Upgrading Jack Orman's tube driver schematic

[frickecello]

Hi... I was searching for a good tube driver schematic and finally found Jack Orman's, I simulated it in Multisim 9 to see if everything was ok although I read a lot of good comments about it. I used the standard signal generator set at 100 Hz@10mVpp (obviously a sinusoidal was used). I noticed that the original signal was not only amplified but also had a strange behavior, at the half of its period it was greatly reduced in amplitude then again reached its peak....

So I studied the circuit and figured that the opamp gain pot had a high value and changed it for a 100k pot, and the simulated... the problem was solved.

I am not sure about what can be improved changing the power supply to 12 V, I think it is there because the recommended heater voltage for the 12ax7 is 6.3V +/-10% (wiring the heaters in parallel) and 12 V not 9V  (wiring them in series like in the schematic), also feeding the opamps with 12 V gives them more headroom and therefore they are harder to produce clipping distortion...

Anyway... I had a hard time deciding if it was practical to build a dedicated 12 V DC power supply for my tube driver....

Finally, this is what I did:

Changed the gain pot for a 100k pot.

I fed the circuit with two 9 volt batteries, to get +/- 9V, feeding the opamps with a total of 18 V (doubling the original headroom), (+Vcc 9v, -Vcc -9V)

To achieve the recommended heater voltage, I made a simple voltage divider with the positive 9V rail, (R1 = 22k, R2 = 51k) obviously connecting pins 5 and 4 in parallel with the 51k resistor (VR2= 6.3 V) (pin 9 is common for the heaters so if you ground it you connect them in parallel)... I did this because it keeps the circuit true to the original design, but correcting the heater voltage and giving the opamps more headroom... (If you still want to feed the circuit with 12 V, connect the heaters in series, you will get less headroom..)

Changed the old 4558 opamps for a single dual TL072 (TL082 sounded good too)...

Finally I made a dedicated +/- 9V power supply XD

This simple mod, makes the Tube Driver a better sounding pedal.

Im currently working in the filter pot.... Hope this help!

Best Regards.

[tcobretti]

I fed the circuit with two 9 volt batteries, to get +/- 9V, feeding the opamps with a total of 18 V (doubling the original headroom), (+Vcc 9v, -Vcc -9V)

This is interesting.  Did you wire one batt's + to +Vcc and the other batt's - to -Vcc?  I've never heard of that.

[frickecello]

Yes, imagine you have 2 9V batteries, name them A and B, you must connect A battery positive terminal to +Vcc and its negative terminal to ground, then connect B battery negative terminal to -Vcc and its positive terminal to ground... Its a common practice when you need maximum signal swing but also need portability...

[d95err]

Hi... I was searching for a good tube driver schematic and finally found Jack Orman's, I simulated it in Multisim 9 to see if everything was ok although I read a lot of good comments about it. I used the standard signal generator set at 100 Hz@10mVpp (obviously a sinusoidal was used). I noticed that the original signal was not only amplified but also had a strange behavior, at the half of its period it was greatly reduced in amplitude then again reached its peak....

So I studied the circuit and figured that the opamp gain pot had a high value and changed it for a 100k pot, and the simulated... the problem was solved.

I am not sure about what can be improved changing the power supply to 12 V, I think it is there because the recommended heater voltage for the 12ax7 is 6.3V +/-10% (wiring the heaters in parallel) and 12 V not 9V  (wiring them in series like in the schematic), also feeding the opamps with 12 V gives them more headroom and therefore they are harder to produce clipping distortion...

Anyway... I had a hard time deciding if it was practical to build a dedicated 12 V DC power supply for my tube driver....

Finally, this is what I did:

Changed the gain pot for a 100k pot.

I fed the circuit with two 9 volt batteries, to get +/- 9V, feeding the opamps with a total of 18 V (doubling the original headroom), (+Vcc 9v, -Vcc -9V)

To achieve the recommended heater voltage, I made a simple voltage divider with the positive 9V rail, (R1 = 22k, R2 = 51k) obviously connecting pins 5 and 4 in parallel with the 51k resistor (VR2= 6.3 V) (pin 9 is common for the heaters so if you ground it you connect them in parallel)... I did this because it keeps the circuit true to the original design, but correcting the heater voltage and giving the opamps more headroom... (If you still want to feed the circuit with 12 V, connect the heaters in series, you will get less headroom..)

Changed the old 4558 opamps for a single dual TL072 (TL082 sounded good too)...

Finally I made a dedicated +/- 9V power supply XD

This simple mod, makes the Tube Driver a better sounding pedal.

Im currently working in the filter pot.... Hope this help!

Best Regards.

Did you actually build this, or did you just simulate it? Some things here sound a bit strange.

First of all, a 12AX7 needs 150mA (at 12.6V) or 300mA (at 6.3V). Using a voltage divider and the values you suggest would only let through a few milliamps, so the tube would never reach operating temperature (unless I've misunderstood you solution).

Second, because of the high heater currents required, a couple of 9V batteries would be drained really fast. Probably too fast to be useful.

There are several different Tube Driver schematics around. Checkout the one at www.generalguitargadgets.com, it has some of the features you suggest, like the bipolar power supply, running at +/-12V.


EDIT: The Tube Driver gets most of its distortion from the opamps (from what I've heard); the tube serves more to smooth out the distortion and add a bit of tube mojo. What you saw in the sim was probably the opamps clipping (which is correct according to the design). Reducing the gain pot means there will be much less gain available from the pedal.

[frickecello]

Thanks for your comment! I started this thread because I want the tube driver to sound as good as possible, and finally correct all the known issues it has, thanks for your correction, you are right, using a voltage divider the heaters will never burn, so I will change the +/- 9v power supply for a +/- 12 V power supply... Anyway I was just trying to comment that the correct heater voltage for the 12ax7 in series is 12 V not 9v...

[d95err]

Thanks for your comment! I started this thread because I want the tube driver to sound as good as possible, and finally correct all the known issues it has, thanks for your correction, you are right, using a voltage divider the heaters will never burn, so I will change the +/- 9v power supply for a +/- 12 V power supply... Anyway I was just trying to comment that the correct heater voltage for the 12ax7 in series is 12 V not 9v...

Yes, it is rather odd that the Tube Driver schematic at amzfx runs at 9V, including the heaters (although there is a comment that running at 12V is probably better).

Good luck with the tube drivin'!