Five Tube Stereo Amplifier

4 watts output max. per channel, 8 watts bridged.

This 5 tube stereo amplifier is based on the output stage of your typical late 1940's 6-tube AM radio amplifier design. Its simplicity invites a little experimentation from those who enjoy tinkering.

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.

After doing some recent work recapping my old Trutone, I thought I would freshen this page up a bit. As you can see, I have redrawn the schematic and simplified it greatly by removing the tone controls (something I should have done in the first version). I have also changed the tube line-up to more accurately reflect the prototype. I have also corrected a few typos. In older versions of this page, the output transformer impedance was given as 7600 ohms, it is actually 3300 ohms.

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.

Parts List

Don't forget hook-up wire, sockets, switch, and a chassis or circuit board. All tubes are octal type and will require an octal socket.

CAPACITORS
ID	Function	Capacitance	Rating
C1, C4	B+ decoupling	200mmF Silvered Mica	600v
C2, C5	AF coupling grid 6V6	.047mF Metallized polyester	600v
C3, C6	Output cathode bypass	22mF Electrolytic	25v
C7	Filter	47mF Electrolytic	450WVDC
C8	Filter	33mF electrolytic	450WVDC
C9	Line bypass	.01mF Ceramic	1400v
RESISTORS
ID	Function	Value (ohms)	Wattage
R1A & B	Volume Control	500K Dual Pot	Audio taper
R2, R6	Feedback	1 meg	1/2
R3, R7	Plate load	470K	1/2
R4, R8	Grid bias	220k	1/2
R5, R9	Output cathode bias	330, wire-wound or metal oxide	2
R10	Voltage divider	750, wire-wound	20 watts
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)
3300 Ω @ 400 cycles	4 or 8 Ω @ 400 cycles
Tube	Function
6SQ7-GT	AF Amplifier-Left
6V6-GT	Output-Left
6SQ7-GT	AF Amplifier-Right
6V6-GT	Output-Right
5Y3-GT	Full-wave vacuum rectifier

Notes

I recommend carbon composition resistors, but, if you find them too drift prone, carbon film resistors should be used instead.
For best audio reproduction results, 90db/watt speakers or higher efficiency should be used.
Shield any hook-up wires used in the signal path between the input jack and the first audio tube.
A choke coil with a DC resistance approximate to 750 ohms may be substituted for R10.
R10 will generate considerable heat, and care should be taken in its location. I used a large adjustable wire wound resistor for this and mounted it above the chassis. A choke coil in its place would give better performance and more closely resemble the electromagnet field in the prototype.

Reader's Contributions

2-7-14 George in Ohio, who also built the amp featured on the 9-tube amp page, emailed me about this amp and included these three photos and had this to say:

"Wes, I built the 5 tube stereo too, about 3 years ago. Told my wife it is hers. She uses it on her computer. It has wonderful tone, and big sound. She loves it, and listens a lot to streamed music. I highly suggest this as an easier project for first timers. The card is for scaling purpose. Thanks again George"

Thanks George for sharing the photos of the beautiful amp you've built! As you can see, big tube sound does not need to take up lots of space.

Updated 2-7-14

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