Shaka Tube debug - Help needed....

[vin]

Hi there thrill-seekers.

I've done a bread-board of Aron's Shaka Tube to try something different and have a play around with a 12AX7.
The first thing I did after building was test voltages as per Aron's thread and noticed that I wasn't getting the requisite +12v and -12V at V+ and V-. It was: 0 @ V+ and -1v @ V-

Here are the clues:

• To debug: I disconnected most of the circuit leaving the power section ONLY. ie: everything in the schematic from the 12VAC power supply to the V+ and V- in the diagram above but the problem remains.
• My AC supply is putting out 13V- I need to check back on its power rating.
• Voltage across the diodes is 13V
• My 10Ohm resistor gets unusually hot.
• All my components are as marked (except I'm using 47K where 50K resistors are required).
• I'm looking suspiciously at those diodes. I've checked about 5,098,765 times and they are polarised correctly. As are the capacitors.

I'm pretty new to this and have never done an AC circuit before so it's quite likely that I'm doing something fairly stupid. 

Am I missing something obvious? ...or should I provide more information? Feel free to fire whatever questions at me

[vin]

um.... anyone?

[the_random_hero]

Is something shorting out so all the current is flowing to ground? That might explain why your resistor is getting rather hot.

[vin]

No. The resister isn't heating up today - red herring there. Perhaps it was something else.

My Transformar doesn't have a current rating. Just says 12V~ Max 6VA.
Is it possible that there isn't enough current to get through the diodes or something? It heats the 12AX7 plates up just fine.

This is frustrating- There's only so many times you can keep checking 9 components.

[the_random_hero]

The VA rating is a volts x amps. Since it's a 12V transformer, it's current output is 500mA (fairly basic math). This is plenty for running your pedal (150mA for the heaters with plenty of current left on tap).
With the power disconnected, work through each component of the power supply and find out where the voltage stops.
Can you provide a picture or two of just the power section as this might be able to help us spot where the problem lies.

[vin]

Sure thing.



The outer blue rails are joined at the bottom and are ground.
Left and right red rails are V+ and V- and are not joined (duh).
Blue and red leads coming in from the side are from the PS.

Voltage stops at the diodes. ie:
the intersection of the 2 diodes and the 10R resistor is 14.2v
After the diodes the voltage is .04v and -.04v

I'll upload another shot soon so you can see connections from a different angle.

Thanks for the help

[vin]

View from the top.



I pulled the rest of the circuitry for clarity.

[vin]

anyone?

[vin]

I will post a photo of myself begging on my knees if that helps...

[aron]

Some of the caps look like they are inserted the wrong way. All positives should be faced towards the power supply/diodes.

[vin]

Thanks. I'll try it when I get home.
... but The schem has the + facing ground on the V- side

[aron]

Correct, but V- is not ground.

[vin]

OK... Confusing.... but I think I get it. Caps are reversed in -Ve voltage land.

Muchos gracias, Aron

[aron]

Wait, we are talking about the power supply only right? V- is connected to the  - of the electrolytic. GND or ground is connected to the + side of the caps for V-.

[vin]

Yup.
The schem threw me as in the (-Ve)  half of that circuit has "+" of the cap connected to ground. where your post says it is actually the "-".
Is that not correct- or is it a convention that I'm not aware of? I've only worked DC circuits till now.

[aron]

Oh, it's my mistake. Hmmm. You are correct. Triple check those caps and let us know.

[Paul Marossy]

The VA rating is a volts x amps. Since it's a 12V transformer, it's current output is 500mA (fairly basic math).

Uh... wouldn't that be 720mA then with a 12V 6VA transformer?

[anchovie]

The VA rating is a volts x amps. Since it's a 12V transformer, it's current output is 500mA (fairly basic math).

Uh... wouldn't that be 720mA then with a 12V 6VA transformer?

Nope. 12V x 0.5A = 6VA.

[Paul Marossy]

Oh, I see. You're really dividing the amps by volts, not multiplying them. I always wondered about that VA thing on transformers... 

[bancika]

No, you're multiplying amps by volts VA (V*A)