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
You will notice that I have finally redrawn the schematic after having it for years in the crappy hand-drawn version.
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.|
|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 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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