[R-390] R390 B+ Expected Tolerance

[David Wise]

175/180 is less than 3% error.  I would be delighted to be that close.
What's the tolerance on the divider resistors?  One also has to consider bias voltage variation in the tube that senses the error.

As tubes warm up and begin conducting current, B+ drops, which reduces the current into the VR tubes.  The VR tubes warm up also but the B+ effect dominates.

Regards,
Dave Wise
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Barry <n4buq at knology.net>
Sent: Friday, November 18, 2022 8:11 AM
To: R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] R390 B+ Expected Tolerance

I replaced C606 and C608 with new paper caps but still only getting 175V instead of 180V.

David Medley's page on the VR circuit states that once the VREF tubes have ignited brightly, their brightness should reduce somewhat after a short while.  I don't think I'm seeing that behavior and am wondering whether I still might have a problem with the reference voltage to V607.

I've tested the 6082s on my (uncalibrated) Hickok 752A and, while the readings there could be off, they don't show any obvious signs of problems and I haven't found any other resistors in/around that part of the regulator to be out of spec far enough to make this difference.

Any other suggestions as to what to look for here?

Thanks,
Barry - N4BUQ

> Jacques, Jim,
>
> Thanks for the replies.
>
> I didn't check C606 or C608 so I have something I need to do now!  Hopefully
> that'll get the regulator back up to 180v.
>
> Thanks again,
> Barry - N4BUQ
>
>> I discovered that the regulated B+ is running about 175v instead of 180v in my
>> R390.  I found R625 had drifted from 1k to 1.2k and, since I think that was
>> originally a 1%, I replaced it.  I now get very close to 3.7k at the cathode of
>> V609; however, that didn't change the regulated voltage.
>>
>> I have five 5651s and except for one of them, virtually any two will produce
>> 175v.  If that fifth tube is in place, I get about 185v.  According to my
>> Hickok 752A, the fifth one appears to have a different current draw at its
>> regulating voltage and that might account for the difference.
>>
>> I have a spare 6082 that I've tried swapping in either socket with no
>> differences there either.
>>
>> Running at 115VAC input and getting close to 300v unregulated input to the
>> regulator.
>>
>> It leaves me curious as to how close to 180v is within specification.  I've
>> searched the manuals and I can't seem to find a tolerance for that.  Anyone
>> know?
>>
>> Thanks,
> > Barry - N4BUQ
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