Tuesday, June 23, 2015

More Oscilloscope Tests.... now on my Simple20

Hey Merlin, if you are reading this I have real screen caps from my Rigol this time ;-)

I decided I wanted to see what the waveforms look like for my scratch-built Simple20 guitar amp. I made this amp in the Summer of 2014. (You can see a description of the Simple20 guitar amplifier at the end of this post.)

I used a function generator app on my Android phone to get me a 400Hz signal at about 80mVp-p for testing. As you can see, I got a nice and clean sine wave from that setup.
Here is the color-key for these traces in the screen caps...

Yellow: input signal from phone (50 mV/div)
Light Blue: post 1st gain stage coupling capacitor (50 mV/div)
Pink: post 2nd gain stage coupling capacitor (input to master volume control) (5 V/div)
Dark Blue: post phase inverter (one side of long-tailed-pair) coupling capacitor (50 V/div)
Note that the volts-per-division settings are different for each trace

This first picture is with the preamp volume  at 100% and the master volume at 0%. As you can see, each gain stage keeps the signal nice and clean... no clipping or distortion of the wave is evident at all. I do notice that not everything is exactly 180-degrees out of phase. I'm not sure why that is the case. I want to investigate that further. I also would have expected the Lt Blue trace to be more than only about twice the peak-to-peak voltage of the input signal. As you can see the voltage then jumps way up to 21.6 Vp-p after the second gain stage with the volume on full-blast.
Preamp Volume @ 100%, Master Volume @ 0%

Now with the other settings equal, I started to turn up the master volume. Here the MV is at about 75%. Looks pretty clean on the phase-inverter output. However, from real-world experience with this amp, things are really crunchy at this point so all that crunch must be from the power tubes at this setting. Notice the phase inverter is sending about 52 Vp-p to its power tube.
Preamp Volume @ 100%, Master Volume @ 75%

Here the master volume is at 100%. Notice now we are driving the phase-inverter into pretty heavy clipping on the peaks of the wave. The troughs are still pretty smooth but they are starting to noticeably flatten. From real-world experience this is pretty heavily distorted to the point where I don't particularly care for the sound. It's not blocking distortion or anything... just not pleasing to my ears is all. You can see the phase-inverter is putting out 104 Vp-p at this setting. It is contributing quite a bit to the distortion of the amp and at that voltage level, the power tubes are probably distorting like crazy too. My next experiments will involve breaking out my 100x probes and looking at the waveforms coming out of the power tubes at these same settings.
Preamp Volume @ 100%, Master Volume @ 100%

Background Information on the Simple20...
The Simple20 is a mating of the AX84.com "Core Projects" Simple Preamp and  20-Watt Push-Pull Power Amp that uses a long-tailed-pair phase-inverter and 6V6 power tubes. The amp was originally designed for use with 12AX7 preamp tubes. I currently roll JAN Phillips 12AT7 tubes for both the preamp and the phase-inverter and JJ 6V6 power tubes. I added a one-tube reverb circuit to the amp and I run it all into a 12" Warehouse Guitar Speakers (WGS) ET65. I used Edcor power and output transformers. This amp sounds absolutely AMAZING. When that WGS speaker got broken-in this thing just came alive. I get A LOT of comments from people on how nice it sounds - quite often from other guitar players who wonder what the heck it is. I mostly use it with my PRS Single Cut with their #7 pickups. This is the amp and guitar I use live for the band I play in: the Sonhouse Blues Band. It has an amazing clean tone, a fantastic edge-of-overdrive tone which I can dial in directly or it is also easy to switch-in with a nice clean boost pedal. It can get nicely nasty when pushed hard and with the right pedals this amp will do anything very very well. For a clean boost, I currently use the EH Soul Food pedal.

This is a picture of the inside of my Simple20 and how I hooked-up the probes for these screen caps:
Probing the Simple20

The second tube socket from the right is unused. There are some components on some terminal strips at that socket but they are currently not connected to anything. I was experimenting with adding a third gain stage. I really liked the results but I needed to go back to the original topology for my next gig. It was a great learning experience though! The third tube socket from the right is the reverb tube. All of the reverb components are on the terminal strips right there at the socket. The next tube is the phase-inverter and then the two power tubes. Looking at the rear panel of the chassis, at the top right you can see the dwell and reverb level control pot knobs, the reverb send and return jacks and the hole on the rear of the chassis where the reverb transformer wires enter and leave the chassis. The white chicken-head knob there is the speaker impedance selector switch.

The turret boards are as follows going left to right:  Power supply (horizontal components, solid-state rectifier.) Then comes the power amplifier section, a gap, and then the blue board holds the preamp's components.

I absolutely LOVE this amp. I have to say though, if someone offered me $1000 for it, I'd sell it to them and build myself two more!!! One just like this one and one in a 50-watt 6L6 or KT77 version.

Thanks for looking and I would love to hear your comments below!


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