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5751 SRPP / KT88 Monoblock Tube Amp Kits

 Giovanni Militano    Canadian Flag     To email Gio, type out the email address.

5751 SRPP / KT88 Push-Pull Monoblock Tube Amplifier Kits

One of the advantages of hosting a hobby website is that you meet people through email who have similar interests as you. Ever since we posted Bruce's first OddWatt project on the site I have communicated with a number of DIY hobbyists who have built the various OddWatt amplifiers and have had nothing but very positive comments about the sonic qualities of these tube amps. The kits are available in two versions, KT77 rated at 15 watts and KT88 rated at 25 watts which I decided to try.

Odd Block KT88 Series 1 Tube Amp Kits

The monoblock tube amp kits arrived about two weeks after placing the order in two double boxed packages, each package weighing about 6.4 kg (14 lbs.). The kits were well packaged and there was no damage to the contents. Not shown in the photograph is the comprehensive assembly manual which was provided through email. All the parts required to build the tube amp are included. For tools you will need a soldering iron, screwdriver, pliers and wire stripper / cutter. The enclosure is powder coated steel and includes a removable vented cover which can be used to keep curious hands and paws away from the very hot vacuum tubes.

5751 SRPP / KT88 Push-Pull Tube Amp Kit

Photograph 1: OddBlock KT88 Series 1 Tube Amp Kit Contents

The manufacturers specifications for the kit are as follows:

  • Rated Power Output: 25 watts (RMS)
  • Input Sensitivity: 1.5V (at rated power)
  • Input Impedance 100 k-ohm
  • Input Terminal: Line Level RCA
  • Speaker Output Taps: 4 and 8 ohms
  • Distortion: less than 0.5% through half power, 1% at full power
  • Frequency Response: 8Hz to 20 kHz +0/-1db at any rated power
  • Signal to Noise Ratio: less than 80dB
  • Tubes: Gold Lion KT88, NOS JAN Philips 5751
  • Supply Voltage: 120VAC 50/60Hz
  • Power Consumption: 130 watts (each monoblock)
  • Weight: 6.3 kg (each monoblock amp)
  • Dimensions: 205 mm (W) X 225 mm (H) X 300 mm (D)

SRPP and Class-A Self Inverting Push-Pull Topology

The amplifier schematic is shown in Figure 1. Please note that this circuit is © OddWatt Audio and permission to host the schematic on this site has been provided by OddWatt Audio. You are free to use the schematic for personal, non-commercial use.

5751 SRPP / KT88 Push-Pull Tube Amp Schematic

Figure 1: OddBlock KT88 Series 1 Tube Amplifier Schematic

The voltage gain (input) stage is a common Shunt-Regulated Push-Pull (SRPP) amplifier using a 5751 miniature dual triode valve. Other input tubes can be used in the driver stage, the closest match being a 12AX7 which will provide more gain. 12AU7, ECC82 and ECC802S can also be used but will not be able to provide sufficient gain to drive the KT88 tubes to full power. Bruce indicated that the 5751 provided the best measured performance from the lot of driver tubes.

The output stage is a Self Inverting Push-Pull (SIPP) amplifier which is based on the Compact Hi-Fi Power Amplifier by Melvin Leibowitz from 1961. Unlike conventional push-pull output stages, a phase inverter is not necessary here. Eliminating the phase inverter stage removes an entire active circuit from the signal path. There is only one DC blocking capacitor in the entire signal path of the circuit. The elegance and sheer simplicity of this amplifier circuit will appeal to audio enthusiasts who prefer less components and circuitry in the signal path.

A constant current source (CCS) consisting of a LM317HVT voltage regulator IC is used to force the output stage into class-A operation. The bias current can be adjusted by changing the current setting resistor and this will allow the use of many different power tubes. For those who like to roll tubes, a switch be used to readily allow adjusting the bias current. With the Group "B" tubes (6550, KT88, KT90) the design is conservative and keeps the tubes at about 75-80% of their ratings which will extend the life of the tubes.

