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.

Top of Completed AmpTop of Completed Amp Click here to see all the pictures in the gallery.

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:

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.

Measurements

Conditions:

  • 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)

Frequency Sweeps

I set the level to provide 5% distortion @ 1 kHz before doing the sweeps, allowing the comparison to a reference.

NU 6L6G Sweep: Power and Distortion vs. Frequency with the NU 6L6GNU 6L6G Sweep: Power and Distortion vs. Frequency with the NU 6L6G RCA 6L6G Sweep: Power and Distortion vs. Frequency with the RCA 6L6GRCA 6L6G Sweep: Power and Distortion vs. Frequency with the RCA 6L6G

Sound Clips

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.

  • Walk Away - This demonstrates what a clean tone with slight overdrive sounds like
  • ACDC riff - preamp gain cranked, nice little rock sound

re:

Hola! I'm a fair lady but I soo love guitars! Dang! Jammin' with my guitars have never been great if it weren't for my various signature electrics! Not to brag but I have my beloved Fender Strat, Washburn X series and Ibanez Roadstar! Ain't it great lovin' music with all your fave equipments?

Single-Ended (Class A) 6L6 Guitar Amp

Thanks so much for sharing your knowledge. BTW the amp has a beautiful tone. I am looking at building a 5AR4...6SL7...6L6 SE but I am hoping to try a PSE (parallel) with two 6L6s. I am a lowly harmonica player plagued (as we are) with feedback, and guitar players with amps that go up to 11. I am fairly new to amp building but am part of a group of experienced builders. Excellent work !

Nice Amp ... Nice Riffs!

Good job with the amp. How about a few more sound clips? I like what I have heard so far.

Credit for putting the AMP together

Nice work! I give you alot of credit putting together this guitar amp. Rock on!

Thanks!

Thanks!

Looks like the author has

Looks like the author has large experience in the subject. Thank you for the info.

WOW! I've been looking for

WOW! I've been looking for an octal front SE 6L6 based amp for a harp amp. I hope you don't mind If I judiciously borrow from your intelectual properties. The project I'm intending is not for profit, It's for a long time friend who's asked me to build him an amp. I love the fact that you designed this "Bare Bones", It's allowed me to modify the design to EXACTLY the way I imagined. The fact that it can have such a SANITARY CLEAN sound gives me all the room I need to go from that to Dirty Delta Blues.

I do have a few questions about your PS design and why you chose to keep your B+1 and B+2 voltages so low. is this for higher clean headroom?

Thank you for your Ingenuity

Ray

Thanks!

Feel free to borrow from the design. Glad someone can use it.

Well, B+ is close to 400V and that's the plate voltage, so that will mostly determine clean volume. I was using a 6L6G, and those early 6L6's can't dissipate as much heat as the newer 6L6GC's, so that was the reason for B+1 being 272V. As it is, I was probably a little over spec with just the quiescent power. If you use a GC, you can get more power out of it.

6L6 single ended

Hi there ...

I read on the Valve Wizard site that when designing SE output stages, the anode voltage should never be more than half of the max rating. Most sources give the 6L6GC a max of 500v. So that lead me to believe that 250v would be the max plate voltage for 6L6 SE. However here on your design you are close to 400v and on most other sites the typical 6L6 plate voltage for SE operation is around 350v ... Can you shed any light here for me?

Thanks, Jason.

Max Plate Voltage "Rule"

Hi Jason,

I have never heard of such a "rule". The max plate voltage rating on a tube is a DC voltage rating, not AC, and it doesn't matter what type of output stage it is: Single ended, push-pull, class A or AB. In fact, Fender, among other amp makers, commonly exceeded the max plate voltage datasheet specs, most notably on several 6V6 amps. I believe the max plate voltage rule is just a guideline. Tubes have manufacturing tolerances and electrodes in the tube have maximum breakdown voltages according to electrode spacing. Also, increased plate voltages will result in increased secondary emissions which will degrade the performance of the tube.

I think a more important guideline is maximum plate power dissipation. If you adhere to that, you'll have reasonable output tube lifetime. If you adhere to both plate voltage and plate dissipation specs, you'll build yourself a reliable amp with few if any output tube failures.

Good Luck, Imre

ACDC riff - broken link

i tried to download the ACDC riff sample, but i came out with a broken link...

...but the clean sound is great anyway!

Fixed

I fixed the broken link. I also noticed that on the clean file, there was some distortion. This was in the recording setup, not the amp. Sounds like the microphone preamp was overloaded. Thanks!

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