Wow! Now don't think I am bragging but... This amplifier kicks ass.
I built it last night and I am listening now to it with my Fisher tuner hooked up to it.
My first amp only uses two tubes but it acts as 4. The preamp is a 12AX7 with a 6N6 output stage. A bass - treble tone control circuit is connected between the sections of the 12AX7. Now comes the interesting part. The 6N6 tube is a dual section tube. The first triode is a driver and the second is the output. The first section's cathode is connected to the grid of the output stage. There is a built in ballast resistor. You might call this an integrated circuit tube in a sense. This makes for a very easy hook up. Instead of a 6N6, a 6AC5 driven by a 6J5 could be also used.
The power supply is made with a small Hammond power supply transformer, part number 269GX with a 225-0-225 @ 65ma plate secondary and a 6.3 volt filament winding at 2.5 amps. This provides enough power for this amplifier. The rectifier is a full wave made with two diodes. A robust filtering system follows using four 22 uf capacitors and three resistors.
The preamp is the 12AX7 tubes connected using no cathode bypass capacitors. No need in pushing the stage gain in this little amp. There is a negative feedback loop from the output transformer back to the cathode of the second section of the 12AX7. Just a little extra more that makes this such a nice amplifier.
The tone control circuit is one I found on Jogi's German Tube Site. I used only the section between the two tubes. I could have used the whole circuit, but I already had it in my head how most of this was going to be.
I did some experimentation with the tone circuit. I found a program that would show the response when components are changed. It can be found at Duncan Amps. Download the TSC program and then open with this file: 12ax6-6n6-tone.tsc. (you can right click and save target as...). Start the program and load this file. Pick the "James" tab on the tonestack program. This change as shown on the schematic increased the bass response control. That one resistor change improved this amp, in my opinion.
The Output transformer is a Hammond 125D, a 10 watt unit with the impedance set at 8000 ohms. It is a good general purpose transformer. If you need a wider frequency response Hammond makes those too.
The tube shield probably wasn't necessary but It looked nice. The knobs are some old style, the same as I have used on my crystal sets. I like those knobs very much. The chassis is a Bud aluminum chassis measuring 9x7x2 inches. It is slightly big but allows me to mount the front panel controls without crowding. This is the first chassis project that I have built in about 25 years. Good thing I kept all my Greenlee chassis punches!
The wiring going to the volume pot and to pin 2 of the 12AX7 should be shielded. The tube shield doesn't appear to be needed. I wired it that way, just in case of hum problems but this amp is pretty quiet. The layout isn't critical. I added some pictures to show clearer detail of the power supply section and the tone control area. I wired the power supply section and tested it before I built the rest of the amplifier. Doing a nice, slow job pays off in it working right away with no rats nest wiring.
How does this work? Fantastic! This amplifier started working the moment the tubes warmed up. The tone controls work very well. Presently I have don't have an audio generator or distortion analyzer to measure performance. So all is done with the ear test. I am using this with a small Polk Audio speaker I have had for 15 years or so. There is more than enough audio for me. If you like building things like this, I can highly recommend this project to you.
Should you build this? Yes, if you have some experience making tube projects that run on the ac mains. If you don't have this experience, it is recommended you find a friend close by that can check your amp before you plug it in. If you have small children around, a metal plate fastened to the bottom would make this amplifier safe.
Would you like to see how others have built this little amplifier? Here is a fine example.
Under The Chassis
Amplifier Back View
Power Supply Wiring Detail
Tone Control Wiring Detail
Disclaimer: My copyright only extends to the JPG schematic drawing images themselves. I make no claim to the circuit design.
12AX7-6N6 Amplifier Parts List
Resistors: 1/2 watt unless otherwise noted 2 - 100 ohm, 1 watt (1/2 watt is fine here) 1 - 470 ohm, 5 watt (500 ohm) 1 - 2.2k 1 - 3.3k 1 - 6.8k 2 - 10k 3 - 100k 1 - 120k (use 100k) 2 - 220k 2 - 470k Pots: 1 - 100k Log (Audio) Taper Volume control with power switch 2 - 1meg Linear Taper Capacitors: 1 - 27pf disc ceramic 1 - 150pf disc ceramic 2 - .0022, 600 volts mylar (2,2nf) 1 - .022uf, 600 volts mylar (22nf) 1 - .01, 1000 volts disc ceramic (10nf) 1 - .047uf, 600 volts mylar (47nf) 2 - .1uf, 600 volts mylar (100nf) 4 - 22uf, 450v electrolytic Transformers: 1 Hammond 269GX Power Transformer 1 Hammond 125D Audio Output Transformer Misc: 1 - 12AX7 Tube 1 - 6N6G Tube 1 - Miniature 9 pin tube socket 1 - Octal Tube Socket 1 - 7x9x2 inch aluminum chassis, Bud AC406 1 - 0.5 amp, 125 volt fuse 1 - Panel mount fuse holder 1 - RCA Phono Jack (Input) 2 - Johnson Binding Posts (speaker) 3 - Knobs 1 - Neon Pilot Lamp 2 - Terminal Strip, 6 lug, T G T T G T (T= Terminal, 1 - Terminal Strip, 4 lug, T T G T T G=Grounded terminal) 1 - Terminal Strip, 3 lug, T G T 2 - Terminal Strip, 2 lug, T G 1 - Neon Pilot Light 5 - Grommets (2 for each transformer and one for power cord) 2 - 1N4007 Diodes 1 - 2 wire AC cord - Screws and nuts, hook up wire