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A few years ago I built a Tube guitar amp / pedal / preamp with a 12ax7 tube (and some other pin compatible tubes). I don't remember what schematic I used to make it and I wasn't skilled with electronics then but surprisingly sound does come out of it, and I really like the distortion when the tube is overdriven.

Circuit Diagram It looks like I've made some whack decisions when building it:

  1. I have no idea how I arrived at the resistor values, as well as their wattage
  2. The capacitors don't look like they are the right type or values
  3. The heater connections look weird, whats with the grounded middle?
  4. The pots look like they are 1M each, Is that sane?
  5. The amp is really quiet, even turning all pots to maximum with line level input. It distorts slightly, but I was expecting it to be massively distorted (in a good way) when turned up to the max.
  6. Whats with the 100uF Electrolytic caps on the cathodes of the triode?
  7. Where should the ground be connected in this setup. I'm getting intermittent hum when I connect a guitar to the input, and speakers/amp to the output, but fine if I play music to it from my phone
  8. Why the two resistors around the output potentiometer (see Update below)

I would love for someone who is familiar with tube amplifiers to look this over and help me work out right values and parts.

Here is an album with couple of pictures of the insides, Its not pretty.

Update:

Crud, I forgot about the two resistors across the output potentiometer. From the 2nd picture from the bottom of that album, the resistor across output and ground is 100k and one across capacitor and output is 1M

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  • \$\begingroup\$ You might want to measure some voltages and mark them on your schematic. The B+, cathode voltages, grid voltages, filament voltages (use DC range), and plate voltages. 9 voltages total. \$\endgroup\$
    – Spehro Pefhany
    Aug 22, 2014 at 15:28
  • \$\begingroup\$ Will do. Should these be measured without an input signal, and without output load? (in my case load is a set of powered PC speakers) \$\endgroup\$
    – Yarek T
    Aug 22, 2014 at 15:46
  • \$\begingroup\$ Start with with no input. \$\endgroup\$
    – Spehro Pefhany
    Aug 22, 2014 at 15:50
  • \$\begingroup\$ B+ is 135V, U1 cathode is at 1.2v U2 cathode at 0.7v. Both grids are at 0v. heater is a bit weird. One side is 4.5v, other is 4.1v, that is, pin 4 is 8v above pin 5, and pin 9 is somewhere in the middle of that. I don't think this is ideal, as the heater should be run at 12v (or 6 if they are in parallel) \$\endgroup\$
    – Yarek T
    Aug 22, 2014 at 16:13
  • \$\begingroup\$ I have a pretty crappy meter, these readings are maybe 10s of mV off \$\endgroup\$
    – Yarek T
    Aug 22, 2014 at 16:14

1 Answer 1

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Recommended configurations for the 12AX7 are below:

http://pdf1.alldatasheet.com/datasheet-pdf/view/121279/GEC/12AX7/+02Q_8AVEDwa8AEbw+/datasheet.pdf

Since the cathode resistor is bypassed for audio frequencies by the 100uF capacitors, gain will be set by the transconductance of the tube under the operating conditions, as shown in the above diagram. Lowering the plate resistance, especially on V2, will allow you drive a lower input impedance input.

Below is your original circuit with your voltage measurements and calculated plate voltages shown.

enter image description here

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  • \$\begingroup\$ I found this diagram earlier too, I found it hard to read, however, because I have two stages, and the values don't quite make sense. My Rks are 330k and 380k, should they be 0, or 1-4k ? \$\endgroup\$
    – Yarek T
    Aug 22, 2014 at 16:56
  • \$\begingroup\$ what about the two C's too. Are the ones i'm using now ok? I highly doubt they are rated for 100-200v. They are just normal caps I use for microchips and that \$\endgroup\$
    – Yarek T
    Aug 22, 2014 at 16:57
  • \$\begingroup\$ Your Rks are 15K and 4.7K which means the tubes are running at much lower current than optimal. It's possible your B+ supply is so weak that the optimal values won't work for you. \$\endgroup\$
    – Spehro Pefhany
    Aug 22, 2014 at 16:58
  • \$\begingroup\$ The plate coupling capacitor voltage rating should be something like 200V. The input coupling cap and the cathode caps can be low voltage (10V or 25V). \$\endgroup\$
    – Spehro Pefhany
    Aug 22, 2014 at 16:59
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    \$\begingroup\$ Try lowering the plate resistances in proportion. \$\endgroup\$
    – Spehro Pefhany
    Aug 22, 2014 at 17:03

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