HIGH POWER PUSH-PULL MONOBLOC AMPLIFIER

POWER TRANSFORMER – The power transformer is a New Old Stock, hermetically sealed, military grade unit supplying 500 volts B+ with a Mullard GZ34 rectifier tube. The GZ34 tube has a delayed output voltage to allow the output tube filaments to fully warm up before B+ voltage is applied thus extending output tube life. Also, the power supply is a CLC unit employing two oil caps and an inductor rather than the lone 50 uF electrolytic used in the HF-60.

OUTPUT TRANSFORMER – The output transformer is the humongous (14 lb.) T-269 60 Watt unit made by Rex, the same manufacturer as the excellent little Olson Ultra-Fi transformers. No one seems to know for sure, but several have suggested that the T-269 is an early Tamura design. Primary impedance is 6,600 ohms with UL screen taps and 4, 8 & 16 ohm secondary impedances. It is a very wide band transformer with flat response well beyond the audio band. Extra effort was made to assure exceptional high frequency stability as this is of greater importance than having response flat out to 100 kHz or thereabout.

One improvement is use of Zener diodes (about 35 volts) and a regular diode in series with each power output tube screen. This operates the screens at a lower voltage and also below the plate voltage. Too high screen voltage is where modern day EL34 output tubes start running into trouble.

OUTPUT TUBES – A NOS quad of (factory matched) Svetlana Gold Top tubes are supplied. These are selected at the factory and reckoned to have much better quality control than new EL34s being manufactured today. Several other tubes could be used such as the Philips 7581 or similar tubes. Like the EICO HF-60 amplifiers, the output stage operates in fixed bias. A balance control and a bias control is provided to allow proper calibration of the output stage operating point. This needs to be carefully set as the output tubes are operated at near their maximum ratings. Note that 2 Watt molded pots (like Ohmite AB) are used – not the less expensive small carbon comp pots as used in the original HF-60.

DRIVER STAGE – Like the HF-60 the 6SN7GTB tube is used for the driver stage. The HF-60 employed the driver stage as also the phase inverter. Unequal plate resistors were used in an attempt to balance the drive signals to the output stage. American Tube Amp disapproves of this aspect of the HF-60 design. The driver is strictly a driver stage with precisely matched plate load resistors. The correct values were used to provide maximum drive voltage at minimum distortion.

INPUT STAGE – The EICO HF-60 utilized an EF-86 pentode input stage, a tube not particularly well regarded by the ‘golden ears’ crowd. In the past some have recommend that the EF-86 be run in triode mode. However, in our design a 12AX7 tube was used to provide a linear triode gain stage and also a separate cathodyne phase splitter. One advantage of this design concept is to allow the listener to use a 12AX7 tube brand of preference.