Pedal for Vox AC15

[Kesh]

My friend is somewhat frustrated by his Vox AC15. It has no master volume, so overdriving the thing means it has to be loud.

I looked at the schematics and it seems to have a single stage, half of a 12AX7, before the volume control.

I was wondering if it would be possible to build a clean boost pedal to overdrive this first stage, and use the volume control that comes after to cut volume while retaining overdrive.

I know the valve can swing a couple of hundred volts (the B+ was not marked, but less than 350, judging by cap rating), and has mu of 100, so potentially I could drive the input with a swing of 9V with a rail-to-rail op amp in a battery powered pedal and it would overdrive it.

So this is my thinking, is there anything I've overlooked? I'm not totally familiar with how tubes behave, being pretty new to them.

[bean]

The AC15 is a pretty unpredictable beast with overdrive pedals, I've found. Some sound great and others sound just terrible. And, it is pretty conditional on the volume output of the amp, too.

I just finished a recording session with my hand-wired Vox and what I used was a FET based boost and a Timmy clone. The boost worked out well (it's the FatPants boost one on my website---there is etchable artwork in the project). The Timmy was pretty good, but not mind blowing. This was also running the Vox volume at about 4 or 5....I would have cranked it way further but the engineer had control over that, ha ha.

[petemoore]

  I'm going to get another FET Booster [stratoblaster or see ROG/Jfet testing].
  I gave 'em to my brother when they were easy for me to make, he broke the pot/BP switch so sent it for repairs, I was reminded how well this ~simple 'secret' works.
  Meanwhile in the AC15 re-issue...speaker upgrade [yay !
  OT and PT Transformer, and Sp upgrades made volume settings 2 and 3 louder, and cranking = the amps distorting properties got a good bit 'deeper' without sounding ragged or 'spikey'.
  The amp is kind of 'high strung' anyway and tends to be mostly clean to begin with, I use a DIST+ [+adjustable tonecap across the diodes] to make the dirt.
  The preamp won't really distort or distort nicely, it's the output section [tubes/speaker] that gets distorted when cranked, again 'high strung' has a lot to do with the VOX voicing, what guitar pickups too, this amp isn't as versatile a distorting beast as some that are designed with disto in mind.
  I cancel spikey highs with a pot/capacitor between the output tube grids, because the inputs are out of phase...but it's not that much different than what the Treble/Bass controls can achieve.
   The best I was getting from it stock was not very cranked, using distortion boxes to do most of the distorting.

[ashcat_lt]

I don't generally have this problem with my AC4. 

I found that hitting the input with anything even approaching 9V makes it just fall apart.  Haven't honestly messed with it much, but the couple times I tried I was not happy with the results.

[bean]

Yeah the AC4 is a different can of worms. I can throw almost anything at that...it's my main testing/prototyping amp.

[Unlikekurt]

I'd feel a little uncomfortable in a studio setting where I wouldn't be permitted to play my instrument or set up my amplifier as I desire.

[roseblood11]

The phase inverter has a lot of influence to the overdrive sound. If you decrease the volume before it, your sound will suffer. Why not add a post phase inverter master volume? It's quite easy, I needed 30 minutes to add it to my AC15 H1TV. As I wanted to keep the wires as short as possible, I placed the pot at the bottom side of the chassis.

All you need is a 220k (or 250k) stereo pot and two 2,2M resistors. Don't leave the resistors out, they are important in case the wiper looses contact.

source: http://www.unet.univie.ac.at/~a9525719/php/amp_ac/de/

[Kesh]

If it was my amp that's exactly what I'd be doing, but he will not let me touch the innards.