I have a SE amplifier with some sort of autobias. I don’t have any schematic. I would like to check what the bias is without taking the box apart.
I think this:
Tube-Town Online-Store - Bias-Messadapter Oktal, Fertigadapter tt-probe8
has a 1 ohm in series with the cathode. Would this affect any known autobias circuits so my measurements will be incorrect?
I think this:
Tube-Town Online-Store - Bias-Messadapter Oktal, Fertigadapter tt-probe8
has a 1 ohm in series with the cathode. Would this affect any known autobias circuits so my measurements will be incorrect?
Do you mean bias or idle current? These terms are often interchanged but have VERY different meanings. Idle current can be determined by measuring the voltage from cathode to ground, then dividing by the resistance from cathode to ground. Bias is the voltage between cathode and grid and can usually be measured directly.
Terminology: You ARE setting bias. But you're MEASURING idle current, which is controlled by the bias voltage. And typically you don't care what the bias voltage is - you want to set an operating point at a specific current.
Yes the probe does change the bias and the results are not exact. But the error is on the order of about one quarter of one percent. So there is some error but it is trivial, to small to notice with most DMMs
To put this in perspective, in the tube era, before transistors, parts with tolerances better than 10% were rare.
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