Comparing the Nonlinearity of a Triode and a Bipolar Junction Transistor

[Leo_Gnardo]

We can probably blame him for the spread of the misinformation, but I don't think he came up with many ideas. He was probably just repeating something one of his "gurus" told him. Now we can all take a guess as to who.

It's all right there in the preface: Aspen Pittman claims to be NOT an engineer. Just a very good salesman with a lot of friends who let him reproduce their articles & other writings in "The Tube Amp Book." So we find some very good advice interspersed with dubious claims, twice told tales, half baked hints, engineering faux pas, plus some out and out rubbish. And of course gorgeous photos of Aspen's guitar & amp collection. The second half of the book, all schematics, has proven to me to be the most useful.

[Enzo]

I bought Pittman's book, mainly for the pictures and the schematic collection.

[Justin Thomas]

Two best bits I took away from The Tube Amp Book:
1. Play a tube amp LOUD.
2. DON'T LOSE YOUR BIAS!!!

Justin

[loudthud]

Here's the results of an experiment I did. The amp is solid state with a simple switchable diode clamp on the input to simulate grid current. Turns out there are a number of patents that claim the use of the diode as an "invention". There is a resistor in series with the diode because you have to tune the clipping to the source impedance of whatever is driving the amp. The amp is clipping pretty hard into square wave territory. The diode changes the duty cycle of the square wave output.

What you are looking at is the output of my distortion analyzer coupled to a spectrum analyzer. On the left a symmetrical square wave with even order harmonics suppressed. On the right the non-symmetrically clipped wave has much more even distribution of harmonics.

Note: As Steve Connor predicted, the diode causes DC to appear on the output of the typical solid state power amp that uses bi-polar power supplies and a DC coupled output stage. You can measure the DC with a standard DVM while the clipped square wave is being produced.

[soundmasterg]

The part that I heard about the solid state distortion = odd harmonics and tubes distorting = even harmonics that resonated with me that often seems to be left out by the magazine articles was that solid state produces lots of high order odd harmonics, which don't harmonically relate to the fundamental, while tube stuff tends to produce lower level odd harmonics that relate more strongly to the fundamental and not so many of the higher stuff, along with some even harmonics. I don't have the equipment to test the theory myself but that always seemed reasonable to me. Both can be made to sound good though...CCR's sounds on most of their catalog are solid state Kustom's and those sound great, at least on the recordings. Likewise the hybrid amps the Beatles used on Revolver and the solid state ones on Sgt. Pepper's sound great too.

Greg

[Gingertube]

Where did that even and low order harmonic thing come from?
If you take a bottom E on your git and call it 82 Hz and look at the harmonic distortion products. Hz values below are rounded a bit.


1 - E Fundamental = 82 Hz
X2 - 164 Hz is an E an Octave Up
X3 - 246 Hz is a B which fits an E Chord - that is it is muscially related
X4 - 328 Hz is another E an ocatve up again
X5 - 410 Hz is a G# also part of an E Chord, musically related
X6 - 492 Hz is another B
X7 - 574Hz - 1/2 way between a C# and a D - Musically unrelated
X8 - 656Hz - another E
X9 - 738Hz is an F# but doesnt fit an E Chord
X10 - 820Hz - 1/2 way between a G and a G# - Musically unrelated
X11 - 902Hz - Halfway bwtween a A and an A# - Musically unrelated

The first 6 harmonics are musically related.
Those musically un-related harmonic products are called "Quint" and you don't want them produced.
So avoid 7th, 9th, 10th, 11th and higher harmonics.

It is the higher odd order ones which sound objectionable.

Cheers,
Ian

[Malcolm Irving]

Modern music uses the ‘equal tempered scale’. This means that all octaves are exact, while all other intervals are a bit ‘out’ from the harmonic series. The scale is listed here:

https://pages.mtu.edu/~suits/notefreqs.html

For example, E2 is 82.41 Hz and its 3rd harmonic is 247.23 while the nearest note on the scale is a B which is 246.94. Obviously, that is pretty close. But for higher harmonics the situation gets worse, e.g. the 5th harmonic is 412.05 while the G# on the scale is 415.3

[Mike Sulzer]

I think that a key difference between tube and ss stages is that the tube stage has significant distortion even below the clipping level, especially if the level is so high that clipping occurs during part of the guitar waveform. BJT has little distortion below clipping because, as has been shown here, the transconductance is so high that feedback (within a stage, around several, or both) must be used to control the gain. It usually is used in JFET circuits as well, but it appears that it could be avoided with the right design.

