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Old 3rd July 2005, 02:47 AM   #1
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Default Tube noob needs some help on PSU

Hi. I ran a search and couldn't find much in the way of help on this, so I thought I'd post and see if the more experienced tube builders could help me out.

I'm presently building my first tubed linestage, based around the 5687.. It's a simple one that was published in Audioxpress a year or two ago. My problem is that the B+ for the tube is way too high -- it's supposed to be ~220vdc and I'm measuring 308vdc unloaded.

The schematic is pretty simple. I'm using a 24VAC/1.2 amp wall wart to feed the secondary of a radio shack 25.2VCT/430ma transformer. The measured AC output on the primary of the radio shack transformer is ~113VAC. This feed a bridge rectifier, which in turn feeds a voltage doubler (2x 1000uF, 200VAC capacitors). The output of the voltage doubler goes into a 1uF/250VDC polypropylene cap, then a Hammond 156L, 5h 75ma choke, and is followed by a 470uF, 250VDC cap. There's a 100k/3 watt bleeder resistor across the voltage doubler.. I've also tried adding an additional one after the 470uF cap.

According to the calculations that I've read for a voltage doubler, my DC voltage of 308VDC is correct... 113 VAC * 1.414 gives 159 VDC, then feeding the voltage doubler should give me ~320VDC, not including the rectifier voltage drop or any drop from the choke.. So 308VDC unloaded sounds about right.

Under load, the B+ drops down to around 250VDC, but this is still pretty high, since the article says that the voltage should be 220VDC. The only difference in parts is that the author used a 24VAC/830ma wallwart, and the one I'm using is rated at 1.2 amp (it puts out ~28VAC unloaded).

So the basic questions here are:

1) Will the 250VDC harm the 5687s? Or should I not worry about it?

2) If I should drop the PSU voltage, what would be the easiest method? A simple resistor? Is there an optimal method?

Sorry if this is a really noobish question.. This is my first try doing a tube amp with everything wired point-to-point myself. I'm sure it's an elementary question to you guys, I just not confident enough in my skills yet to know which way is the best to proceed.

If a schematic is needed or helpful, please let me know and I'll try to get a copy scanned in of the PSU section.

Thanks!
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Old 3rd July 2005, 05:43 AM   #2
SY is offline SY  United States
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I'd be more worried about the 250V cap than the tube.

FWIW, the Raytheon data sheet shows a 330VDC maximum for the 5687WA.
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Old 3rd July 2005, 06:02 AM   #3
PRR is offline PRR  United States
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> Under load, the B+ drops down to around 250VDC, but this is still pretty high, since the article says that the voltage should be 220VDC.

A 13% "error" is small in tube work. +/-20% variations in voltage are routine in tube-gear service notes. Most tube-stuff will work (perhaps at lower level) with +/-50% variation. Preamp stages generally give lower THD at a specific test signal when run on higher B+.

As SY says, some of your caps are under-rated. Adding series resistance won't stop the voltage from rising to 300V at switch-on, before the tubes warm, unless you run an absurd bleeder load. The ideal fix is to get higher-volt caps.

I don't recall the design, but I would not worry about the tube at all. Preamp and voltage-amp stages tend to run the tubes at about half the B+: 300V B+, 150V across the plate resistor, 2V on the cathode, the tube only sees 148V. This is well within the rating of the most common 300V plate-stuff. Even if the design calls for only 50V drop in the plate resistor, at 250V B+ you'll only have 200V across the tube.
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Old 3rd July 2005, 07:51 AM   #4
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Thanks so much for your input guys. Makes me feel much better. I just fired it up after finishing it and it sounds great. The 250VDC on the 250VDC cap does have me concerned.

Would it be either of your opinion that getting a higher voltage capacitor in that location is a must-do? I know that it's probably surge rated to 300-350vdc, but I would like this little guy to last now that I've completed it.

Thanks again!

Mike
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Old 3rd July 2005, 11:46 AM   #5
SY is offline SY  United States
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It is a must-do. If there's any heat around, use 105C-rated caps.
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Old 3rd July 2005, 09:56 PM   #6
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Will do. I'll be ordering some new ones tomorrow. I always buy 105C rated caps when they're available (I prefer overbuilding).

Thanks again, SY!
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