TFR Electronics

TFR SE-88:

  The Epiphone Electar Tube 10 is a great platform for mods and can be picked up for around $100. It’s a relatively simple single-ended amp with a 6L6 power tube and a single 12AX7 preamp. The circuit is similar enough to the AX84 project P1-eXtreme that it can be converted to one fairly easily. From empirical evidence the stock PT is capable of happily outputting enough extra current to add either another 12AX7 or change the power tube another octal (EL34, KT, etc).

  I was happy with the gain from a single 12AX7 preamp so I opted to replace the output transformer and go with a KT88 power tube for some more volume. I went with an OT20SE output transformer from Matt at Musical Power Supplies. This OT is at the upper end of the current limit in my setup (just over 90mA). Matt said it should be fine, but offers a custom OT20SE+ for higher currents as well.

 

Jan 2022 Update: Added correct voltages to the schematic, added a 3H choke to the PS filtering instead of a 100r 5W resistor, and removed the standby switch.  On the costmetic end I replaced the grill cloth, removed the E logo, and had a custom control plate made.  Sadly I measured wrong so the control plate is a little off but it looks decent.

 

 

Most of these changes are taken from the AX84 P1-eXtreme project  project with a few of my own ideas mixed in.

 

  1. Circuit changes:
    1. Changed R2 to 33k
    2. Changed R3 to 1.5k
    3. Changed C1 1uF 25V
    4. Changed C2 22nF
    5. Removed R5
    6. Changed R6 to 220k
    7. Added a 100pF HV capacitor from the top of R6 to the VR1 wiper
    8. Changed R7 to 820R
    9. Changed C4 1uF 25V
    10. Removed R11 (Removed NFB)
    11. Removed C14 (Removed NFB)
    12. Replaced C5 with a jumper
    13. Changed R9 to 47k
    14. Changed C7 to 10nF
    15. Added 5.6k control grid resistor for V2
    16. Changed C10 to 100uF 50V
    17. Changed R11 to 390R 5W (adjust value for bias)
    18. Changed R12 to 1k 5W
    19. Replaced the linear taper Gain and Volume pots with audio taper pots.
    20. Removed wiring to the preamp out jack (this cut down on noise a bit)
    21. Replaced C3, C9, and C12 with 47uF 450V Caps
    22. Changed R10 to 47k 2W
    23. Changed R13 to 1k 2W
    24. Removed Standby Switch
    25. Added additional PS Filtering before C12. I was able to mount all these parts on the PCB (execpt the choke) using preexisting holes with some creative component mounting.
      1. 47uF 450V Cap to GND.
      2. 220k 2W Resistor to GND.
      3. 3H 100mA Choke between the new capacitor+ and C12+.
      4. Removed C13
    26. Elevated the PT heater center tap from GND to the power tube cathode.
    27. Speaker:
      1. Weber Signature 8S (Ceramic)
  2. Non-circuit changes,
    1. Removed the E logo and replaced the grill cloth with an oxblood cloth.
    2. Routed cutouts in the lower rear panel to make the cabinet open back and to allow room for the KT-88 to fit with some breathing room.
    3. Repladed the OEM pilot light with a Fender style jewel light.
    4. Designed and added a custom control plate.