My New Oscilloscope with my 18-Watt Lite Ib

I finally pulled the trigger and bought an oscilloscope. I was going to buy a PC-based USB scope but read some horror stories about people frying their laptops and PC's so I chickened-out and bought a desktop digital storage oscilloscope. I went with the very popular Rigol DS1054Z. I've been playing with it using some pretty low level stuff from a signal generator just to figure out how to acquire a signal and get it triggered properly.

Tonight I finally got up the nerve to hook it up to my 18-Watt Lite IIb tube guitar amp that I made. This is a very nice amplifier based on the Marshall 18-Watt amps of legend. These amps are known for having pretty much all of their overdrive from the phase inverter and the power amp. I did some tests and verified this does in fact seem to be the case. I fed it a 100mVp-p 400Hz sine wave, verified at the source that it was in fact a nice smooth sine wave at all volume settings with the preamp tube in parallel mode.

Next I hooked up two channels of my scope to the phase inverter, one channel on each side. This was a really good exercise for me because it really illustrate very cleanly exactly what the phase inverter does.

Here is a picture of the phase inverter output at a pretty low volume. Based on my actual experiences with the amp, this setting would be very clean albeit not anywhere near full volume level. Each side of the phase inverter is its own color so you can clearly see that the two signals are exactly 180 degrees out of phase. You can also see that the sine waves are nice and smooth... no real overdrive here to speak of, or if there is any it is extremely mild...

(Yes I know my scope has the capability to capture a screen-shot but I haven't played with that feature yet... sorry.)


This next picture is with the volume turned up to a nice medium "crunch" setting based on experience. You can see the tops of the waveforms are beginning to flatten-out - that is the phase inverter starting to "overdrive." Very cool to see it coming from my own amp...

This last picture is really cool and leads me to a couple of questions. It is of the amp turned up pretty high... not quite on eleven...

(I had to shift the vertical center up a bit to get it all on the screen as you may have noticed.)

First thing I notice is that not only are the peaks getting much flatter, but you can also see that the troughs of the sine waves are now getting deformed but in a different way than the peaks. This I need to research... I think I remember Merlin Blencowe writing about this in his awesome tube preamp design book... I will go check on that.

The other thing I notice is that the yellow trace does not extend down quite as far as the blue trace. This is somewhat noticeable in the second picture but here it kind of jumps out at me. Is this a result of the 12AX7's triodes being "unbalanced?"

Another interesting thing to note is that these are all traced with the tone knob all the way down. With it on any other setting, the traces got really jagged-looking. Wondering about that too.

Would love to hear your thoughts on this. You can comment below or join my oscilloscope thread on ax84.com's forum.