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Novice Voltage Regulator Question
Novice Voltage Regulator Question
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Old 17th December 2007, 06:36 AM   #1
slor is offline slor  United States
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Default Novice Voltage Regulator Question

I'm hard at work rehabbing an Ampex 601 preamp module. Most of the work is done, but now I want to build a regulated DC power supply for the filament circuit.

A fellow modder was kind enough to give me a schematic for the circuit:

Click the image to open in full size.

Makes sense to me, more or less. My real question is about heatsinking the LM350 regulator: The tab on the regulator is punched to allow it to be connected to a heatsink, but is also paralleled to the "Voltage out" pin. Would connecting this tab to a heatsink, and connecting the heatsink to the chassis, effectively short the output to ground? A former poster here:

Aikido Power Supply

cautions to isolate LM350 from heatsink and chassis, so clearly there's something here I'm misunderstanding.... Excuse my inexperience, this is my first stab at such a project.

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Old 17th December 2007, 06:42 AM   #2
aletheian is offline aletheian  United States
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You need to use an insulating pad.. either silicone or mica. If you don't want to order them, you can get them at radio shack if you have one close.
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Old 17th December 2007, 07:35 AM   #3
arnoldc is offline arnoldc  Philippines
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You must also use an insulating washer to isolate the screw
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Old 17th December 2007, 11:32 AM   #4
SY is offline SY  United States
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Novice Voltage Regulator Question
What's the AC voltage of the transformer winding you're proposing to use? Current rating? Using a doubler is fine if there's sufficient current rating, but if there isn't...

A better scheme might be to use a more prosaic 317 regulator with a power resistor bypassing 50-60% of the current around it.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 17th December 2007, 11:54 AM   #5
woody is offline woody
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A couple of questions come up. What will the voltage drop be
across the regulator and what is the current draw. In other words
what will the power dissipation be for the regulator? If your
power disipation is only 2 or 3 watts you might mount the device
to the chassis with the proper hardware. You would need a insulating sholder washer that fits through the whole in the device and an insulator. If you only need one of these let me
know and I will send you a pair of them. I am trying to change
my holiday sperit. Just send me your adress.


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Old 18th December 2007, 06:54 AM   #6
slor is offline slor  United States
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Default New Information, New Questions


Thanks much for your replies. Embarassingly, soon after posting this I found an online source for purpose-built insulators and heat sinks. (Though next time i will certainly visit the Shack, should have tried there first.)

As for the specs of the circuit, bear with me, as I'm new and inexperienced. My understanding is that the filament circuit should be putting out roughly 6VAC. However, because most of the tubes in this module get pulled, when I fired it up I was measuring nearly 8VAC. So I need to drop 2VAC (though I don't know if I'll need to lose the same amount once it's rectified).

I haven't looked up the current draw of the tubes I'm heating, though I will once I get a spare moment. I'm running a 5879, a 12AY7 and two 12AU7.

Another thing I have to educate myself on is the routing of the filament circuit. Currently, it's set up in the "usual" (that is, parallel, right?) fashion for AC. I've been advised that I can leave the negative side of this new regulated filament supply floating (not tied to chassis ground, right?) or tie it to chassis ground for potentially lower noise. In any event, aside from removing the now-unused tubes, is there any rewiring of the filament supply needed?

Thanks much,
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Old 12th January 2008, 02:07 AM   #7
slor is offline slor  United States
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Default LM350 or LM317?

Thanks much for the advice folks; I hooked everything up today and found that there's too much of a voltage drop, the highest I can get is around 5.7. Can anyone recommend a way to boost this? Substituting an LM317 for instance?

Thanks much,
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Old 12th January 2008, 02:40 AM   #8
poobah is offline poobah  United States
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It would help to know the voltage of the INPUT to the regulator as well.

BTW... 5.7 is not the end of the world... within 10%. The tube experts 'round here may know something good about being at the minimum of the voltage (cathode temperature) spec...

OH BTW... you should tie one output leg, or the other, of your DC heater supply to something low impedance... what exactly to tie it to depends on the cathode voltages and the limits imposed on heater bias relative to them. The heater circuit should at a AC ground... with a correct DC bias. Keeps one tube from talking to another... or other weird things.
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Old 12th January 2008, 02:40 AM   #9
ticknpop is offline ticknpop  Canada
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Novice Voltage Regulator Question
try lt 1085 with lower drop out/ seems to me there are some discrete circuits in back issues of audio Xpress with lower dropout but I can't remember which issues -
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Old 12th January 2008, 07:07 PM   #10
slor is offline slor  United States
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Default Hmm....

Thanks much for the replies; after I posted I found this handy calculator:


Perhaps by varying the value of the resistor between the adjustment pot and the output voltage, I can get closer to where I need to be. (By the way, I tested the preamp with the filament voltage at 5.7v; it sounded weak and terrible!)

Unfortunately, there's a lot I don't yet understand about regulator circuits (this is my first). The voltage coming off the transformer is 7.1, and I assume the diodes induce a voltage drop. But before I built the regulator, I was reading nearly 8v on the tubes!

Also, I'm afraid I don't understand your adivce poobah:

"OH BTW... you should tie one output leg, or the other, of your DC heater supply to something low impedance... " Are you talking about a bypass capacitor? As it is one leg of the filament circuit is tied to the voltage output of the LM350, the other to chassis ground....

Thanks much,
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