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DIY SE KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier

Alex Gendrano  Philippines Flag   To email Alex, type out the email address.

DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier

Here are some notes and photographs of a single-ended (SE) KT88 tube amplifier which I built. The amplifier design has been based largely around Mikael Abdellah's KT88 SE Tube Amplifier design from 2003. A schematic of the se KT88 amp that I have built is shown below in Figure 1.

KT88 Tube Amplifier Schematic with 6N1P driver

Figure 1: 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 amp can also be used with the 6L6 and EL34 / 6CA7 tubes. I have added a switch that allows me to switch the ampifier between triode-strapped or ultra-linear operation (the original circuit is only 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.

5U4GB Tube Power Supply Schematic

Figure 2: 5U4GB Tube Power Supply Schematic


Construction - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp

Dry Fit - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp

Photograph 1: Dry Fit - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp

Enclosure Construction - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp

Photograph 2: Enclosure Construction - DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amp

Finished 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 stero SE tube amp, 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.

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