TomMcNally wrote:I am using a matched quad of new ElectroHarmonix EL-84's. Running a little hot, adjustment range at the test points is
about .67 to .47 ... can't get any lower.
What do you think ?
... tom
Holy guacamole, those are some roarin' tubes! They don't seem to match the classic datasheet curves at all.
10V zener + 10 ohm current sense (times 67mA) + variable pot @ 0 ohms = 10.67V (+/- 200mV)
10V zener + 10 ohm current sense (times 47mA) + variable pot @ 100 ohms (times 47mA) = 15.17V (+/- 200mV)
So you have an adjustment range of 10.67V to 15.17V on the selfbias adjustment. Verify that your 10V zener is actually 10V and that my computed voltages match yours at the cathode, pin 3. Maybe just verify this at the 15.17V setting, as we don't want to burn those babies up. If it does turn out to be 'hot' tubes, get a few sets of four of these, 12V, 13V & 15V zeners (all in stock):
78-BZX85B12-TAP
78-BZX85B13-TAP
78-BZX85B15-TAP
You can use the first class mail shipping option on this small order and it should be very cheap.
If you really have 15V at the cathodes at 47mA, I would go straight to the 15V zener. The 12V and 13V may not be enough:
(12V + 10 (times 35mA) + 100 ohms (times 35mA) = only 15.85V
Let me know, Tom. If these EL84s are going to be all over the place, then I might just change the 100 ohm variable pot to 200 ohm to save headaches. You could also do a quick fix by changing your 10 ohm resistors to 100 ohm. I originally had the 12V zener in mine, but changed to 10V zeners due to low current (less than 35mA on all my Reflector 6pi14pi tubes).
Shannon