First Build - PS Voltage Question?

roundel325 · 12th August 2010, 02:45 PM

I built the following power supply as part of my first tube amp. The with the tubes installed, the power stage B+ measures 315v. A little higher than I was expecting, but that's ok. The real surprise is the B+ for the driver stage is also measuring 315v. The 1.2k 25w dropping resistor isn't providing any voltage drop, and doesn't get warm at all. My ohmmeter verifies that it is 1.2k, so I don't think the resistor is faulty. I've checked my wiring a dozen times and I'm pretty sure it's correct. The only departures from the attached schematic are that I added bleed off (bypass) resisters across the 50uf caps.

What might I be doing wrong?

Jared

llwhtt · 12th August 2010, 02:59 PM

If there is no load on the output of the 1.2K OHm resistor there will be no current thru it and therefore NO voltage drop. You mentioned something about a bleeder resistor, what size resistor. If there was a 20ma load as on your schematic your voltage drop should be 24V, without a load no voltage drop.
If you have nothing connected to the supply ALL of your voltages will be high.

Craig

Mr. Zenith · 12th August 2010, 03:38 PM

One other reason your voltages are a bit high is that input capacitor. The image is a bit pixellated on my mobile phone, but it looks like you have a 5U4 rectifier feeding a 100 uf capacitor. That cap needs to be knocked down to about 33 uf or so, or you risk arcing in the rectifier.

kavermei · 12th August 2010, 05:08 PM

If you measure the voltage drop across the dropping resistor, you can work out the actual current drawn by the driver stage. Maybe your 20mA estimate turns out to be way off?

Kenneth

roundel325 · 12th August 2010, 06:04 PM

Craig -- The bleed resistors are 1Meg ohm. These voltages were measured with the tubes installed and fully wired except the input tube (6sn7) grids are floating (100k input pot not installed yet). No speakers were connected to the amp. Why would you expect a 24v drop with a 20ma load when PSUD calculates a 120v drop? Is the software not accurate.

Zenith -- Thanks for the cap suggestion. I will adjust it.

Kennith -- My 20ma estimate is for both halves of a 6sn7 @ 180v. Hopefully not too far off.

llwhtt · 12th August 2010, 06:20 PM

.02 times 1200 equals 24. In your explanation you said nothing about an amplifier only the power supply. The 1 Meg resistors will only provide .3ma of current draw. Are you sure the 1.2K is correct? To get a 120V drop you'll need 100ma of current. If the software program can't do simple math I'd say it's bad.
Try grounding the input grid and see what happens. 20ma might be a tad high. Do you get any voltage across the 1.2K resistor?

Craig

DF96 · 12th August 2010, 06:42 PM

PSUD2 says I2 is drawing 100mA, although the circuit says 20mA. Something wrong there.

In real life, with no grid connections who know what current might be drawn? Usually too much! Are heaters on? No current with heaters on and HT on means something is open circuit.

llwhtt · 12th August 2010, 06:48 PM

I am currently modifying some old Atma-Sphere MA1s, replacing the 9 pin tubes with four 6SN7s. all four together pull less than 20ma.

Craig

roundel325 · 12th August 2010, 07:10 PM

Ok. There was something glitchy with my PSUD install. I reinstalled it and now I'm getting more reasonable values. Looks like a 6k dropping resistor is what I'll need for a 20ma draw (6000*.02=120v drop). Can someone advise what a 6sn7 will draw at 180v with a 240k plate resistor and a 6.8k cathode load?

DF96 · 12th August 2010, 07:31 PM

Almost nothing, less than half a mA? 240K alone across 180V is only 0.75mA. 6.8K cathode resistor will almost cut off 6SN7. Are you sure these are the right values? I would have guessed an order of magnitude lower!

12th August 2010, 05:08 PM	#4
kavermei diyAudio Member Join Date: Jan 2005 Location: Lokeren, Belgium	If you measure the voltage drop across the dropping resistor, you can work out the actual current drawn by the driver stage. Maybe your 20mA estimate turns out to be way off? Kenneth __________________ Never send a human to do a machine's job. --Agent Smith

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12th August 2010, 02:59 PM	#2
llwhtt diyAudio Member Join Date: Jun 2008	If there is no load on the output of the 1.2K OHm resistor there will be no current thru it and therefore NO voltage drop. You mentioned something about a bleeder resistor, what size resistor. If there was a 20ma load as on your schematic your voltage drop should be 24V, without a load no voltage drop. If you have nothing connected to the supply ALL of your voltages will be high. Craig

12th August 2010, 03:38 PM	#3
Mr. Zenith diyAudio Member Join Date: Aug 2009 Location: Central NC	One other reason your voltages are a bit high is that input capacitor. The image is a bit pixellated on my mobile phone, but it looks like you have a 5U4 rectifier feeding a 100 uf capacitor. That cap needs to be knocked down to about 33 uf or so, or you risk arcing in the rectifier.

12th August 2010, 06:04 PM	#5
roundel325 diyAudio Member Join Date: Mar 2004 Location: Ohio	Craig -- The bleed resistors are 1Meg ohm. These voltages were measured with the tubes installed and fully wired except the input tube (6sn7) grids are floating (100k input pot not installed yet). No speakers were connected to the amp. Why would you expect a 24v drop with a 20ma load when PSUD calculates a 120v drop? Is the software not accurate. Zenith -- Thanks for the cap suggestion. I will adjust it. Kennith -- My 20ma estimate is for both halves of a 6sn7 @ 180v. Hopefully not too far off.

12th August 2010, 06:20 PM	#6
llwhtt diyAudio Member Join Date: Jun 2008	.02 times 1200 equals 24. In your explanation you said nothing about an amplifier only the power supply. The 1 Meg resistors will only provide .3ma of current draw. Are you sure the 1.2K is correct? To get a 120V drop you'll need 100ma of current. If the software program can't do simple math I'd say it's bad. Try grounding the input grid and see what happens. 20ma might be a tad high. Do you get any voltage across the 1.2K resistor? Craig

12th August 2010, 06:42 PM	#7
DF96 diyAudio Member Join Date: May 2007	PSUD2 says I2 is drawing 100mA, although the circuit says 20mA. Something wrong there. In real life, with no grid connections who know what current might be drawn? Usually too much! Are heaters on? No current with heaters on and HT on means something is open circuit.

12th August 2010, 06:48 PM	#8
llwhtt diyAudio Member Join Date: Jun 2008	I am currently modifying some old Atma-Sphere MA1s, replacing the 9 pin tubes with four 6SN7s. all four together pull less than 20ma. Craig

12th August 2010, 07:10 PM	#9
roundel325 diyAudio Member Join Date: Mar 2004 Location: Ohio	Ok. There was something glitchy with my PSUD install. I reinstalled it and now I'm getting more reasonable values. Looks like a 6k dropping resistor is what I'll need for a 20ma draw (6000*.02=120v drop). Can someone advise what a 6sn7 will draw at 180v with a 240k plate resistor and a 6.8k cathode load?

12th August 2010, 07:31 PM	#10
DF96 diyAudio Member Join Date: May 2007	Almost nothing, less than half a mA? 240K alone across 180V is only 0.75mA. 6.8K cathode resistor will almost cut off 6SN7. Are you sure these are the right values? I would have guessed an order of magnitude lower!

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