Good quality components are used throughout the amplifier circuit. The DC blocking capacitor is a Russian made paper-in-oil (PIO) type marked K40Y-9 (0.33uF / 630V) which sounds good and are popular among audio hobbyists. Feel free to experiment with different capacitor or drop in your usual favorite. The resistors are carbon film. The audio output transformer is an Edcor CXPP25-MS-8k, rated at 25 W.

Power Supply - Odd Block KT88 Series 1 Tube Amp

A schematic of the power supply is shown below. Like the amplifier schematic, the power supply circuit is © OddWatt Audio and permission to host the schematic on this site has been provided by OddWatt Audio. You are free to use the schematic for personal, non-commercial use.

Power Supply Schematic KT88 Push-Pull Tube Amp

Figure 2: Power Supply Schematic - OddBlock KT88 Series 1 Tube Amp

Mains power enters the amplifier through an IEC socket located at the rear of the amp. The IEC socket includes a 3 Ampere fuse and an EMI filter. The power transformer is an OEM manufactured by Edcor with ratings of 180V-0-180V at 250 mA and 12V at 4A. Power supply capacitors are Panasonic ECG series 500 volt electrolytic and Solen Polypropyene. The large power resistors are Vishay/Dale wire wound and the remaining resistors are carbon film. The High-Tension (HT) supply is rectified with STTH5 ultrafast high voltage rectifiers and uses CRC filtering. A 12V DC supply is used for the tube heaters. An adjustable delay circuit is used to delay the HT supply on power up.

KT88 Tube Amp Kit Construction

A comprehensive 27 page assembly manual full of photographs (26 to be exact) was sent via email. The detailed step-by-step instructions walk you through the construction the tube amp kit. With just basic soldering skills you should be able to piece a kit together in a few evenings. The kit contains many pieces and starting off by organizing all the parts makes following the instructions much easier. Before you start to mount the hardware, you need to scrape away some of the paint at the IEC socket and at the power ground. The powder coat finish is very thick so I used a small rotary tool to remove the paint at the two grounding points. Full details are provided in the instructions.

Tube Amp Chassis and Parts

Photograph 2: Monoblock Tube Amp Chassis and Small Parts

Following the step-by-step instructions I started to assemble the kit. The sockets provided with the kit are Belton tube sockets.

KT88 Tube Amp Kit Chassis

Photograph 3: KT88 Tube Amp Kit Chassis

I did make a slight deviation from the instructions. The 25-ohm pot (used to adjust the bias) was rotated 180 degrees to allow direct connection to the test points using the 1 ohm resistor leads. Also, there was a note in the instructions indicating that if the metal body of the PIO capacitor touches the chassis it may cause unwanted noise so for extra precaution I used electrical tape to cover the housing while leaving the markings exposed. Use masking tape to label the unconnected leads so you don't make a wrong connection later on.

point-to-point (p2p) construction - KT88 Push-Pull Tube Amp

Photograph 4: Point-to-Point Construction of KT88 Tube Amp Kit Chassis

The CCS and rectifier diodes for the low-tension (LT) heater supply are located on the reverse side of the printed circuit board (PCB) and line up with the chassis ventilation. The CCS heatsink protrudes outside the chassis. To get a good thermal connection I used silicone heat sink paste between the LM317HVT regulator and the heatsink. After stuffing the PCB the final connections are made.

KT88 Push-Pull Amp Kit PCB

Photograph 5: Final Connections - KT88 Tube Amp Kit

Warning: This project uses high voltage. Contact with such voltages can cause injury or death. If you are not familiar with high voltages and how to safely build high voltage equipment, then please do not attempt this kit. Please take time review the safety instructions in the manual and check online sources for high voltage safety.

Final Connections - KT88 Push-Pull Tube Amp Kit

Photograph 6: Final Connections - KT88 Tube Amp Kit

The next step was to power up and balance the bias between the tubes. This is done by using a multimeter inserted into the test points at the top of the amp and adjusting the 25-ohm potentiometer which protrudes from the top of the enclosure. After an hour of operation you should check the balance again. Mine had drifted just a few mili-Amperes (mA) and one month later it had only drifted about 0.3mA.

