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5751 SRPP / KT88 Monoblock Tube Amp Kits

Giovanni Militano

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Screenshot 3: 1 kHz Square Wave Response - 17 Watts

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.

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.

Last update 1 September 2014. Created 30 November 2009. ©
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