Low Power (Apartment Level) Tube Power Amp 10-30 mW10-30 mW Guitar Power Amp 
with Genuine Power Tube Distortion (using EL84's) (NOT using preamp tubes such 
as 12AX7!)

IMPORTANT NOTE! This circuit has been refined to include the following
changes as of 1/12/97:

1. 100 pF capacitors have been added in parallel with the feedback
resistors in the gain stages to prevent the possibility of oscillation
that may occur in some circuit layouts.


2. The inverting stage in the phase splitter is fed from the output of the
noninverting buffer.  A change in op-amp loading due to this results in 
being able to drive the tubes more heavily for an improvement in tube
distortion characteristics.


NOTE! This is the correct schematic as of 1/12/97.  If you downloaded this
schematic prior to this date, please update your downloaded version.




     !
___560pF_ ______|\     _15k__________|\
         |      |  \__|    |         |  \___.47uF__
       1meg   __|  /  |   47k      __|  / |       |
         |   |  |/    |    |      |  |/   |       |
         *   |__33k___|    *      |_100k__|       |
             |        |           |       |       |
             |_100pF__|           |_100pF_|       |
             |                    |               |
           4.7k                 4.7k              |
             |                    |               |
             *                    *               |
                                                  |
  _____________15k________________________________|
 |          |
 |         47k
 |          |                 ____________________
 |          *             ___|___                 |
 |____|\                  ___________             |
      |  \_ ___100k______________    |            |
    __|  / |              _______    |            |
    | |/   |             |      |    |            |_
    |______|                    |    |              )| ____
           |                    |    |              )|(
           |                    |    |   B+         )|(
           |        *--68-------|    |___|__________)|(
  _________|          ohms      |    |              )|(   TO SPEAKER
 |                              |    |              )|(
 |                              |    |              )|(_____
 |    *__|\              |______|    |             _)|
 |       |  \_____100k___________    |            |
 |_10k___|  /  |          ___________|            |
       | |/    |          _______                 |
       |___10k_|             |____________________|



SCHEMATIC NOTES:

1.  Tubes are EL84's.

2.  Op-amps are TLO84's.

3.  * is ground.

4.  B+ for the tube output section is 10 to 15 volts.  Op-amps require a
    split supply of +/- 10 to 15 volts.  The tube B+ is typically the
    positive rail for the op-amps.  For best results, this voltage should
    be regulated, or at least well filtered.  The positive and negative
    supply voltages should be within 1 volt of each other.

5.  The tube heaters should get their usual voltage (6.3 volts).

6.  Supressor grids of the tubes are are not shown, and are internally
    connected to the cathodes.


Technical notes:

1.  The 560 pF input capacitor reduces fundamental content.  Otherwise, the
    fundamental content of lower strings dominates and results in a "hoarse"
    distortion effect when playing chords.  Those who build this circuit
    are encouraged to adjust this value to suit their needs.

2.  The output tubes are operating in pentode mode, with the screen grids
    connected to B+.  Triode mode involves the screen grids receiving the
    same AC voltage the plates receive, as well as the same DC voltage.

3.  The output transformer has a primary impedance of 6k.  Any primary
    impedance close to this should work.  It may be necessary to change
    the cathode resistor if the transformer primary impedance is
    different.  The output transformer should have good frequency
    response, since no negative feedback is used.  Small, low power output
    transformers are likely to have inadequate low frequency response.
    Recommended output transformers include the Hammond Cat. # 1608
    (8K center tap primary).

4.  The resistors between the phase splitter op-amp stages and the output
    tube grids are necessary to eliminate the "reversal" aspect of
    "positive grid" effects.  Those who desire the more extreme positive
    grid effects should use a lower resistor value or none at all.

5.  Tubes should be matched.  Tube mismatches become more apparant at lower
    supply voltages.

6.  Those familiar with effects of changing the cathode bias resistor are
    encouraged to experiment with this.  Higher cathode resistor values
    near or above those typical of typical EL84 output stages (above 100
    ohms) are likely to result in a worse distortion tone and lower output
    stage gain.  Lower cathode resistor values can cause positive grid
    effects to occur more easily.


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