Updated Acrosound KT66/807 Amplifier Circuit

From the 1955 Acrosound transformer catalog, KT66/807 amplifier Williamson type circuit with updated circuit and component parts list, ultra-linear or triode operation.

Circuit drawing updated by showing connection points with a dot to avoid confusion, lines that don't connect were left 'hooked over' other lines.

The original 5V4 rectifier was changed to a 5U4. The 5V4 is not currently being made and only available as new old stock.

C1 in the high voltage supply was increased from 10uF to 330uF for better performance. Using a snap-in type 35mm diameter electrolytic allows using a standard capacitor clamp to mount C1 on the chassis top side.

A 200K ten watt bleeder resistor was added to the plate high voltage supply for safety reasons.

The eight henry 200mA plate supply choke was changed to a more readily available six henry 200mA choke.

The .25uF capacitors were changed to .33uF, .25uF are hard to find.


Although KT66 datasheets specify that each tube has its own cathode resistor this circuit ties the cathodes together. There is a 100 ohm potentiometer in the cathode/grid circuit that allows some balancing of the tubes.

The hum balance pot wiper connects to the output tube cathodes. This method feeds phase adjustable 60HZ to the cathodes to 'cancel' inherent amplifier hum. If a transformer with a center tapped 6.3V filament winding is used it may be more effective to ground the center tap and omit the 100 ohm two watt hum balance pot. Do not have both the center tap grounded and the hum balance pot connected, doing so will upset output tube cathode bias.

When the amplifier is operational, disconnect the capacitor/resistor feedback from the output transformer secondary. Being extremely careful, with the amplifier on and playing at low volume connect the feedback to the output transformer secondary. The audio level should drop a bit. If the audio level does not go down or increases a bit, then most likely the feedback is not properly phased. Try reversing the output transformer secondary so the other end of the secondary is common ground. Repeat the test.

The triode output option is shown on the bottom right of the circuit drawing.

Unless otherwise indicated, all resistors 1/2 watt

updated acrosound KT66/807 amplifier circuit


1 - 5U4GB
2 - KT66 or 807
2 - 6SN7
5 - Octal tube sockets

2 - 100 3 watt
1 - 300 10 watt
1 - 470 1/2 watt
1 - 560 1/2 watt
2 - 1K 1/2 watt
2 - 10K 1/2 watt
3 - 22K 1 watt
1 - 33K 1/2 watt
1 - 47K 1/2 watt
2 - 47K 2 watt
2 - 100K 1/2 watt
1 - 200K 10 watt
2 - 470K 1/2 watt
1 - 1M 1/2 watt
2 - 100 2 watt Potentiometer

1 - 68pF 500V mica
4 - .33uF 630VDC
1 - 10uF 600VDC
3 - 20uF 600VDC
1 - 220uF 100VDC
1 - 330uf 550VDC

1 - 6H 200mA Choke - Triad C-14X
1 - 10H 50mA Choke - Triad C-3X

T1 - Output Transformer
Primary 7,000CT to 8,000CT
50 - 60 watts
Primary 4,000CT
25 - 30 WATTS
Secondary 4, 8, 16 ohms

T2 - Power Transformer
800VCT @ 200mA
5V @ 3A, 6.3V @ 4A

EDCOR Transformers
T1 - Output Ultra-Linear
Primary 7,600CT 50 watts
Secondary made to order

T1 - Output Triode
Primary 4,000CT 25 watts
Secondary made to order

T2 - Power - XPWR011
800VCT @ 200mA
6.3VCT @ 5A
5V @ 3A
Requires .05 ohm 3 watt resistance
in series with 6.3V filament.

Finding transformers requires some searching. EDCOR is one possible source. On output transformers, EDCOR usually supplies a single output impedance. You can inquiry about having the transformer made with 4, 8 and 16 ohm output taps. A single 8 ohm output will work without upsetting the feedback loop. Another possibility is having custom transformers wound at Musical Power Supplies.

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High Voltage Audio
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DISCLAIMER assumes no responsibility for damage, injury or otherwise related to any use of information on this site or given by other means. Many electronic circuits, in particular vacuum tube circuits, operate with dangerous voltage and current. Always excercise care when working with electronic circuits.