G7PSK,
Thanks for replying, please don't misinterpret my response, as you can't express intonation when writing. I appreciate anyone that takes the time to read my post and respond.
I do understand heater concepts and applying 6.3V to a filament, which in turn, heats the cathode, which in turn, emits electrons. I've set that up
on a proto-board many times to test the heaters on several different valve types. (6922, E188CC, ECC88, PCC88, etc) I have the specs for many dual triode valves as they range from 300mA to 600mA. When testing with a 6.3V 600mA trans, naturally the secondary voltage varies with the valve. For the tester I want to get as close to the 6.3V spec as I can. I have 120V on my mains. The transformers I've tried want 115V on the primary. Hence I inserted a 150 ohm thick film resister in series. Between that and a 5 ohm POT, I can dial in the heater voltage for any dual triode valve I wan to test.
Now I'm off to tackle the other, important have of the circuit. I knew that the testing circuit had a 6BN8 in it (see schematic), and that the pin assignments were different. 4 and 5 heaters seem to be standard. What Ian pointed out that I did not pick up, was the 6BN8 is a dual Diode and Single Triode. That tidbit went a long way in helping me understand negative grid bias. Key word… "helping me understand" This is the negative DC bias I was reading about but I couldn't understand where it was coming from. I kept thinking. "Don't I need a diode for that?"
Ian,
Wow. thanks for annotating the schematic and for pointing me in a direction where I can understand the circuit. I've included the passage from the manual that describes the function of SW-5. It is the Grid Emissions Test. I've been rereading it for some time to connect the dots. It ties the 6BN8 and Valve under test grids together. I am repeating whats in the manual to help the concept sink in. Gris emission are caused by residual cathode particles that land on the grid that intern turn the grid into an emitter. I get the concept. What I have to learn is how the tests detects the emissions via the circuitry.
As noted above, I am going to decouple the 6BN8 heaters from the test circuit. The valve under test already was isolated via the heater selection switch that selects one of the taps to supply the appropriate voltage.
That leaves the 30V bugger and the 120V mains. I know from the valve spec sheet that the normal operating plate voltage is 90V - 120V and they can max at around twice that.
The 30V supply goes to SW-4B of the 3PDT "Quality" test. SW-4B disconnects the 6BN8 Grid from the tube under test grid and gives it 30V. I'll figure out why when I get the other part down.
My thought is, rather than direct mains, inserting an isolation trans. Otherwise I'll look at other possibilities. I'd like to keep the transformers to 2 rather than 3. I know I could use one heater trans for both the 6BN8 and the tube under test. But they are small so I'm splitting them.
Here's a cleaned up schematic based upon your suggestions. The 30V part is probably incorrect. I'm hoping the 120V mains into pin 2, V-1B is ok w/o being tied into the heater circuit.
So let's call that my question for the day. Can (should) I pull those 2 apart? I've included the Emissions test description that applies to the left side of the schematic.
You've sent too much time on this. Please do not feel obligated to read the test info. I provided it in case you were curious.
Can't thank you enough.
Couldn't help but smile when reading the earth ground thread. Not just saying this, but I think I understand you perspective. I can retain tidbits and waves and phase mostly make sense without going into geometric equations.