7 Tube Stereo Output Amplifier

20 watts max output per channel. 

This is a 7 tube 20 watt amplifier that must be used in conjunction with a pre amplifier having volume an tone controls. For this, I recommend this 6 tube pre-amp. With this amplifier you may use 6BQ5 output tubes, or 7189 output tubes. The 7189 is a lower distortion version of the 6BQ5 which also offers a few more watts of output. A 5U4-G rectifier is used so that the power supply will have plenty of pep to run the amp. Feedback is also employed to cut down on distortion. Even with this feedback, the amplifier is very responsive and very fast.

Schematic:

 

The amplifier uses a 6EU7 in each channel as an amplifier and phase inverter. Also, a balance control may be optionally used with this amp to balance the channels.

Parts list

Resistor ID Value Function, notes
R1, R15 470K 1/2W 5% Input load
R2, R16 47K 1/2W 5% Input isolation
R3, R17 2200 1/2W 2% matched Cathode 
R4, R24* 330 1/2W 2% matched Bias, see R25
R5, R18 5600 1/2W 5% Cathode
R6, R19 39K 1/2W 5% Grid-leak voltage divider
R7, R14 220K 1/2W 5% Plate load
R8, R22 470K 1/2W 5% Grid-leak voltage divider
R9, R23 3900 1/2W 5% Feedback isolation
R10, R21 220K 1/2W 5% Grid leak
R11, R20 220K 1/2W 5% Plate load
R12 100 ohms 5 watts W.W. Output cathode
R13 750 ohm pot, 5 watts hum adjust
R26 10K 5 watts filter

* R25 750 ohms 1/2 watt pot, balance control. Optional. If used, omit R4 and R24.

Capacitor ID Value Function, notes
C1, C4 100mmF 500V AF grid, silvered mica
C2, C5 .0047mF 630V AF coupling, metalized polyester
C3, C6 .047mF 630V AF coupling, metalized polyester
C7 .01mF 1400V Line bypass, ceramic
C8A 10mF 450V Filter, electrolytic
C8B 30mF 450V Filter, electrolytic
C8C 40mF 450V Filter, electrolytic
C8D 20mf 25V Output cathode bypass, electrolytic

C8 A-D amy be individual units, if you prefer.

Choke ID Inductance (henries @ 1000 cps) Current, notes
L1 1 henry 130 ma @ 330 volts, U-bracket

 

Power Transformer, T1
Primary Sec. 1 Sec. 2 Sec.3 Sec. 4
117V AC 60cps .92A 600V CT 130ma 5 volts 3 amps 6.3 volts 4 amps 6.3 volts 4 amps

My power transformer came from a large console and had separate filament windings for a tuner and an amplifier. connect sec. 3 to the 6EU7's (one side grounded),  and connect Sec. 4 to the 6BQ5's and R13 as shown in the diagram. Sec. 2 goes to the 5U4-G filaments.

Output Transformers T2, T3
Primary Impedance Primary DC resistance Secondary Impedance Secondary DC resistance
8000 ohms,CT @ 400 cps 300 ohms total 4 ohms @ 400cps*    0.3 ohms #

* Match to speakers you are using     # May be different if your impedance is not 4 ohms.

Tube line-up:

Notes:

  1. Speakers: I have been using 85db/watt speakers to great success, but 90db/watt speakers or better would give best results.
  2. The rectifier tube should be mounted vertically, with plenty of room to breathe.
  3. Heavy jacks or binding posts are recommended for speaker connections.
  4. You may want to install a fuse in the primary circuit of T1.
  5. Keep component leads short and filament wires close to the chassis.

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