The sound of a signal is a result of the harmonic content. If a stage is used near clipping, or especially above clipping part of the time, then the tone should be a function of the harmonics produced (below clipping). This would mean that tubes and ss should sound different even if you make the clipping responses similar.

[loudthud]

Consider the various tube/solid state hybrid offerings. A 12AX7 front end on a bass amp is somewhat common. It sweetens the sound but being at the beginning of the signal chain doesn't contribute much to hard clipping. The only guitar amp I can think of with a tube front end was something called a Legend(?) from the 80's. Never played or worked on one, I think it had two 12AX7s.

Then there is MusicMan. First version had a full tube power amp with opamp preamp. Didn't sound horrible but a little on the cold side. Second version got rid of the tube phase inverter. Not an upgrade in my opinion. A little colder sounding. Final version kept the same power amp but added a tube distortion generator. That made the amp somewhat usable, better distorted tones available. When overdriven the opamp preamp generates mostly odd order harmonics. The tube phase inverter (1st gen) might have generated some even order harmonics if it was driven hard enough. The distortion generator tube (3rd gen) also contributed some even order harmonics. None of these really sounded like an all tube amp under all conditions. Nobody is building clones of these amps.

[Justin Thomas]

Had a Legend for a while, 3x12AX7. Played several MusicMans, currently trying to unload a Peavey Deuce VT. All of these fall short compared to my Fenders. The Legend I consider different from the "tube preamp" hybrids of today, because they just stick a single preamp tube in with SS for the rest. The Super Champ XD I find "passable" at best, when put up against a real Champ with a couple pedals - you just can't dial the digital out, and I tried lots of speakers.

Even for bass I prefer an all-tube amp, and more so in the power amp. I tried a Fender TV Bassman a few years back, the top-shelf one. Putting that $1300 hybrid against that crusty old Bassman 100 in the corner for $400, no comparison.

And even my little Spitfire clone powering a 4x10 homemade crap-speaker column that my buddy got for $20 on CL being fed by my friend singing into an SM-57 elicited this response from our dobro player when our singer asked me to turn up his vocals a bit: "Oh, hey - I didn't even notice you were singing through a Mic. I just thought you were singing loud!"

I support tubes for pretty much everything... it may not be 20-20k, but it's warmer and easier on the ears to me.

Justin

[soundmasterg]

Consider the various tube/solid state hybrid offerings. A 12AX7 front end on a bass amp is somewhat common. It sweetens the sound but being at the beginning of the signal chain doesn't contribute much to hard clipping. The only guitar amp I can think of with a tube front end was something called a Legend(?) from the 80's. Never played or worked on one, I think it had two 12AX7s.

Then there is MusicMan. First version had a full tube power amp with opamp preamp. Didn't sound horrible but a little on the cold side. Second version got rid of the tube phase inverter. Not an upgrade in my opinion. A little colder sounding. Final version kept the same power amp but added a tube distortion generator. That made the amp somewhat usable, better distorted tones available. When overdriven the opamp preamp generates mostly odd order harmonics. The tube phase inverter (1st gen) might have generated some even order harmonics if it was driven hard enough. The distortion generator tube (3rd gen) also contributed some even order harmonics. None of these really sounded like an all tube amp under all conditions. Nobody is building clones of these amps.

There were also the Vox hybrid amps from the 60's, the UL 7120/4120 and 730/430. They were and are very rare amps but that is the sound of Revolver from the Beatles and that's a pretty good sound. A little more noticeably harsh then the all tube stuff but better than an all solid state amp.

I wonder if those new vacuum channel transistors might have any audio applications down the road? The last update I saw from them was back in April and they had posted curves that looked a lot like a triode.

Greg