DIY SE KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier
Alex Gendrano  
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DIY Single-Ended (SE) KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier
Here are some notes and photographs of a single-ended (SE) KT88 tube amplifier which I recently built. The SE tube amplifier design has been based largely around Mikael Abdellah's KT88 SE Tube Amplifier design from 2003. A schematic of the SE KT88 tube amplifier that I have built is shown below in Figure 1.
Figure 1: Single-Ended (SE) KT88 Tube Amplifier Schematic with 6N1P driver stage
The amplifier section is generally the same as Mikael's, with the main change being that the cathode resistor on the output tube has been increased from 470 ohms to 560 ohms. This reduces the plate disipation of the tube and the amplifier can also be used with lower rated output tubes like the 6L6 and EL34 / 6CA7. You can also use KT66, KT77 and 6550 tubes with this circuit. For my SE amplifier build I have included a switch that allows me to select between triode-strapped or ultra-linear operation (the original circuit by Mikael uses ultra-linear). Also, the power supply to the 6N1P preamplifier section has been beefed up with the addition of a choke.
Power Supply - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
The power supply is based around a 5U4GB dual tube rectifier and is shown in Figure 2 below. My friend Arnold Cruz helped me modify the PSU circuit from the original done by Mikael. For more information about tube power supplies, see Power Supply Design for Vacuum Tube Amplifiers. The tube rectified power supply is common to both audio channels.
Figure 2: 5U4GB Tube Power Supply Schematic
Construction - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
Photograph 1: Dry Fit - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
Photograph 2: Enclosure Construction - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
Photograph 3: Finished DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
Remarks - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp
After I finished wiring the stereo SE tube amplifier, I fired it up and what a glorious sound! It has better sound than my Push-Pull KT88 commercial amp. Maybe due to the SE operation.
You can swap the KT88 output tubes with 6L6, or EL34 / 6CA7. The 6N1P driver tube can be swapped with a 6DJ8 / ECC88 / E88CC. I have tried Siemens EL34 and new production tubes JJ EL34 and Electro-Harmonix EL34EH. A friend lent me a pair of NOS GEC KT88 and the amp sounded really great! The KT88 tubes sound better, but they are quite expensive, the EH EL34 is one-third the price and it sounds good too! My favorite selction of tubes for this amp are an Amperex Bugle Boy 6DJ8 (in place of the 6N1P) and Siemens EL34 on triode mode.
More Single-Ended KT88 and 6550 Tube Amplifiers
- SE KT88 Tube Amplifier Schematic (6N1P driver) by Mikael Abdellah - (Photographs)
- SE KT88 Tube Amplifier Schematic with 12AX7 driver - Jean Hiraga
- SE 6550 Tube Amplifier Schematic with 12DW7 driver - (Photographs)