DIY tube overdrive pedal

[BigBadWolf1171]

If I ever get ahead of my other projects, I gotta try one of these....

what made it interesting to me (other then it doesnt look to hard to build), is i use a solid state xxl peavey amp, and it allows you to put something in the preamp (pedals etc).while the amp is a real good SS amp, it has no real "tube sound", so one day i took my ibanez tube king pedal (has a 12ax7 tube) and plugged it in the preamp and maaaannnn was i suprised at how it sounded, like a totally different amp. So anyway i was wondering if there was a way to make this pedal with 2 or 3 or more tubes in it, as i could run it in the preamp, but ive never attempted making a pedal before.any idea?

[jim p]

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.

[tunghaichuan***]

Edit: the Westbury tube OD runs at high voltages and doesn't operate in starved plate mode.

Another project to check out is the Real McTube OD. I built the first version that came out in Feb. 1999 issue of Electronics Now. The newer version can be found here: http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

[jim p]

I did up this quick schematic for a tube overdrive using the 6GM8 tube. Do to the low grid resistor required with this tube there is a jfet buffer a J211 as the input which is also used as a tone control. The schematic program does not have symbols for pots so R3-R4 represents a 10 k pot. Also R19-R20 and R21-22 is a dual 100k pot used as the gain control. While R23-24 is a 1 meg audio pot for the output level control. The two triodes have current source loads in the plate circuits using 2N5457 jfets with 750 ohms in series with there source. What is not shown is the power supply section which is a rectified 12 volt AC supply that is also used for the heater with an approx 18 ohms 5 watt resistor in series. The final output buffer FET could be optional if you are connecting to a tube amplifier, required with a solid state amplifier. C10 and C11 are brightness (treble boost) that has been added and one or the other could be removed or values could be increased or decreased to your preference. You may want to add a resistor in series with the tone pot (R3-R4) to limit the maximum gain with the tone at maximum.

Took a break to see about the cost for the tube and not sure but looks like they are going up in price. The tube you can use is a 6GM8/ECC86/2N27P getting to be 20 bucks each.

Here is the basic schematic
http://i1020.photobucket.com/albums/...8overdrive.jpg


Here is a page on space charge tubes
http://www.junkbox.com/electronics/l...agetubes.shtml

This whole thing made me look around and what I think would be away to go would be to modify one of these. http://pro-audio.musiciansfriend.com...amp?sku=180581
I need to check out the schematic but for 30 bucks you have the box the tube the power supply hard too beat, just need to see what it would need to be used for a guitar.

[jim p]

I was just checking through things and on the posted schematic R24, R23 that represents a 1 MegA pot is shown wrong. The wiper should only be connected to C7 so its resistance relative to the tube is always 1 Meg.

[jim p]

Got the ART Tube MP Studio today and have opened it up to have a look around. The plate current of the 12AX7 operating with just 48 volts on its plate is only 65uA with R61 a feedback resistor removed. There is approx 5uA of grid current so with the 20k grid resistor the grid is -0.1 volt and with a 2k cathode resistor the cathode voltage is 0.129 volts. The gain of the tube is approx 40. The second triode in the tube is used as a cathode follower. Figure I will cut traces on the PCB connect an emitter follower to the first triode plate for a high impedance to low impedance buffer. Then make a common cathode stage out of the second triode. With the high grid current the grid resistor values used will need to be 50k or less can probably grid bias the tube with the cathode tied to ground. So if I get a gain of 40 from each tube should have an overall tube gain of 1600.

[jim p]

I have been working on the tube section in the ART Tube MP and this is what I have so far. The stock circuit uses one of the triodes as a cathode follower but for an over drive I wanted both triodes to be common cathode gain stages. When operating the 12AX7 in starved plate there is 5uA of control grid current so the grid resistor value needs to be approx 50k ohms or less. Also the plate impedance of the tube is probably approx 120k ohms or better and with the high value of plate resistance used I connected an emitter follower to the plate as a high impedance to low impedance buffer to maintain high gain. I used a KSP44 as the transistor buffer the preamp comes with a MPSA29 which may be an even better choice being a 100 volt Darlington pair. Each tube stage has a gain of approx 40 so it is better then what I was getting using the low voltage 6GM8 tube. In the attached schematic I have 2k emitter resistors with 1uf bypass capacitors to reduce the low frequency gain. I also needed to load the plate of both tubes with 30pF to stop oscillations. If possible the collectors of the emitter followers should be connected to a supply greater then the plate supply so they will not go into saturation before the tube. You can operate the tube with no cathode resistors with a grid resistor of approx 39k. The heater supply in the ART is only 5.7 volts DC with the low plate current you do not need to heat the cathode as much as in normal operation. I can not be sure but the low heater voltage may also reduce the grid current. The input impedance of the tube is low due to the grid resistor value so if you were to make a preamp from scratch you should install a jfet op amp or jfet buffer before the input to the tube for a high input impedance.

Just to add trying the circuit with another amp it is a bit bright so may change the first bypass back to 100uF.

Here is the schematic of the tube section
http://i1020.photobucket.com/albums/...tetwostage.jpg

[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.