Problem with tube regulator?

trombone · 5th September 2010, 02:12 AM

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 · 5th September 2010, 02:20 AM

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

Richard Ellis · 02:42 AM

Are your heater voltages well regulated?? This is a good candidate for 'sand' regulation!

__________________________________________________ _____Rick.........

bob91343 · 5th September 2010, 04:47 AM

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.

5th September 2010, 02:12 AM	#1
trombone diyAudio Member Join Date: Jan 2005 Location: Boston, MA	Problem with tube regulator? 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. __________________ Trombone

5th September 2010, 02:40 AM	#3
Richard Ellis diyAudio Member Join Date: Sep 2006 Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.	Are your heater voltages well regulated?? This is a good candidate for 'sand' regulation! __________________________________________________ _____Rick......... Last edited by Richard Ellis; 5th September 2010 at 02:42 AM. Reason: More thoughts

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5th September 2010, 02:20 AM	#2
llwhtt diyAudio Member Join Date: Jun 2008	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

5th September 2010, 04:47 AM	#4
bob91343 diyAudio Member Join Date: Mar 2010	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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