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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.
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.
Last edited by jim p; March 21st, 2010 at 05:41 PM.
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.
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.
Modifications to the tube circuit in the ART Tube MP
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.
Still checking things out on the starved plate circuit. The stock setup on the ART has a heater supply of 5.7 volts so I shorted the diode in the ground leg of the three terminal regulator causing the control grid current to drop to 3uA from 5uA. So I may try reducing the heater voltage to the tube a bit more to see where the tradeoff between gain and grid current is. The gain of a single stage is looking to be approx 31 with the maximum signal at the control grid before distortion at 40mVolts peak to peak. So if you build your own box you will need to buffer the signal to the tube due to its low impedance and reduce its amplititude if you want a clean sound before going into distortion.
So you don’t need to boil off very many electrons with a plate current of only 60uA I have too look around but thought I saw a curve on heater voltage and electron emission.
If I can reduce the grid current more a higher value grid resistor can be used thus increasing the input impedance of the tube.
I'm following your progress on this with great interest, Jim. A couple years back, I bought one of those $30 ART tube preamps on a whim and haven't done anything with it yet. If you get your project running well & sounding good (we're gonna need a clip or two here, as I'm sure you realize!), I may give it a go.
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here is a 6GM8 overdrive schematic
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