17-11-2016, 09:27 PM
Because carbon track pots do not like voltage and current! Thinking about it, a higher value of grid bias pot. would not work as well as the 5K one, because it would need a much finer adjustment to get the exact required voltage. I found that out when trying the 10K one, but put it down as "scratchy!" I am going to use a small, cheap digital multimeter for anode and screen current, and build it into the tester.
All this came about through me studying the circuit diagrams of commercial valve testers. After I had syphoned off all the numerous switches and transformer windings, there was very little in them! My variac takes care of the HT and screen voltages, and although variacs are expensive items, mine wasn't because I only gave £5 for it on a car boot sale. As for the LT supplies, I can get whatever I want from the heater supplies in my Avo Valve Tester, as I have put two 4mm sockets in the side of it for that purpose. But for the initial tests earlier today, I used a 6 Volt battery! I will also put a neon indicator on the front panel so I don't accidentally electrocute myself when putting the jump leads across to the valve-holder panel.
Bob
All this came about through me studying the circuit diagrams of commercial valve testers. After I had syphoned off all the numerous switches and transformer windings, there was very little in them! My variac takes care of the HT and screen voltages, and although variacs are expensive items, mine wasn't because I only gave £5 for it on a car boot sale. As for the LT supplies, I can get whatever I want from the heater supplies in my Avo Valve Tester, as I have put two 4mm sockets in the side of it for that purpose. But for the initial tests earlier today, I used a 6 Volt battery! I will also put a neon indicator on the front panel so I don't accidentally electrocute myself when putting the jump leads across to the valve-holder panel.
Bob
