Blackface/Silverface mod for harmonica by Dr. Z.
If you are using a fender blackface or silver as your test bed, try one with a 2 output tube set-up.
  1. Replace the 1M on the 1st input jack with a 5M resistor.
  2. Replace that channels 100k plate resistor with a 470k.
  3. Replace the 1.5k cathode res. with a 2.7k, and leave the 25/25 bipass cap in place.
  4. Decouple the stage with a .047 uf.
  5. Decrease slope res. from 100k to 56k.
  6. Replace the phase inverter 12at7 with a 12ax7.
  7. If it has a tube rectifier, try a big bottle 5u4.
  8. Here comes the big one install 6v6's to replace the 6l6's, and install a dropping res. of high wattage to lower the screen voltage, say 500 ohm 10 watt.
  9. Set the bias to 27ma / tube.
  10. Plug in and give it a blow.