Hashimoto KT-88 UL Push Pull

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KT-88 Push-Pull Project with Hashimoto Transformers

This amplifier was featured on the February 2004 Issue of the "High-Fidelity Audio: Radio Technology" by Mr. Tadaatsu Atarashi.  Mr. Atarashi originally built this amplifier as the fixed bias EL34 Class B Push-Pull monaural amplifier featured on the January 2003 Issue of the "High-Fidelity Audio: Radio Technology".  Then, Mr. Atarashi modified the amplifier to fit for KT-88 AB1 Push-Pull Ultra-Linear operation.   

As you could notice by comparing these two amplifiers, there were very minor changes between them except the differences in power supply circuit.  Instead of the original full-wave bridge rectifier for the B+ power supply, Mr. Atarashi chose the half-wave rectifier for both B+ and bias power supply on this amplifier.  According to Mr. Aarashi, the half-wave rectifiers increase the depth of sound image by eliminating the soft bass sound (need testing to verify his opinion).  For this purpose, Mr. Atarashi used a 30A high-voltage-rated Schottky Barrier Diode.  Mr. Atarshi also connected the anode of this diode to the ground.  By doing this, Mr. Atarashi claims it improves the sound (need testing also).             

Because Mr. Atarashi was modifying two mono-bock  EL34 power amplifiers to the KT-88 version, he compared the sound of amplifiers after just one amplifier was converted to the half-wave rectifiers (another was with a bridge rectifier for the B+ and a full-wave for the bias).  According to this comparison, Mr. Atarashi concluded that the amplifier with the bridge and full-wave rectifiers sounded brighter, also yielded too much bass sound.  On the other hand, the amplifier with the half-wave rectifiers sounded more responsive and realistic.  Evidently, Mr. Atarashi preferred the sound from the half-wave rectifiers over the sound from the full-wave and bridge rectifiers.





KT-88 Class AB1 Fixed Bias Ultra-Linear Push-Pull Data Sheet (40%  Screen tap)


Opt 1 Opt 2
Plate, Screen B Voltages: Va,g2(b) 560 Volts 460 Volts
Plate, Screen Voltage (0 Signal): Va,g2(0) 553 Volts 453 Volts
Plate + Screen Current (0 Signal): Ia+g2(0)  2 x 50 mA 2 x 50 mA
Plate + Screen Current (Max Signal): Ia+g2(max sig) 2 x 157 mA 2 x 140 mA
Plate to Plate Load: RL(a-a) 4,500 Ohms 4,000 Ohms
Control Grid Voltage: Vg1 (approx) - 75 Volts - 59 Volts
Power Output: Pout 100 Watts 70 Watts
Total Distortion: Dtot 2% 2%
Intermodulation Distortion (60&6000Hz 4:1 Amp): I.M. 11% 10%
Pa+g2(0) 2 x 27.5 W 2 x 22.5 W
Pa+g2 (max Signal) 2 x 33 W 2 x 27 W
Voltage between C Grids: V(g1-g1)(ac) crest 140 Volts 114 Volts
Zout 7,000 Ohms 6,500 Ohms

Schematic and Actual Characteristics

bulletSchematic & Harmonic Distortion


bulletInput/Output & Frequency Response



bulletKT-88 Data Sheet from Genalex
bulletKT-88 Data Sheet from GEC 

           Data Sheets are from Frank Philips and Duncan's




























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Last changed: 02/01/15

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Last updated: 02/01/15.