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.  A 5U4-G rectifier is used so that the power supply will have plenty of pep to run the amp. This amplifier is basically a Magnovox amplifier I bought on ebay years ago and rebuilt after building the 6 tube amp. It is really amazing how it sounds, and the circuit is very minimalist, as you can see. Because of the output setup, you must use a quartet of matched 6BQ5 tubes.

There's nothing finer that listening to beautiful music while watching the tubes glow in a darkened room

 

I have noticed, using speakers with 12"-4"-2" sizes, that this is really bottom heavy, even with the preamp tone controls set for flat reproduction.

You will notice in looking at the schematic my preference for the balance control. I find this a convenient way to compensate for less than optimal speaker placement in my living room

Schematic:

Read the notes in the parts list on the balance control.

Above, the amplifier's schematic. In addition to the 6BQ5 output tubes, a 6EU7 dual-triode is used in each channel. This amplifier also uses a curious phase-inverter circuit I have heard described as a "paraphase" inverter.

 

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 Fixed balance, omit when using balance control R25. See note at 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
R25* 750 ohm pot, 1/2 watt linear taper left-right balance control. Omit R4 and R24 when using the balance control. * See Note
R26 10K 5 watts filter

*Note R4 and R24, optional fixed balance control, omit 750 ohm 1/2 watt pot.  The original balance control was mounted inside the tuner. When I began using this amplifier without the tuner, I had to add two resistors in place of the connections to the pots. For whatever reason, I installed two 330 ohm resistors and used the amp this way for years before the rebuilding when I added a balance control pot. I was lucky to find a pot with the same value as the original at a real electronics parts store. Many readers have emailed saying they are having difficulty finding a 750 ohm pot. If this is the case for you, just omit the pot and use the 330-ohm resistors as shown.

Capacitor ID Value Function, notes
C1, C4 100mmF 500V AF plate, silvered mica
C2, C5 .0047mF 630V AF coupling, metalized polyester
C3, C6 .047mF 630V AF coupling, metalized polyester
C7 .01mF 1400V Line bypass, ceramic
C8 10mF 450V Filter, electrolytic
C9 33mF 450V Filter, electrolytic
C10 47mF 450V Filter, electrolytic
C11 20mf 25V Output cathode bypass, electrolytic

Note: When the schematic was redrawn, it was discovered that C1 and C4 were shown in the incorrect location. They should connect pin 7 of each 6EU7 to ground, and not pin 8 as was shown on the old version.

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
117V AC 60cps .92A 600V CT 130ma 5 volts 3 amps 6.3 volts 4 amps

 

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, but it is preferable that the feedback connection go to a 4 ohm output as shown.     # 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 3-prong line cord and connect the chassis to ground. Make sure the fused and switched side of the line is on the hot (black wire) side of the line.
  5. Keep component leads short and filament wires close to the chassis.
  6. Use a common ground point, if possible.
  7. If you really wanted to, you could make this a stand-alone amp by using a 2-gang 500K audio taper pot for the inputs. I've noticed there's plenty of gain in this amp connected directly to a line out device, but you'll want a volume control.

 

Updated 11-20-09

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