PS Problem: How do I Reduce Voltage?

slor · 26th April 2008, 06:00 AM

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!

-Seth

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.

oshifis · 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

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

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

Hmm...no. 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....

-Seth

26th April 2008, 06:00 AM	#1
slor diyAudio Member Join Date: Jul 2007	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! -Seth

26th April 2008, 06:33 AM	#2
Miles Prower diyAudio Member Join Date: May 2005 Location: USA Blog Entries: 2	^^^^ 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. __________________ There are no foxes in atheistholes www.dolphin-hsl.com

26th April 2008, 05:20 PM	#5
slor diyAudio Member Join Date: Jul 2007	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....

27th April 2008, 05:41 AM	#6
slor diyAudio Member Join Date: Jul 2007	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....

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

26th April 2008, 11:23 AM	#3
tubewade diyAudio Member Join Date: Jun 2007	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.

26th April 2008, 12:22 PM	#4
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	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.

27th April 2008, 06:15 AM	#7
sorenj07 diyAudio Member Join Date: May 2006	That is very weird. Have you tried switching the "120V" taps for each other?

27th April 2008, 03:18 PM	#8
slor diyAudio Member Join Date: Jul 2007	Hmm...no. 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.... -Seth

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
how to reduce PSU DC voltage	space2000	Solid State	11	18th May 2008 10:12 AM
Voltage divider to reduce 6v dc to 2vdc	duderduderini	Tubes / Valves	5	26th February 2008 09:28 PM
How to reduce voltage from PSU??	oblerum	Power Supplies	4	10th August 2006 07:04 AM
Reduce wallwart voltage	argonrepublic	Parts	2	19th February 2006 02:51 PM
Help:How To Reduce Transformer Output Voltage?	phill77	Solid State	0	21st April 2003 12:04 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read