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Author Topic: Need some help with C12(copy) PSU  (Read 5347 times)

Clark Muir

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Need some help with C12(copy) PSU
« on: January 07, 2014, 09:48:47 am »

Hello, I could use a bit of help here with a power supply for a c12 copycat. I bought it as a kit, since I am not an EE and thought it would be easy.

The problem is this- the heater's at 6.3v, but B+ drops quickly from 20v to settle down to around 6v and there is no bias voltage at all.
So far I've replaced R1+R2 as well as C2 with no change. In fact, with C2 removed entirely there was no change. I've triple checked everything and it all seems to be in the right place.
I'm kinda stumbling around in the dark, so if anyone could point me in the right direction....

measuring from ground to the diodes I'm getting
D2- 116v/247v
D1- 116v/-.7v
D4- 116v/247v
D3-116/-.7v

Resistors:
R1- 245/11.1
R2- 11.1/5.8
R5- -.65/nothing
R6- ground/same behavior as B+, 6v
R11- nothing/nothing
R7&8- 13.1 all 'round
R9- 6.3/3.6
R10- 1.8/3.6

Capacitors:
C1- 260v
C2- 11.7v
C3- (same characteristic as B+) settles to around 6v
C4- (same characteristic as B+) settles to around 6v
C5- nothing
C6- 13.3v
C7- 13.3
C8- 13.3
C9- 3.6v
C10- 6.3v
C11- nothing
Here is the schematic:
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mbrebes

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Re: Need some help with C12(copy) PSU
« Reply #1 on: January 07, 2014, 01:27:56 pm »

Make sure you don't have a short between the B+ and HEATER.  Looking at your voltages, that is what I would expect.  Measure the resistance between those two points.  If there is not a short there, make resistance measurements between your resistor points and ground.  That might give you a clue as to what is going on.
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Uwe

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Re: Need some help with C12(copy) PSU
« Reply #2 on: January 07, 2014, 01:49:43 pm »

It appears that the values given for resistors R1 (91 kΩ), R2 (91 kΩ), and R4 (100 kΩ - variable) are at least one order of magnitude too large. Depending on the C12 version, the current consumption of the dual triode tube could be between 1.2 mA (C12 VR) and as high as 2.4 mA (original C12). The nominal B+ voltage as present across C4||R6 should be 120V. Applying Ohm's Law will reveal why the voltage collapses as soon as the filament has reached the temperature for the tube to attempt to draw even the puny current of 1.2 or 2.4 mA: R1 + R2 + R4 will result in a minimum of 182 kΩ (max. 282 kΩ), and with the tube trying to draw 1.2 mA plus R6 contributing another 0.25 mA, the voltage drop across this string of resistors alone will trend toward 260 V, and (if there were more) even more for the original C12. When starting out with 260 V at C1, there won't be much left at the tube or biasing circuit.. Please check carefully what the values for R1, R2, and R4 really should be. Perhaps 9.1 kΩ for R1 and R2 are more appropriate values. Then, with R4 still adjustable between 0 and 100 kΩ, the power supply could be adjusted for B+ of the nominal 120 V, regardless whether for an original C12 drawing 2.4 mA or the later C12 VR with its 1.2 mA requirement.
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Clark Muir

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Re: Need some help with C12(copy) PSU
« Reply #3 on: January 09, 2014, 10:04:29 am »

thanks guys, ill let you know how it goes.
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