Updated Acrosound 6L6 Amplifier Circuit

From the 1955 Acrosound transformer catalog, 6L6 amplifier circuit with updated circuit and component parts list.

Original design was for 6L6 or 6L6GB output tubes. Output transformer listed below is for 6L6GC output tubes.

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

C1 in the 6L6 plate high voltage supply was increased from 20uF 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.

Two selenium rectifiers in the screen supply were replaced with one 1N4007 rectifier. The screen voltage will be somewhat higher than the original 270VDC because the 1N4007 has less voltage drop then the selenium rectifiers. Since the screen voltage will remain below the plate voltage this should not be a problem.

The 50K screen bleeder resistor wattage rating was increased from two watts to ten watts. Two watts was marginal.

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

The eight henry 150mA plate supply choke was changed to a more readily available six henry 200mA choke.

The .05uF capacitors were changed to .1uF.

CIRCUIT NOTES:
The 1N4007 rectifier and 10 henry 50mA choke form a choke input filter supplying B+ to the output tube screen grids and 6SL7. There is merit in having the output screen supply isolated from the plate supply, the 1N4007 supplies screen voltage to the output tubes and the 5U4 supplies plate voltage. This provides a more stable screen voltage less affected by plate current voltage variations. Although both screen and plate current comes from the same transformer winding, the lower current screen supply filter capacitor will maintain a steady screen voltage. The fact that screen voltage comes on instantly, but plate voltage has to wait several seconds for the 5U4 to warm up is not a problem. Beam power output tubes employ space-charge suppression such that with plate potential less than screen potential stray secondary electrons are restricted from returning to the screen grid (Reference 1961 RCA tube manual).

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.

Unless otherwise indicated, all resistors 1/2 watt

updated acrosound 6L6 amplifier circuit

PARTS LIST

VACUUM TUBES
1 - 5U4GB
2 - 6L6GC
1 - 6SL7
4 - Octal tube sockets

RESISTORS
2 - 100 1/2 watt
1 - 200 1/2 watt
1 - 250 5 watt
1 - 1K 1/2 watt
1 - 3.3K 1/2 watt
1 - 22K 2 watt
2 - 100K 1/2 watt
5 - 470K 1/2 watt
1 - 510K 1/2 watt
1 - 50K 10 watt
1 - 200K 10 watt
1 - 100 2 watt Potentiometer

CAPACITORS
1 - 500pF 500V mica
3 - .1uF 400VDC
3 - 20uF 450VDC
1 - 100uF 6.3VDC
1 - 100uF 50VDC
1 - 330uf 450VDC

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

T1 - Output Transformer

Primary 6,600CT 30 watts
Secondary 4, 8, 16 ohms

T2 - Power Transformer

700VCT @ 140mA to 200mA
5V @ 3A, 6.3V @ 2.5A

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.

EDCOR Transformers
T1 - Output - CXPP45-6.6K

Primary 6,600CT 45 watts
(screen taps not used)
Secondary made to order

T2 - Power - XPWR155

700VCT @ 200mA
6.3V @ 5A
5V @ 3A
Requires .1 ohm 3 watt resistance in series with 6.3V filament.

Vacuum Tube Amplifier Basics

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