Rich,
Maybe your tube tester is out of calibration. Tek designed circuits so
the
tubes didn't need to be that great. I know there are messages in TekScopes
archives about this.
Yes - they ran the cathode to -150V via a large resistor, rather than the
normal practice of running to ground via a small resistor. Kind of looks
like a constant current source to feed the cathode. This tends to stabilise
the amplifier to tube ageing - so a tube can be a long way down before any
change in performance is seen.
Tek describe this, and using another tube as the current source (where the
effective slope resistance is not the plate resistance of the constant
current tube) in their book "Typical Oscilloscope Circuitry", published
1961, which is a blow by blow description of the 535/545 design principles.
My copy was originally owned by "James D Hawes, Plant 3, Test" at Tek.
Can you imagine any company now publishing a book giving their recipe for
their leading edge products?
Craig
