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
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.
|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.
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