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Old 1st December 2002, 04:11 PM   #1
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Question help! power supply voltage issue

I've just finished the tube power supply for Steve Bench's tube active crossover. The schematic for the supply can be found at

http://members.aol.com/sbench101/#Power

Anyway, this is my first project in over 25 years (first one was a Heathkit radio Dad helped me build when I was just 6), and I am pretty pleased I got this far-- tubes glow, values are *almost* right. So, I want to know two things at this point:

1. On the filament wires coming off the supply I've got 7.3v rather than 6.3. Does this matter? If so so, what should I check and possibly fix? (The schematic gives you a 6v DC and 6.3v AC option; I'm using the latter.)

2. One of the OA2's appears a little "off"-- it glows a pinkish and when I test the B+ wires, I get 146 rather than 150 (with the other I get a perfect 150). I can tell the 146 reading is due to the tube, because I swapped the tubes into the opposite supplies & my readings switched accordingly. Should I just buy another OA2? Do these tend to be finicky? (So, should I buy a few and hope one works?)

Thanks for any help you can give!
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Old 1st December 2002, 04:12 PM   #2
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Default It's the "Supply 1" .gif

Forgot to mention-- It's the "Supply 1" .gif.
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Old 1st December 2002, 06:38 PM   #3
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Hi,

I am no expert, but love tube audio. I believe that you use a resistor to drop the voltage to 6.3v a/c. E=I*R will give you the value you need(R=E/I) I believe in parallel and use a 5 watter or better.

Buy another tube. glow I believe equals leakege, outside air coming in.

Wait till one of the experts gives solid advice before doing anything to your filament wires.
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Old 1st December 2002, 06:49 PM   #4
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Default Thanks

Thanks-- I am wondering, since there's no apparent voltage-dropping resistor in the schematic, if the problem could be the transformers themselves-- I forgot to record their outputs, and maybe they are just supplying the 7.3 (this would be consistent with the problem being identical on both of the supplies filament outputs). I'll check this. Thanks again.
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Old 1st December 2002, 07:22 PM   #5
Joel is offline Joel  United States
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Hi,
The OA2 is a glow discharge gas tube. It's supposed to glow.

And the 7.3V across the filament winding is due to taking the measurement in an unloaded state. After the tubes are drawing current off of it, it will go down - but may still be slightly high due to the 117V primary. Unfortunately, almost everywhere in the US has 120V, or more, coming out of the wall sockets. I have 124V here in Brooklyn. A reading of 6.7V would not be unusual, and is within a modern tube's tolerance.
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Old 1st December 2002, 07:34 PM   #6
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Default Ah-- ok.

Thanks, Joel-- I'm just learning, and I appreciate your help. One thing-- the problem isn't that the OA2 glows, but that one glows substantially pinker than the other, and whichever of the two supplies it's plugged into also shows the off reading for the B+, or at least it's more like 146 rather than 150.

BTW, in my apt, the wall sockets give consistently 126, or higher-- I even asked about this, and I was told, "That is within Ohio's 10% tolerance regulation." I have a TrippLite voltage regulator and its lights are always bouncing around "high."
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Old 1st December 2002, 07:40 PM   #7
Joel is offline Joel  United States
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jlg,

146V is within 4% of 150V. That's a negligible difference when you're talking tube stuff. Most of the classic period tube amps had tolerances for parts and voltages of 10-20%.
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Old 2nd December 2002, 03:07 AM   #8
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Default Woe is me! HUM!

I finally hooked the whole assembly up between my pre-amp and amp (using one channel each for the high and low, since for the moment I have no second-amp for biamping). So, I run the pre-amp output to one of the inputs on the X-o, and I run the X-o's high to one channel of the amp and the low to the other. When I powered up, I got a nasty hum than kept building until I got spooked and shut the whole main power down. I tried a few obvious checks (use other channel of x-o, in case one is bad, e.g.), but no luck. I have been very careful, the readings off the power supply were good, the x-o itself was built on a pre-fab pcb, so miswiring isn't likely there.

The only thing I can think of is a grounding problem-- I'd run the supply and the x-o's ground connections all to a common ground. Any ideas?

I really do appreciate the help here-- any tips are most welcome.
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Old 2nd December 2002, 02:52 PM   #9
Joel is offline Joel  United States
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jlg,

If it's noise that "keeps building" louder and louder - it's a feedback problem, not hum. You have positive feedback somewhere in the chain. Did you build the amp? Does it work fine without the pre and x-o connected?
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Old 2nd December 2002, 03:08 PM   #10
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Default reply to Joel

Hi Joel,

The amp and pre-amp are both commerical-- Acurus A150 for the Amp and Melos MA111 for the pre. Everything works swell until I hook in the x-o as described in my last message. I understand the idea of feedback-- could this be a ground loop? I ask that because a "loop" in my mind is "feedback." I know phono players have grounding problems, but that's just a steady hum, not something that builds.

What could be causing the feedback?
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