EZ125 125 Watt Quad KT88 Amplifier


Project Resources
High Voltage Safety EZ125 Gallery EZ260 Schematic KT88 Datasheet 6SN7 Datasheet 5U4GB Datasheet 0B2 Datasheet DIY Audio Thread Ugly Signal Thread

Project Notes

This is the start of the EZ125 project. It is based on a couple of standard circuits, and I used the same design philosophy as the other ‘EZ’ amplifiers, that is, simple PCB construction and an all in one design. The basic idea was to get to a 125 Watt or more power level in a compact space. The basic design of the amp is a quad of KT88’s (Or KT120’s) driven by a cathode follower and a conventional phase inverter and preamp. The design uses all Octal tubes except for the 0B2. The project is pretty much complete, just need to do the write up and add the missing photos to the gallery. The photo gallery has many pictures right now, just not all of them.

Hang in their, it’s close!

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2 Responses to EZ125 125 Watt Quad KT88 Amplifier

  1. Ferenc Farkas January 6, 2015 at 8:38 am #

    My name is Feri, I live in Hungary, I love the beautiful pipes.
    I would like to build EZ125 125 Watt Quad KT88 Amplifier circuit.
    I have an experience base Williamson switching:


    I want more volume, low THD.
    Experience to say? Something needs to be changed?
    Think about this connection, but I do not like …12AU7 tube. 6H8C(6SN7GT) beautiful.


    LC adapter is not nice to me. Nicer RC Power Supply
    Thanks for the answer!

  2. Sandy Ganz January 13, 2015 at 9:57 pm #

    I have looked at many of the same amp as you have listed on the Google search link. Many are good. The TRex amp also listed has a lot of good feedback. Depending on the power you want the TRex can be a good amplifier. The EX125 runs about 80watts with the old 5U4 rectifier tubes, about 120 with solid state. The voltage on the power transformer is a bit lower then I should have but works fine. The EZ125 actually has way too much power when all said. The cleanest amp I have is the EZ215’s and it sounds super clean except the draw back of being about 20-25 watts depending on configuration.

    I would build a DUAL KT88 and see how that works, the TREX is a well designed circuit and the builder is always on DIYAUDIO.com so you can ask questions. The EX125 has some extra driver tubes that I chose just to give it a try. It was based on a few older amps (Grommes, etc).

    One thing to also remember that with a tube amp you just don’t need the same power as a transistor amp. I can’t really explain it, but 60-80 Watts should be all you need unless you live in a bar!

    For the LC filter, I think it is not a problem to drop the Choke part as long as you are using a good size filter cap(s). Just remember if using tube rectifiers their are limits on the first capacitor and most tube books recommend a ‘R’ after the rectifier. I didn’t use that, but try to limit the size of the first cap. The 5U4 tubes are very rugged so you can get away with more ‘C’ but I’m sure it hurts tube life.

    For the 12AU7 I have not done side by side, but I think it is OK, I have a bunch of odd versions NOS and in the EZ260 I built it sounds good even though it’s only putting out about 40 watts. I do like the Octal tubes (6SL7, 6SN7) but have no problem building with 12AU7.

    On THD, skip the quest for LOW THD. I had built one of the amps incorrectly and when I first hooked it up it sounded fantastic. When I tossed it on the scope I could see a lot of visible distortion due to incorrect components, and after fixing it and making it ‘look’ good I could say I enjoyed the just same! So it not just THD but the sound of the amp itself that makes it good. I like tube sound so I always use the Tube rectifiers and no solid state even though they are very complimentary to be used together.

    And for the record I also like Tone Controls 🙂


EZ260 Tube Amplifiers and Tube Projects