Problem with tube regulator?

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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.
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
What is happening on the control grid of the 8417, pin 5? This should give you a clue as to what is happening overall. The 8417 tube is VERY picky about it's operating parameters, especially the non Sylvania tubes. Is there a particular reason you chose the 8417?

Craig
 

bob91343

Member
2010-03-11 10:43 pm
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.
 
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