Single-Ended (Class A) 6L6 Guitar Amp
I wanted to create a reasonably loud and simple amp without extra crap like tone control. Just a plug and play amp that sounded good with maximum one knob. I also wanted to use a 6L6 and some older types of tubes (octals), just for show.
Schematics / Parts List
- Amplifier Schematic - Just a very conventional simple Class A design. One half of the 6SL7 is used as a preamp, no cathode to provide a little source degeneration and a little more linear amplifification - more gain was not needed anyway. The other half of 6SL7 is used as part of the amplifier inside the feedback loop. Overdriving the input is pretty easy since the cathode bias is only about a volt with a 6SL7.
- Power Supply Schematic - I only had a 2 amp tap on the 5 V winding, so options were limited pretty much to 5Y3GT, 5AR4 and 5V4G. 5AR4's are kind of scarce and the 5Y3's plate impedance is too large - I wanted the B+ to be as high as possible. The 5V4GT is a nice compromise. The filament current is only 2A and the plate impedance is similar to the 5AR4, in fact, some people mention using it as a cheap 5AR4 substitute. The transformer's high voltage winding was 700 VCT.
- Tube line-up: 5V4G, 6L6G, 6SL7GT
- Parts List:
- Hammond 125ESE output transformer
- Thordarson 10 Hy choke
- Power Supply Transformer
- (eventually will list all parts.)
Lab and Test Results
The following test results are from the tube complement shown above in the pictures. These results obviously will vary on the tubes used. Some 6L6's will be more linear than others, more transconductance will create more open loop feedback which will reduce the distortion when the feedback loop is closed and better rectifiers will have a lower plate impedance resulting in a higher B+ voltage.
- V line = 117 V
- B+ = 397 V
- B+1 = 272 V
- B+2 = 219 V
- V 6L6 plate = 393 V
- V 6L6 cathode = 16.3 V
- R cathode = 310 Ohms
- I cathode = 53 mA
- Preamp Gain = 28.4 V/V
- Minimum Distortion @ 1 kHz output = 1.23% (0.3 W output power)
I set the level to provide 5% distortion @ 1 kHz before doing the sweeps, allowing the comparison to a reference.
Please forgive the quality of these recordings, I have a crappy mic preamp and I have to 4x gain the audio clip after recording it, so right there I lose 2 bits of resolution and I'm down to 14. The background noise is from the mic preamp, the amplifier itself is actually pretty quiet with no input, although I forgot to measure this. The slight amount of fret noise on the neck is also a little annoying, not to mention my crappy playing. Audition at your own risk.