the anode voltage slowly up to the 250 Volts specified in the data book. I had to try several different valves until I found one that passed the specified 10.5mA with 250 Volts HT and –8.5 grid bias. Then I turned the grid bias up to –9.5 Volts, whilst maintaining the HT at 250 Volts, and the anode current fell from 10.5mA to 8.2mA, indicating a gm of 2.3 mA/V! I feel that is a pretty good result, as the data book specifies the gm is 2.2.
No doubt the emission can be calculated quite easily in % when supplying the correct grid bias and anode voltage. The other 250V meter is for when I add the variable screen voltage. This will be in the form of a coarse control with 12 positions ranging from 0 Ohms up to 120,000 Ohms, in series with a 10,000 ohm wire-wound pot. Giving me a choice of 0 to 130,000 Ohms in the screen grid circuit. When that is fitted, I should be able to test pentodes, although I will no doubt have to balance anode and screen voltages by first the variable input, and then the adjustable screen voltage controls.Series test points will be fitted in anode and screen wiring, with a shorting switch across each, so I can use external digital meters for the valve currents. The heater is supplied externally.
I expect that someone will tell me that my reasoning is flawed in some way, but at last I feel I am getting somewhere. No photographs available yet, as it is dark!
Bob
