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slor 26th April 2008 06:00 AM

PS Problem: How do I Reduce Voltage?
I'm working on an active tube DI, based on the Altec 1566A:

I had to sub out the power transformer, but the new one is fairly close to the one called out in the schematic: It puts out 120VAC at 93 mA, and the filament is 6.3VAC at 1A. I'm inexperienced at best, but I did follow the schematic and the connections etc. are all good.

The filament side is fine, a little high at 6.8VDC but that seems at least in the ballpark. The problem is the B+; even bumping R9 up to 9500ohms, the lowest voltages I can get are 345VDC at "2" and 240VDC or so at "1."

In short: HELP!!!!!!!!!!!

I'm assuming I'm going to need to introduce a regulator of some kind; I have a spare LM350 but I need a plan!


Miles Prower 26th April 2008 06:33 AM


Since it's a voltage doubler circuit:

2(120 * sqrt(2)))= 339.4Vdc

That's reasonable for this circuit. I wouldn't worry much about it since the load will pull the voltage down a bit, especially since RC filter sections are included for ripple filtration.

tubewade 26th April 2008 11:23 AM

The voltages shown on the schematic are what you should find with a load. If your measurements are without a load I expect all is well.

lcsaszar 26th April 2008 12:22 PM

The voltage rating of C6 should be higher than the unloaded B+ voltage. If you use the common 350V capacitor, there is a risk. Go for 450V if you want to be in the safe area.

slor 26th April 2008 05:20 PM

No, this is loaded!
Thanks for the replies gents; I should have specified: The voltages I'm getting are with tubes installed!

And thanks oshifis, the filter caps are F & Ts rated at 500 I believe....

slor 27th April 2008 05:41 AM

Almost Forgot...
The "120VAC" transformer actually puts out 146VAC, a least unloaded. Is this so far off spec to be worthy of return or replacement? Still seems there's other issues at play here, but....

sorenj07 27th April 2008 06:15 AM

That is very weird. Have you tried switching the "120V" taps for each other?

slor 27th April 2008 03:18 PM I suppose it couldn't hurt. But either way, something seems real off here. Supposedly:

2(146 * sqrt(2))= 412Vdc

which is actually just about what I got with the PT unloaded.

I'm just trying to figure out if this transformer is going to work at all or if there's a way to lose 25V or so right off the bat....


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