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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