A somewhat related question...
As I'm going to fabricate new top plates for mine, how many tip jack test points would it be useful to have accessible from the top, and how many trimpots?
Thanks,
Jim
How fussy do you want to get? OK, here's my list of all the things I can think of that might want checking:
1) Driver plate voltage, one test point and one trimmer. This calls for making a remote trimmer, off the PC board. It is wired as a variable resistor so only two wires are needed, but do shield it, it feeds the driver cathode which is a noise-sensitive point. In another thread I suggested a small extra resistor to make the test point one of low voltage and impedance - I think Grainger and Paully were involved. This enables you to roll tubes without inverting the amp to trim the voltages. Critical for 2A3 direct coupled operation, less so for 300B cap coupling but if you want to get really accurate tube comparisons without other influences ...
2) Input level control. I have long thought this should be placed symmetrically with the hum pot, but next to the driver tube. There is no useful test point, but repeatability is an important consideration. Personally, I'd go for 6dB in 1/2 dB steps, with a fixed series resistor to optimize the system gain structure if needed. This allows balancing the channels for variations in individual tube gain (when rolling tubes for instance). Unfortunately, we keep finding that quality of attenuators is audible, so choosing the form and brand is an open question still.
3) Test point for output tube cathode voltage. A reduction in this voltage indicates aging of the tube and loss of emission. Note that with the direct-coupled 2A3, this voltage will vary much less, and the voltage should be compared to the driver plate voltage in 1) above to derive a useful measure of tube aging.
4) I would install a switch to adjust the output impedance. For the OT-2, this can be a 2-pole 3-throw; I've used a 4-pole 3-throw and paralleled each section to obtain a lower switch resistance and greater switch reliability.
5) If you have an adjustable AC power voltage, some means of assuring that the applied voltage is close to 120vAC would assure operation as close as possible to the design condition. Most tubes will operate reasonably well at +/- 10%, but the better ones will have more optimal operation and lifetime if you can stay within 5%. EML says 4% for filament voltage.
For the seriously OCD, all the tube pin voltages mentioned in the manual could be checked before every listening session. This paragraph is intended to be funny, and is by no means a slur upon our OCD brothers and sisters - we are all misaligned in some way or another. :^)
