This was an experimental project using a 6SN7 as a single ended output tube. Measured power output at 1KHZ is ½ watt sufficient to drive headphones loud and speakers in a small room to a moderate listening level. There are two inputs, each with their own level control in a mixer configuration. Input sensitivity is .5 volt RMS in for ½ watt out. With the tone control set in middle rotation frequency response measured 40HZ to 18KHZ -3db points. The tone control only adjusts the high end, full rotation right is high end boost and full rotation left is high end cut. The input has a low-cut filter rolling off frequencies below 40HZ to help reduce bass distortion. Due to the low output power some limitations are expected.
Power consumption for a tube amp is very low measured at 25 watts. With low power consumption the amplifier puts out moderate heat and can be on for several hours a day without running up a large electric bill. Although ½ watt is not a lot of power the amplifier will drive more efficient speakers loud enough for casual listening. The amplifier was used by this author for several months and found the sound quite pleasing. The amplifier was given away and the new owner also found it enjoyable.
Compared to most stereo amplifiers this project is not very expensive to build. The most expensive item is the custom milled chassis plate used here, a standard chassis would be considerably less expensive.
The chassis is a flat aluminum plate 3mm thick. All holes and lettering were milled to give the amplifier a professional look. The base is a black steel chassis upside down. The chassis flange holes were threaded for securing the chassis plate on top. Rubber feet are attached to the base bottom (chassis top) with screws in threaded holes.
This is a stereo two-channel amplifier using three tubes, a 6SN7 in each channel and a 12AX7 each section used by a channel.
The output transformer is a low cost P-T1750A replacement used in Fender reverb units purchased from tubesandmore.com. The primary should be wired so the red wire goes to B+ and blue wire to 6SN7 plate. Normally the output secondary black wire would go to ground and green wire is speaker +. To insure the secondary is wired correctly for negative feedback connect the R24/C13 feedback loop last. While listening to audio connecting the feedback loop should cause the audio level to drop slightly. If instead the level goes up, then the output transformer secondary black and green wires need to be reversed. The primary impedance of the output transformer is actually higher than it should be, but was a low cost option for an experimental amplifier.
The amplifier uses fixed bias on the 6SN7 output stage allowing to adjust bias for different brands of 6SN7 and compensate as tubes age. Voltage at the 6SN7 output cathode test point should read between .18VDC and .20VDC. This correlates to 9MA to 10MA plate current.
There are actually two feedback loops. The R24/C13 feedback loop helps reduce distortion in the output transformer. The second feedback loop is plate to plate feedback using R24. All total there is only a few db of feedback.
The input low-cut filter is comprised of C5, C6 and R9. The roll-off after 40HZ is a gentle slope.
Tone control wiring, full counter-clockwise rotation is high end cut (pot wiper at .0047uf capacitor). Full clockwise is high end boost (pot wiper at 100pf capacitor).
Power transformer used was purchased from Allied Electronics part number 6K88VG.
PRI: 110-120V, 50/60HZ,
500VCT @ 40DCMA,
6.3VCT @ 2.0A
The completed amplifier chassis plate underside. Mounting terminal strip terminals so the grounded lugs line up allowed running buss wire through the amplifier. Using buss wire for ground points makes wiring simpler and neater. Some may criticize using terminal strip ground lugs and multiple ground points as bad design, but this author has never had noise or hum problems using this method.
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