KT88 SE Stereo Amplifier Custom Graphic

Custom KT88 SE Stereo Amplifier - Hand Wired

Dimensions: 18.5 inches wide by 12 inches deep by 8 inches high
Weight: 33 pounds

Our KT88 SE Stereo Amplifier has lower distortion than a SE triode amp. The saying, “the first Watt is what really counts” can be expanded at American Tube Amp to ‘the first 6 Watts’ – which are pristine in fidelity!

The design philosophy was not to achieve maximum power, rather to over-design a zero-feedback tube amplifier to achieve high reliability, very low distortion and long tube life.

Angular view of ST88 SE Stereo Amplifier
Osciliscope Screen Wave Plot = No Feedback
20 kHz output signal just below clipping displayed with the input signal. No observable distortion at full power.

COMPONENTS:

POWER SUPPLY – The foundation of a good single ended power amplifier is a robust power supply. A NOS Freed hermetically sealed military grade power transformer with a B+ current rating of 315 ma was selected with a DC load of approximately 150 ma.

RECTIFIER TUBE – A new JJ GZ34/5AR4 was selected for the rectifier tube. It likewise is being run well below its 275 ma. maximum rating and was chosen for its slow warm-up time to let the power tubes warm up before applying B+ as well as for its low voltage drop.

INPUT CAP – The input cap is the hardest working cap in the amplifier so a new 35 uF oil cap was used for extended reliability.

INDUCTOR – A NOS 1.5 Henry Ferranti rated at 225 ma was selected for the inductor.

POWER SUPPLY – The foundation of a good single ended power amplifier is a robust power supply. A NOS Freed hermetically sealed military grade power transformer with a B+ current rating of 315 ma was selected with a DC load of approximately 150 ma.

RECTIFIER TUBE – A new JJ GZ34/5AR4 was selected for the rectifier tube. It likewise is being run well below its 275 ma. maximum rating and was chosen for its slow warm-up time to let the power tubes warm up before applying B+ as well as for its low voltage drop.

INPUT CAP – The input cap is the hardest working cap in the amplifier so a new 35 uF oil cap was used for extended reliability.

INDUCTOR – A NOS 1.5 Henry Ferranti rated at 225 ma was selected for the inductor.

OUTPUT FILTER – For the output filter cap a new computer grade 470-uF, 450-volt B+ with ~380 volts unit was selected. The large output cap provides excellent filtering and low-end isolation between channels.

OUTPUT TRANSFORMERS – The output transformers used came from a vintage Japanese amplifier. They perform excellently and are down about a half dB at 35 or 40 kHz.

OUTPUT TUBES – The amplifier utilizes a UL connection at a higher than usual plate load of ~6,200 ohms. The KT88 output tubes are new Genelex factory matched platinum Gold Lion, among the best of newly manufactured KT88 tubes. This set was designed for use in UL connection and they perform best in UL.

View of the back of the KT88 SE Stereo Tube Amp

TECHNICAL DESCRIPTION:

Maximum output power is just over 6 watts. The higher than usual plate load results in better damping (2.4). The KT88s are being run at ~20 Watts plate dissipation while the maximum plate dissipation rating is 42 Watts. This greatly extends the useful life of the tubes. The coupling caps are new 0.5 uF, 625 Volt paper in oil.

The input and output connectors and the aluminum feet are imported, otherwise, everything except the output transformers are new or NOS premium parts run at a fraction of maximum ratings. Output impedances are 4, 8 and 16 ohms. The waveform display shows input and output signals at 20 kHz just below output stage clipping. No discernible distortion is evident – even at maximum output power at 20 kHz. The output load is an 8 Ohm power resistor on the 8 Ohm tap. Total hum and noise levels are low at approximately a quarter of a millivolt on the loaded 8 Ohm tap.

DRIVER STAGE:

No matter how good the output tube and output transformer, a poorly performing driver stage, will result in a poor sounding amp. Conversely, an amp with an excellent driver stage with an average quality output tube and transformer will result in an amp that sounds pretty good. Of course, excellent output tubes and transformers ARE required to achieve outstanding performance.

The driver uses excellent quality NOS 5965 tubes in SRPP configuration. NOS 5965 tubes are similar to 12AV7s, but are known to sound much better. They are biased between 5 and 6 ma. The SRPP is run at ~300 volts B+ derived from the 470 uF cap through two 18 k, 3-Watt resistors to two NOS 40 uF caps, one for each driver channel. One driver tube is used for the gain stages and the other for the upper SRPP tube. This allows the upper tube to be run on a separate filament winding biased slightly positive to its cathodes.

The volume controls are excellent quality NOS 2-Watt molded Ohmite AB pots. Z in is 100 kohms. The tube sockets are new Belton Micalex. The 11×17 inch chassis is heavy gauge steel. The bottom plate is very strong, heavy gauge 6061 aluminum. The feet are solid turned aluminum. Most top side screws are stainless steel.

This amplifier will drive an efficient set of speakers beyond comfortable listening levels. A pair of speakers that don’t sound overly impressive when driven by a solid state amplifier really come to life when driven by this amp. The amp’s frequency response is excellent from 30 Hz to 20,000 Hz.