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Homebrew tube dist. question

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1/27/2000 12:39 PM

Homebrew tube dist. question

I just finished a tube distortion project based on the westbury "The Tube" schematic from GEOFEX. I decided to go with a 5y3 rectifier tube instead of a solid state bridge rectifier. The device works, but sounds kind of dull with low output. Could my mod to the power supply cause this? The power tranny is swiped from a 1950's Hi-Fi, and is a little hot on the voltage, but no more than 15%. In addition, The secondary from this transformer is center-tapped and I'm drawing the voltage from the center tap and one terminal wire, leaving the other terminal disconnected. Should I leave the unused terminal wire unconnected, or should I connect it to ground, as the center tap would be if I were using the full voltage of the secondary? Thanks in advance!

Keith

1/27/2000 3:59 PM

The 5Y3 is specifically designed for for full wave rectification with a power trans that has a CT. It could be used as a half wave rectifier by connecting one of the tranny secondary leads to one of the 5Y3 plates (pins 4 or 6) and grounding the CT, but this will require much more filtering. Lets assume the trans is 300-0-300. Since you mention you considered a bridge rectifier across the 300-0 terminals, that would give no load DC of 423. You can get the same 423V using the full wave arrangement the 5Y3 was designed to be used in. The trans secondary leads go to pin 4,6 and the trans CT is grounded. Look at any guitar or hifi schematic with a tube rectifier to see what I'm talking about.

1/27/2000 4:33 PM

I used the schematic from a tweed Princeton as amy inspiration for the power supply. The distortion circuit calls for about 200V across a bridge rectifier, but I measured in the high 500's across the two terminals of the secondary. Measuring across one terminal and the center tap showed about 236V. So, I put this voltage across the 5Y3. The problem is that the other secondary lead is just hanging there. Is this OK?

1/27/2000 5:25 PM

I think the part you are missing here is a fullwave rectifier creates DC that is 1.4 x HALF the primary voltage. If you hook it up like this:

The trans secondary leads go to pin 4,6 and the trans CT is grounded.

the no load DC created will be 1.4 x half the 500's you quote above.

If you use a half wave rectifier like it sounds you are trying to do (using only one of the secondary leads) the no load DC will be the same voltage, but will require more filtering since there are AC pulses at 60 hz in a halfwave, instead of 120 Hz like the fullwave (or a bridge rectifier). This is not recommended.

Check out this link for some power supply basics:

http://www.ampage.org/htac/TAME1

1/27/2000 6:01 PM
keith	Thanks, Scott, I'll look this up and try your suggestion. It does seem to inject hum into the signal path. Maybe the filtering consideration is the culprit. Keith

2/3/2000 10:29 PM

keith;

The tube rectifier will just be a real PIA. The 12ax7 tube will pull about 7 mA under full load, which is not enough current for the rectifier to sag at all, making it moot to even put one in.

Just as a side note, you don't need such a large PT anyway since you are pulling so little current. You could use say a 240V PC mount tranny, and zener it down a bit, since the zener wouldn't be that large.

I wouldn't think excess voltage on the plates would cause the pedal to be dull. The tube may run with more headroom though.

This is a wild guess, but maybe you have some HF oscillation?

Brian

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