I have a pretty standard tube regulator in my preamp. It uses a 12AX7 error tube an 8417 series pass, and a 100 vold zener reference. It is set for 337 DC, regulated from 650. I never noticed this before--perhaps it's always been doing this--but after being on a half hour or so, it is up to 347 DC. It may continue rising after that: I've never checked. Maybe it's always done this. Or maybe this is a new problem. My questions are: Is this typical of tube voltage regulators; and if not, where generally should I look for the problem? Or have I not supplied enough information.
The problem is clearly the zener diode. The 100 volt units have a terrible positive temperature coefficient.
There are several possible fixes. The simplest is to replace that diode with a string of lower voltage units. Another is to use a VR tube as a reference instead. The third, and most complex, is to temperature compensate the circuit. I wouldn't do it that way.
As for zeners, the lowest temperature coefficient is around 5 Volts but that requires 20 diodes in series. You can also buy compensated diodes, more money, but not at 100V so you'd still need to put some in series.
There are several possible fixes. The simplest is to replace that diode with a string of lower voltage units. Another is to use a VR tube as a reference instead. The third, and most complex, is to temperature compensate the circuit. I wouldn't do it that way.
As for zeners, the lowest temperature coefficient is around 5 Volts but that requires 20 diodes in series. You can also buy compensated diodes, more money, but not at 100V so you'd still need to put some in series.
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