DIY tube overdrive pedal

[DeanEVO_Dude]

The tube in the circuit looks to be for decorative purposes only and all the battery does is light the LED. I drew the circuit out and the only pins on the tube doing anything are the heater pins 4 and 5 passing the signal. So at best the heater element is being used as an inductor in the signal path. There is no current supplied to the heater so no space charge of electrons in the tube so how could the tube work? If you want to make a tube overdrive with low voltages they used to make tubes for car radios that work with a 12 volt supply. I have built an overdrive with one before and am now building a vibrato/tremolo and overdrive box with one now a 6GM8.
I can post a schematic of the posted circuit to give you an idea of what it is, some people would call it a signal canceling noise enhancing circuit.

Hi jim,
Do you know if those are those the same tube used in the ZVex Nano?

By the way, thanks for all the tech-talk info in the "Crate V5 mods" thread, great info. I am picking mine up this weekend ($50 used at GC).

[jim p]

The 6GM8 is the same size as a 12AX7. In the Nano they use miniature tubes and a have a switch mode power supply to get the plate voltage of approx 250 volts. The smaller a tube the shorter its lifetime also the tubes in the Nano are soldered in so? If you want a low wattage amp I would build a Firefly for a lot less money. Ted Weber has the transformers cheep and the board is 20 bucks.


You could also consider adding a variable voltage regulator to the Crate V5 if you are modifying it for a low volume overdriven sound in that mode the power tube should last a long time.

[jim p]

I never used the ART as a mic preamp just cracked it open and started modifying so I don’t know about microphone problems. On the box I could not buy the parts for what it costs.

Where I am at right now with the modification.
As I posted before I converted both tube stages to common cathode with a transistor buffer connected to there plates. On the + 20db gain switch it is a double throw double pole that is tied common across the two switch sections. I cut the traces on the PCB between the two sides re ran the broken trace to the wiper and used the other side to connect just one common cathode stage to the output in low gain and both common cathode stages in series in high gain. I made the bypass cap on the first common cathode 220nf for brightness the second stage has a 10uf bypass. I am adding 500pf from the plate to ground on the second common cathode to lower hiss from the preamp I have 30pF on the first common cathode I may increase that also.
I found the VU meter gives you a good indication of the amplititude of the output signal at zero it is approx 1 volt RMS. So if you do not want a signal going into the amplifier connected to the box to cause overdrive in it keep it at or below zero on the meter.
I changed the tube to a Groove Tube so it is a Sovetek 12AX7 saw a post where they tried different tubes in an overdrive pedal and found the Sovetek to be best. I did try an ElectroHarmonix, a 12AT7 and a 12AZ7 and the Sovetek sounded best to me.
I will post some pictures of the guts when I get a chance I may not show the specific cuts and jumpers. Behind the PCB the tube is mounted to there is a gap of 0.4 inches for new components. I don’t see enough room for a footswitch in the stock box. If you took out the XLR connectors blanked off there holes you may have room for a footswitch at the top of the box. You could also build an external signal bypass footswitch or maybe add a relay in the box for signal bypass.

If I get a chance I may try to change the compressor output limit circuit it just kills the highs.

[jim p]

Well I am done cutting up the ART for a tube overdrive for now. What I have is I cut the traces on the bottom side of the PCB to free up pins 6(plate) 7 (control grid) and 8(cathode). I have removed R51, C40and R61 from the PCB. I installed a 2k cathode resistor from pin 8 to pin 9 (heater ground) a 300k resistor from pin 6 to the junction of C10+ and R37 the plate supply and a 20k resistor from pin 7 to ground (grid resistor). I installed a KSP44 transistor with the base connected to pin 1 the plate of the first triode with a 10k emitter resistor and the collector tied to the +48 volt supply. A MSPA29 instead of the KSP44 might be a better choice you can use any NPN transistor that will allow 60 volts from emitter to collector. At the emitter of the KSP44 I have a 100nF coupling capacitor with 100k resistor in series to the control grid pin 7 of the second triode. The emitter is also connected to the low gain position of the normal/+20db switch on the PCB. I cut free one side of the normal/+20db switch to select either one triode in low gain or both in series for high gain. The wiper of the gain switch hooked to the triode outputs is connected through a 10k resistor to the base of Q4. The collector of Q4 is connected to the +48 volt supply where it comes into the PCB. The other contact on the gain switch is connected to the plate (pin 6) of the triode. On both triodes I have connected a 150pF cap to ground to reduce hiss (high frequencies) when the gain is near maximum. Cathode bypass cap C34 was changed to 220nF for brightness. I changed the stock China 12AX7 to a Sovetek, Groove Tube it sounded better over driven plus may have had feedback with the stock tube. If you wanted to add a brightness control you could increase the value of R64 at the input to the preamp and install a pot with a capacitor in series across the new resistor value.
Here are pictures of the mods
http://i1020.photobucket.com/albums/...nARTstudio.jpg
http://i1020.photobucket.com/albums/...nARTstudio.jpg
and schematic of the triode section
http://i1020.photobucket.com/albums/...strevision.jpg

From what I have measured you have approx 0.4 inches between the bottom of the PCB and the cover so room enough for the new components. To play it safe I have put plastic tape on the inside of the bottom of the cabinet. I may put RTV on some of the parts when I am all done.
In the future I may try to change the compressor circuit it just kills the highs, maybe just replacing R77 with a capacitor of the right value will do the trick.

Will have to get some help to post a sound clip of the results in the future.

I would E-mail ART for a schematic to see what is what.

[jim p]

I found this while I was looking around on starved plate operation http://www.instructables.com/id/The-...-Tube-Booster/
I can’t be sure of how well it works but I would look for control grid current which will be a negative voltage at the control grid because that current will be flowing through your guitar pickups if you plug straight in.

[jim p]

I still need to finish off the box I started with a 6GM8 that is an overdrive and tremolo but just not up for drilling holes today, so. I grabbed up a wall wart 9 VAC transformer from an old dial up modem and built up a voltage multiplier for 55 volts DC to check out one of the 12FQ8 tubes I got in the other day. I found a patent at Google by the Wurlitzer Company for tone generators that used these tubes and the tube has Wurlitzer on it. With the tone generator they could use one plate for the oscillator and take the signal at the other plate so it would not load the oscillator down. With a plate load of 500k, a 100k grid resistor and the cathode at ground I am getting a gain of approx 50 from the tube with both plates tied together. The grid current is 800na with one plate and drops to 600na with the two plates connected. The plate current is approx 60ua. The heater voltage is 10.2 volts AC. The maximum input signal before distortion is 500mv peak to peak, signal level at saturation is 1.2 volts peak to peak. The tube has a gradual slope going into cutoff, wish I had a 12AX7 waveform to compare it too I am sure the 12AX7 would look different more squared off. I need to connect up a jfet buffer on the input due to the 100k grid resistor and a emitter follower at the plate to not load it down to maintain the gain then see what the tube sounds like.

[jim p]

Has anyone out there tried the ART mod? I am going to post a sound clip of how it sounds soon I hope. I was going through buying the required parts caps, diodes and a transformer from Mouser for the power supply coming to approx 15 bucks without shipping. Still need a tube and tube socket figure 10 bucks, then switches pots a box, hard to beat the 30 bucks for the ART. Anyway be interested to hear if anyone has tried this out aside from me, thanks.