Finished 5751 SRPP  KT88 Push-Pull Tube Amp Kits

Photograph 7: Finished 5751 SRPP KT88 Push-Pull Tube Amp Kits - Front and Rear

Testing and Measurements - Monoblock KT88 Push-Pull Tube Amp Kit

After balancing the bias I hooked up the amps to a pair of back loaded horns with Fostex FE206E drivers (96 dB / 1 W) and checked for hum. With highly sensitive speakers like this hum is pretty much a show stopper for me. With my ear right up to the driver there was no hum - wonderful!

I was hoping to get about a 60 second delay at power up, however I could not get more than about 30 seconds. Bruce indicated via email that a minor fix to the delay circuit is underway and more details will be available shortly.

I could not get a good temperature reading on the tubes, but expect them to be in excess of 200 degrees Celcius (C). The heatsink for the LM317HVT was about 101 C (the maximum operating temperature for the LM317HVT voltage regulator IC is 125 C). The laminations of the power transformer and audio output transformer were at 65 C and 45 C respectively. The transformer end bells were somewhat cooler. So if you have small children or pets please put the protective chassis cover to good use.

After about 2 months and over 200 hours of service the amp had preformed flawlessly. At that time I checked the bias again and it was within 1mA.

The screen shots below show some large signals into a pure 8 ohm resistive speaker dummy load. The green trace is the input signal and the yellow trace is the output. Sorry for the poor quality of the input waves.

5751 / KT88 PP Tube Amp 1 kHz Sine Wave

Screenshot 1: 1 kHz Sine Wave Response - 13 Watts

All the sine waves (less than 25 W) that I looked at between 20 Hz and 20 kHz were perfect copies of the input waveform. I measured the gain of the amplifier to be about 10.3 (20.3 dB). At an output level of about 15 W the frequency response was flat between 20 Hz and 13 kHz, and up about 0.2 dB at 20 kHz.

5751 / KT88 PP Tube Amp 100 Hz Square Wave

Screenshot 2: 100 Hz Square Wave Response - 17 Watts

Note that the 100 Hz input waveform was not perfectly square to begin with. The 100 Hz square wave response was pretty much an amplified copy of the input waveform.

5751 / KT88 PP Tube Amp 1 kHz Square Wave

Screenshot 3: 1 kHz Square Wave Response - 17 Watts

5751 / KT88 PP Tube Amp 10 kHz Sine Wave

Screenshot 4: 10 kHz Square Wave Response - 17 Watts

The square waves did not show any signs of overshoot or ringing. The 10kHz square wave response is quite typical of what is expected from a transformer coupled output stage.

Despite the simplicity of this tube amplifier circuit, the measured performance is very good.

Listening Impressions - Odd Block KT88 Tube Amp Kits

With the kit supplied JAN Philips 5751 drivers and Genalex - Gold Lion KT88 output tubes the sound was superb. The response is very smooth, there is plenty of detail and that wonderful warm tube sound. The soundstage is wide and the images are solid. Most surprising was the very full bottom end which can sometimes be missing with tube amps.

Photograph 8 below shows the KT88 Monoblock Tube Ampifiers with a NAD C516BEE CDP and a Cambridge Audio DacMagic Plus. The headphones are Sennheiser HD595 and the speakers used for listening were a pair of DIY rear-loaded horn cabinets with high sensitivity Fostex FE206En drivers. The power conditioner at the bottom is a Panamax MR5100.

KT88 Monoblock Tube Amps with NAD C516BEE and Cambridge Audio DacMagic Plus

Photograph 8: KT88 Monoblock Tube Amps with C516BEE and Cambridge Audio DacMagic Plus

I had a number of different 12AX7 tubes on hand that I tired out with the monoblock amplifiers. The 12AX7 did sound decent enough, but did not sound as refined as the 5751. On the output stage I was able to try a quad set of Winged "C" (SED) 6550C tubes which also sounded very good. The 6550 did not have as solid of a bottom end as the Gold Lion KT88, but I did find that prefer them with guitar rich music.

The Oddblock amplifier kits can be purchased directly from OddWatt Audio. For those who would prefer to source their own parts and build a DIY version of the OddWatt amplifiers, full details are available on the DIY 12SL7 SRPP / KT88 Push-Pull Monoblock Tube Amp and the DIY 6SL7 SRPP / KT77 Push-Pull Tube Amplifier project pages.