Post by radioholic on Jul 24, 2022 16:19:00 GMT 12
This is my home-made PLUTO valve tester. The transformers and panel meters were from my 'junk' box of components kept for a rainy day, and it cost me next to nothing to build. For the heater supply I used a heater transformer from a defunct Taylor valve tester, but an alternative solution could be a DC heater supply using a voltage regulator such as the LM338. The unit I buit has a 100W bulb in series with the power supply as a safety measure but this is probably not necessary as there is a fuse, so I have left it off the schematic.
The unit provides variable regulated anode and screen voltages up to 250V and 125V respectively, and grid voltage up to -30V. It can deliver anode current up to 60mA and both anode and screen supplies are protected against current overload. Anode, screen and grid voltages, and anode current are monitored with analog panel meters.
Valve socket arrangement: The unit I built has a large number of valve sockets all connected together. However in retrospect this is not the best way to build it, because the more sockets are connected, the greater the tendency to oscillation. I have added ferrite beads to many of the sockets to prevent oscillation but it would be better to have several removable panels of a few sockets (say 3 at most) connected together. The tester could have a recessed slot for the panel in use to sit in.
Diode testing: A 6.3V 50Hz AC voltage is applied to the diode anode, and the grid voltmeter is used to measure the rectified current. The diode load comprises a 2.7K resistor plus the grid voltmeter and its series resistor. Full scale reading (3V) corresponds to a diode current of 1 mA.
Gas test: Pressing the GAS TEST switch introduces a 500K resistor into the grid circuit. In a gassy valve, positive ions are attracted to the grid and generate negative grid current (current flowing out of the grid towards the power supply) and the voltage drop across the 500K resistor makes the grid less negative, which causes an increase in anode current. Good valves may show a small increase (usually less than 10%) in anode current. The acceptable upper limit must be determined for each type of valve empirically. Note: if pressing the GAS TEST button causes a decrease in anode current, this suggests the valve is oscillating.
Heater continuity test: Depressing the heater continuity switch will cause the piezo buzzer to sound if heater continuity is intact. (It is only necessary to do this test if the valve appears "dead".)
Anode voltmeter switch: The current drawn by Va (0.1 mA at full scale) will contribute to the measured anode current; this will be insignificant for most valves, but in case of valves with very low anode current Va can be disconnected using the anode voltmeter switch.
I have tested hundreds of valves with this tester and it works very well. I have compiled a table of expected values for many valves, which I will send to anyone who is interested.
I would appreciate any comments or suggestions.
The unit provides variable regulated anode and screen voltages up to 250V and 125V respectively, and grid voltage up to -30V. It can deliver anode current up to 60mA and both anode and screen supplies are protected against current overload. Anode, screen and grid voltages, and anode current are monitored with analog panel meters.
Valve socket arrangement: The unit I built has a large number of valve sockets all connected together. However in retrospect this is not the best way to build it, because the more sockets are connected, the greater the tendency to oscillation. I have added ferrite beads to many of the sockets to prevent oscillation but it would be better to have several removable panels of a few sockets (say 3 at most) connected together. The tester could have a recessed slot for the panel in use to sit in.
Diode testing: A 6.3V 50Hz AC voltage is applied to the diode anode, and the grid voltmeter is used to measure the rectified current. The diode load comprises a 2.7K resistor plus the grid voltmeter and its series resistor. Full scale reading (3V) corresponds to a diode current of 1 mA.
Gas test: Pressing the GAS TEST switch introduces a 500K resistor into the grid circuit. In a gassy valve, positive ions are attracted to the grid and generate negative grid current (current flowing out of the grid towards the power supply) and the voltage drop across the 500K resistor makes the grid less negative, which causes an increase in anode current. Good valves may show a small increase (usually less than 10%) in anode current. The acceptable upper limit must be determined for each type of valve empirically. Note: if pressing the GAS TEST button causes a decrease in anode current, this suggests the valve is oscillating.
Heater continuity test: Depressing the heater continuity switch will cause the piezo buzzer to sound if heater continuity is intact. (It is only necessary to do this test if the valve appears "dead".)
Anode voltmeter switch: The current drawn by Va (0.1 mA at full scale) will contribute to the measured anode current; this will be insignificant for most valves, but in case of valves with very low anode current Va can be disconnected using the anode voltmeter switch.
I have tested hundreds of valves with this tester and it works very well. I have compiled a table of expected values for many valves, which I will send to anyone who is interested.
I would appreciate any comments or suggestions.
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