New DIY'er : A question about opamps

[Hemmel]

Hello everyone,

I recently started to create effects from schematics on a breadboard. After having a lot of trouble with radio interference, everything seems to be working fine now !

I got a schematic from the Beavis Board Project that uses a 12AX7 tube. My breadboard has a DC of 9V (with a pot to lower it) but the schematic needs a 12V DC.

I was thinking of using an opamp (TL071 for example) to boost the voltage to 12V. Is this possible ?

Here's a schematic of what I mean : http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/TL071.PNG.html

Also I was wondering if I would necessarily need to connect pins 4 and 7 (Vcc+ and GND) on that chip ?

Thanks,
Hemmel

P.S.: Great site ! Happy to have joined !

[goamp]

Hi Hemmel, the simple answer to your question is 'no', unfortunately. The 12 volts are needed for the tube's filament (heater), which will draw a current of 150 milliamps. An opamp simply cannot provide that much. But there is a way of running the heater supply on 6.3 volts. You have to connect pin 9 of the tube to positive and both pins 4 and 5 to ground. Then I would use a 6 volt regulator (7806) to lower the voltage from 9 to 6 volts. I hope this is clear enough for you.
Cheers, keep on experimenting.

[Hemmel]

Thanks goamp. I thought that maybe using smaller valued resistors might do the trick for the current. Wont it ? Doesn't V = RC ?
So if I need 12V and I have xR and yC, wont a smaller x give a larger y ?
Of course, I'm not considering anything about the opamp since I'm really new to these chips.
Maybe if I used a TL072 instead ?

Still, you say running the heater on pin 9 of the lap can be done with 6.3 volts. I guess that's easily obtainable with resistors direct from the AC adaptor.

In one of my schematics (called "Westbury W-20 "The Tube"") it asks for a transformer capable of giving 284V !! It's reduced to 65V before it hits pins 1 and 8 of the lamp... Also, there's a strange series of diodes I've never seen before, right next to the transformer's input, it's like 4 diodes forming a square. What is this ?

[duck_arse]

the 4 diodes will be a bridge rectifier. it converts the ac from a transformer into a series of dc pulses, which are then generally smooothed by a filter cap. the result is a usable power supply.

[Hemmel]

Ah ! Ok. I've heard of bridge rectifiers, I just didn't know what their schematic symbol was. Now I know ! And knowing is half the battle.
G.I.Joe.

Thanks duck_arse !

I've re-checked the Beavis Tube schematic and I was wrong, it uses a 9V input. So I wont have to do anything to get 12V. All I need now is a lamp 

I can't wait to finish my first pedal.

Thanks for all your info guys.