PRR Vari-Mu powersupply question
« on: August 12, 2009, 07:13:54 PM »
i just finished building my power supply for the 12au7 vari-mu pioneered here by PRR.
my problem is i am getting the voltages +,- 20v dc for the opamps (i need +,- 16vdc)
and the heater supply is giving me 20v dc as well(i need 12v dc).

the problem lies in the power transformer i got is a little out of spec and giving me 15.23vac instead of 12 so i am getting 20v dc on the output of the bridge rectifier.

my questing is can i knock the voltage down buy just increasing the 220 ohm resistors in the op amp power supply
 and the increasing 16 ohm resistor in the  heater supply?

how would i go about calculating larger resistor sizes? (i have no electronic education more or less trial by fire i would say. but would like to know what i am doing)
would i need higher wattage resistors?

here is a link to the schematic and pcb layout i am using.
http://www.diyfactory.com/projects/diy12au7comp/diy12au7comp.htm

Kind regards
Robert


 
by day a: mechanical engineering student. by night: a homework slave who finds time to build gear.


PRR

Re: PRR Vari-Mu powersupply question
« Reply #1 on: August 13, 2009, 12:54:01 AM »
Hook up a heater. Are voltage still high?

(No, "20V" on a 12AU7 will not cause instant death. It may burn-up in days, but I suspect your supply will sag down to ~~13V, and lower when the rest is connected.)

jwk1

Re: PRR Vari-Mu powersupply question
« Reply #2 on: August 16, 2009, 06:10:29 AM »
Hi,

I am making two (stereo) units to go in the same chassis using the this pcb layout.  Would it be possible to run both of them from the same power supply board?  Or should I build both pcb boards and hook them up seperately?

Thanks,

josh
« Last Edit: August 16, 2009, 06:22:17 AM by jwk1 »

Svart

Re: PRR Vari-Mu powersupply question
« Reply #3 on: August 16, 2009, 09:59:17 AM »
Look at the datasheets for your tubes and opamps.   Add up the current draw for each part and then compare to the current capability of your transformer and rectifiers....

If you draw more current than your transformer can source or more than your rectifiers can handle, then no, you can't run two board from one powersupply.


Quote
my questing is can i knock the voltage down buy just increasing the 220 ohm resistors in the op amp power supply
 and the increasing 16 ohm resistor in the  heater supply

Yes, if you need to do it, that is one way to do it but remember that these will start to get hotter, the more resistance you use.

Look up Ohm's law if you want to know how to figure out the voltage drop.  Another way to do it if you don't care to learn electronics theory is to hook up a 5k/5w pot as a rheostat and use that in place of the resistor.  Dial it around until you get your voltage and then measure the resistance of the rheostat.  That's your new resistor value.

Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

CJ

Re: PRR Vari-Mu powersupply question
« Reply #4 on: August 16, 2009, 03:19:14 PM »
if the heater supply does not drop after loading, you can play games, like his HP circuit:
massive wattage heat and resistors is not green , so we want to be "enviro-circuit" friendly", you know, make it an Obama certified circuit.


I'm jus sayin...
« Last Edit: August 16, 2009, 03:23:21 PM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

PRR

Re: PRR Vari-Mu powersupply question
« Reply #5 on: August 16, 2009, 11:29:23 PM »
> Would it be possible to run both of them from the same power supply board?

No.

Ptownkid

Re: PRR Vari-Mu powersupply question
« Reply #6 on: August 16, 2009, 11:51:09 PM »
Wasn't there something wrong with that original PSU layout?

MrZpliff

Re: PRR Vari-Mu powersupply question
« Reply #7 on: August 17, 2009, 02:51:29 AM »
There is a resistor with a wrong value in the PSU. It's in this thread:

http://www.groupdiy.com/index.php?topic=33625.0

I hope to continue my build soon. I started it but I haven't found any time to finnish it yet...
Peace
Magnus
www.bitterend.se

Re: PRR Vari-Mu powersupply question
« Reply #8 on: August 17, 2009, 04:03:06 PM »
i just got the tubes today and the heater voltage dropped down to 12v with tube in there. as did the op amp supply dropped as well with the chips in respective places. aroung 15 or 16v.

what is going to happen if my bias voltage is not dead on 1.5v its hovering around 1.8v. after going to a pile of LED's  

thanks for the reply this is a really cool project.
much respect to all the help. i am taking some circuit analysis classes this year. but i am a mechanical engineering student so i know more about levers and strength of materials. I'm just a cog in the gearbox of life.... i digress  ::)

Cheers
R.
by day a: mechanical engineering student. by night: a homework slave who finds time to build gear.

 

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