This 5 tube stereo amplifier is based on the output stage of your typical late 1940's 6-tube AM radio amplifier design. The operation of the tone controls was changed to a separately variable type, as opposed to the high-medium-low type of control used on amplifiers of this era.
I get a lot of inquiries about this design. Here is a little history of the design: I designed this amplifier as a stereo version of the output section of an old Trutone radio (made c.1947) I was using with my computer in 1999 (I connected the computer audio to the radio's phono input). The Trutone radio has an 8" electrodynamic speaker that I had re-coned in 1997 and sounded really nice. I used this homebuilt amplifier with some old 12" speakers from a Magnovox console. I made very minimal modifications to the design, and used the amp for about two years before parting it out for other projects.
The output tube is shown on the schematic as a 6K6-GT, but a 6V6-GT, or 6F6-GT tube could also be used. I also recommend that miniature type 6AQ5 be used for the output tubes, and that a single 12AX7 be used in place of the two 6SF5-GT's. The 12AX7 requires a 9 contact socket, and the 6AQ5 requires a 7 contact socket. For basing, look up both types here.
In Construction, the 5 volt secondary of the power transformer will go to the filament pins 8 & 2 of the 5Y3-GT tube. The 6.3 volt winding will go to the other tube filaments.
NOTE!!!! This is the only real parts list for this schematic on the internet. All other have value errors which could be lethal!!!
| CAPACITORS | |||
| ID | Function | Capacitance | Rating |
| C1 | AF coupling | .0047mF | 600v |
| C2 | AF coupling grid 6K6 | .047mF | 600v |
| C3 | Output cathode bypass | 20mF Electrolytic | 25v |
| C4 | Treble tone comp | .01mF | 600v |
| C5 | plate B+ bypass | 200mmF Ceramic | 600v |
| C6 | Filter | 15mF | 450WVDC |
| C7 | Filter | 15mF | 450WVDC |
| C8 | line bypass | .01mF Ceramic | 1400v |
| C9 | Bass tone comp | .0047mF | 600v |
| RESISTORS | |||
| ID | Function | Value (ohms) | Wattage |
| R1 | Volume Control | 2.2 meg tap @ 200k | Audio taper |
| R2 | Input isolation | 220k | 1/2 |
| R3 | Input load | 470K | 1/2 |
| R4 | Output grid leak | 220k | 1/2 |
| R5 | output cathode bias | 330, wire-wound | 2 |
| R6 | Plate load | 220k | 1/2 |
| R7 | treble tone comp. | 2.2 meg | 1/2 |
| R8 | output plate decoupling | 1 meg | 1/2 |
| R9 | B+ Filter | 720, wire-wound | 20 watts |
| R10 | Treble Control | 1 meg | linear taper |
| R11 | bass tone comp. | 33k | 1/2 |
| R12 | bass tone comp. | 22k | 1/2 |
| R13 | Bass Control | 2 meg | linear taper |
| Power Transformer, T1 | |||
| Primary | Sec. 1 | Sec. 2 | Sec. 3 |
| 117v 60 cycles | 600v CT @ 80ma | 5v @ 2A | 6.3v @ 4A |
| Audio Output Transformer T2, T3 | |||
| Primary (impedance) | Secondary (impedance) | ||
| 7600 Ω @ 400 cycles | 4 or 8 Ω @ 400 cycles | ||
| Tube | Function | ||
| 6SF5-GT | AF Amplifier-Left | ||
| 6K6-GT | Output-Left | ||
| 6SF5-GT | AF Amplifier-Right | ||
| 6K6-GT | Output-Right | ||
| 5Y3-GT | Full-wave vacuum rectifier |
Notes
Use this table to guide troubleshooting the finished amplifier. ∞=infinity.
| Location | Voltage to Ground (chassis) | Resistance to Ground (chassis) in Ohms |
| Junction R9 & C7 | 270 DC volts | ∞ * |
| Junction C6 &R9 | 317 DC volts | ∞ * |
| Pin 3, 6SF5-GT | 0 | ∞ |
| Pin 5, 6SF5-GT | 90 DC Volts | ∞ |
| Pin 8, 6K6-GT | -10 DC volts | 330* |
| Pin 5, 6K6-GT | 0.5 volts | 220k |
* Resistance may vary widely depending upon the condition of the electrolytic capacitor in circuit.
The design values in this circuit allow you some flexibility in choosing the tubes used. This table will help you in making those choices. Most of these tubes are not direct plug-in replacement for those shown on the schematic, and will require different socket wiring, or different sockets all together. The tubes shown in the schematic are all octal types.
| Original tube type | Octal, Locktal | 7-pin miniature | 9-pin miniature |
| 5Y3-GT | 5Y4GT,5AX4GT†,
5AZ4(Locktal),
5R4G, 5T4 |
NONE | 5BC3A* |
| 6K6-GT | 6F6, 6V6GT, 7B5(Locktal), 7C5(Locktal) | 6AQ5 | NONE |
| 6SF5-GT | 6F5, 6B6G, 6T7G, 6Q7, 6SQ7, 6SZ7, 7B6(Locktal) | 6BT6, 6AV6, 6BK6, 6AQ6, 6ER5, 6ES5, | NONE |
| Dual-triode substitutions for 6SF5-GT | 6SL7GT, 6SU7, 6SC7, 7F7(Locktal) | NONE | 6EU7, 12AX7, 12DF7, 12DM7, 12AD7, 12AY7, 12BZ7, 12DW7, ECC85, 12AT7 |
Notes
†requires a 5v, 2.5A filament supply.
* requires a 5v, 3A filament supply.
Some tubes, such as the 6AV6 and the 6SQ7 have diode units in addition to the triode. The diode plate terminals should be connected to the cathode terminal.
The 6SC7 is a single cathode dual triode. Since both 6SF5 cathodes are connected to ground, this will not be a problem.
Some octal tubes are available only in glass (GT), some are only available in metal. Those tube types that are available in both glass and metal may have slight differences in the internal connections between the two types. Generally, the metal shell of metal types should be connected to ground
Some tube types are only available in the older style shouldered glass (G). These types are physically larger than GT types.
Locktal types are octal type tubes that lock in place. They require a different type of socket than standard octal tubes. Locktal tubes are not in great demand, so they are inexpensive. Also, locktal tubes are recommended in portable equipment.
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