Friday, April 21, 2017

Finished main wiring, tested by measuring a 6AU6


I finished wiring the voltage loops for all the regular connections from the circuit board. After very carefully checking every connection with a continuity tester, I discovered that when I swapped out the original selector switch for a new one yesterday, I miswired the filament connections. With that fixed, it was time to run a trace on sample tube, a 6AU6.

Leveraged tester that has become the curve tracer
The controls are simple - there are nine rotary selectors, one for each of the possible pins on a tube socket. They are switched from positions 1 to 11, but only some of the positions are active. Switching the dial for a given pin to the following numbers hooks it up to the associated voltage from the circuit board inside:

  1. n/c
  2. filament side A
  3. filament side B
  4. n/c
  5. cathode
  6. control grid
  7. screen grid
  8. anode
  9. n/c
  10. n/c
  11. n/c
Only five sockets are wired up - one each loctal, octal, and 7 pin mini, plus two 9 pin mini types. The remainder of the sockets on the plate, as well as the meter, is superfluous. The following picture shows which switch controls each pin, highlights the working sockets, and obscures the unused parts. You can see the connections to the circuit board hanging out of the bottom, as I haven't finished the mount yet.

Marked controls and active sockets only
I fired up the 6AU6 and measured curves - I really like the circuit and GUI of the uTracer 3+. I have some more wiring to accomplish, however. First, there are two top cap connectors, for tubes that have the anode or the grid connection on the top of the tube. I have to hook these to grid and anode voltage lines and remember they are also hot during any measurement. Second, I have to create loops for positions 10 and 11 of the rotary switches to carry the screen and anode voltage reservoirs out to tube pins.

The normal mode of operation is for the circuit board to build up the target voltages in the screen and anode reservoir capacitors, but only connect those to the screen and anode pins for milliseconds at a time while making measurements. Certain unusual measurement situations may require a steady voltage, not a short burst, which is the purpose of positions 10 and 11. 

The boost converters can't supply more than 3ma steadily, so this connection to continuous screen and/or anode reservoirs will only work for low current applications. The archetypical case for using the reservoir voltage is to test magic eye tubes. This will not be a frequent mode for me, in fact I may never use it, but the wiring should be in place. 

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