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Simple ohm's law question I think...

joeriz

joeriz

AK Subscriber
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I have recently installed an SDS labs cap board in my Dynaco PAS-3x. This board replaces the quad can cap, the selenium rectifier, and the 2000uf electrolytics under the rectifier. The board works perfectly but here's why I'm posting.

I've listened to the preamp for roughly 2 hours since installing the board. The upper registers seem somewhat more strident now. I'm wondering if this might be due to a) not enough break-in time on the new board (I kind of doubt this but I'm open to opinions), or b)the fact that the heater voltages on the 12AX7s went from the Dyna-designed 11V to the 12.6V a 12AX7 would normally see. I'm assuming that this is due to the fact that the voltage drop across the rectifier diodes is less than that across the old selenium.

I might want to drop this voltage back down to the original 11V. If I decide to do this, how do I calculate the value of the dropping resistor. I'm sure this is a simple ohm's law calculation but I don't know how to do it. I need to know the required resistance as well as the appropriate wattage rating of the resistor. How do I go about doing this calculation? It seems to me I only know my target voltage but I don't know R (that's what I should be solving for) and I don't know I (I don't think I know it anyway :scratch2: ).

Thanks!

Joe
 
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R + V/I

I don't have a book here, at the computer, but you add the current for all four tubes, divide the current into the voltage you want to drop, 1.6V, and that gives the reistance you need.

BTW, P = I**R. The I** is the current squared. You want a resistor rated 3 times higher than the power it will use.
 
Thanks, Grainger. I appreciate it.

But current is the part I'm confused about. When you say "add the current for all 4 tubes", how do I find the current?

Thanks!
 
I will look it up for you. . . .

And the answer is: 0.15A per tube, 0.6A total.

But I looked at the schematic, here:

http://www.curcioaudio.com/pas_mnl.pdf

Each circuit board has the heaters wired in series, a 25.2V arrangement (22V in the case of the old rectifier). Unless you wired the heaters differently, the new board should put out somewhere between 22-25.2V for the two boards (phono and linestage).

Have you put a meter on the output of the new board?

Maybe you can measure the voltage from pin 4 to pin 5 on any one of the tubes? All should be the same.
 
So here is a related question: If he is supplying more than one tube with the voltage, would you use a resistor for each tube (Thus requiring a lower wattage resistor) or one larger wattage resistor for all the tubes, and why would you pick either option?

I would guess that the resistor for each tube would be better, and you could "tune" the values if needed, but its just a guess, what do I know...
 
Have you put a meter on the output of the new board?
Click to expand...

No, not on the output of the board but on the heaters of the 12AX7s. They all measure almost exactly 12.6v so everything is as it should be.

you add the current for all four tubes, divide the current into the voltage you want to drop, 1.6V, and that gives the reistance you need.
Click to expand...

So, in this case, 1.6/0.6 = 2.67 ohms? Does that sound correct? Also, according to what you said earlier P=I**R so (0.6)(0.6)(2.67) = 0.96 so it looks like I'd need a 3 watt resistor?

...or...am I missing something (entirely possible). Thanks again.
 
joeriz said:
No, not on the output of the board but on the heaters of the 12AX7s. They all measure almost exactly 12.6v so everything is as it should be.



So, in this case, 1.6/0.6 = 2.67 ohms? Does that sound correct? Also, according to what you said earlier P=I**R so (0.6)(0.6)(2.67) = 0.96 so it looks like I'd need a 3 watt resistor?

...or...am I missing something (entirely possible). Thanks again.
Click to expand...

That answered all my questions. You can purchase 3W resistors. So only one will be necessary. And I didn't take out a calculator but everything looks right.

Go for it!
 
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