Knoppson Tube Power Amplifier, KPA-91E
Data per mono unit [drawings]:
Rectifying tube: Philips GZ34 (Full Wave Rectifier)
Amplifying tube: RCA 7247 (Dissimilar RF Triode)
Power Tubes: JJ KT88 (Power Pentode)
Speakers: 4-8 Ohm
Power Output: 20W (Class A)
This amplifier uses Edison Bias and has no feedback.
It has a potentiometer for 100Hz hum calibration.
The potentiometer has however been removed while
50Hz heater hum is more prominent but low still.
Further hum rejection seems to be possible using
screening of driver tube (7247).
This is my fifth attempt in building a commersial power amplifier. I have put lots of energy into making them look
nice. I really like the copper plate which has been both polished and coated to keep it's brilliance but most of
all I like the fact that it uses a rectifying tube for the high voltage and that the heater is AC-fed. The use of
a rectifying tube is far from a disadvantage because it prevents high voltages to build up across f.i capacitors
before the amplifying tubes are heated up. The transformers are both designed by me. The OPTs are produced by
Noratel and the mains transformers are produced by Dernfu, China.