...vacuum tube THEORY?

[Old Tele man]

...the Child-Langmuir 3/2's Law has an invariant exponent of 3/2; real-world vacuum tubes, however, only follow that exponent over the middle portions of their "S-shaped" conduction curves, as I mentioned earlier.

...maybe a separate posting on the effects of ODD- versus EVEN-harmonics upon fundamental tone summations should be separately initiated?

[Joe Bee]

Duh, I don't know what made me do it, but I made a quick implementation of a Fourier series to plot out the power spectrum of the x^3/2 nonlinearity and it's reconstruction from Fourier components. Units are linear, not the common logarithmic dB scale, so there are a LOT of amazingly high harmonics. If I didn't do it wrong that is.

Steve, say something!

[Steve Conner]

Yes, you are right, there is no such thing as a 3/2 harmonic! The medication must be wearing off.

I've experimented a lot with trying to get soft low-order clipping out of solid-state devices. Here are some things that I found to work well:

A small MOSFET or JFET with its drain connected to its gate

A high voltage diode from a microwave oven

Both of these actually have a much softer knee than the grid-cathode "diode" of a tube! I compared them against 12AX7s and 12AU7s. So when a tube is driven into grid current, it generates a lot of high-order harmonics. It's the other end, when the grid is driven negative into cutoff, that has the soft characteristic.

My conclusion is that many people's favourite tube tones probably contain a lot of seriously hard clipping, and the low order harmonics of the tube's linear region have little to do with it. In other words, I propose that dirty blues and rock tones are basically PWM square waves like you would get from a synth, and the only really important parameter is the duty cycle and how it varies with the degree of overdrive.

I used the MOSFET clipper in an amp design, and it sounds rather, well, soft, dark and mushy compared to overdriven tube crunch.

My gut feeling has always been that you could make an amp out of JFETs, MOSFETs and BJTs that couldn't be told apart from, say, a '59 Bassman, in a blind test, even though the harmonics generated before hard clipping set in would be quite different. AFAIK, Craig Anderton used to do this at lectures: he would bring a tube guitar amp and a solid-state one, play through both and challenge the audience to tell them apart. They always failed.

Even so, if I wanted to clone a '59 Bassman I'd still use tubes :P

[MattT]

...with current passing from cathode (abbreviation K or C) to plate/anode (abbreviation P or A).

OTM has forgotten more tube theory than I'll ever know...but...isn't it electrons that pass from cathode to anode? While current actually goes from anode to cathode?

I know it seems like semantics but I can easily visualize and explain how a vacuum tube works when I think in terms of electron flow...but I can't when it comes to current flow.

[Steve Conner]

Well, current is made of electrons that flow from negative to positive! When we think of so-called "Conventional" current, flowing from positive to negative, that's just a convenient fiction.

[MattT]

I realize that Steve...that's sorta the point. When you think of it one way vs. another...it makes a difference.

To put it another way...try to explain how a vacuum tube works using 'conventional current' and not using electron flow. It's a lot harder...for me anyways.

[Old Tele man]

OTM has forgotten more tube theory than I'll ever know...but...isn't it electrons that pass from cathode to anode? While current actually goes from anode to cathode?

I know it seems like semantics but I can easily visualize and explain how a vacuum tube works when I think in terms of electron flow...but I can't when it comes to current flow.

...oops! my USN schooling is "showing"...USN schools taught electron flow (ie: negative-to-positive movement), whereas most other schools tend to teach current flow.

...so, obviously, I was referring to "electron" current flow (wink,wink)!

[MattT]

...that's just a convenient fiction.

I think it's extremely inconvenient! It downright sucks!

[Steve Conner]

Well, that's the point I was trying to make! You can't explain how a vacuum tube works with conventional current, because conventional current doesn't exist.

[MattT]

Well, that's the point I was trying to make!

Oh, sorry...I didn't realize you were trying to make a point that I had just made one post earlier.

[Old Tele man]

My gut feeling has always been that you could make an amp out of JFETs, MOSFETs and BJTs that couldn't be told apart from, say, a '59 Bassman, in a blind test, even though the harmonics generated before hard clipping set in would be quite different.

...like this:


...from an unknown British electronics magazine many years ago...anybody remember seeing it?

[Steve Conner]

Well, I'd never seen that before, but it's the kind of thing I was thinking of... From the look of the typeface, the magazine was Electronics World, or Electronics & Wireless World, or whatever it was called at the time.

[Old Tele man]

...continue on to TRIODES?

[Joe Bee]

I am not sure if it is really as easy as that. Over at runoffgroove.com, there is a bunch of people trying to replicated classic valve circuits (mostly preamps) with JFETs, and they seem to have a lot of trouble biasing the devices correctly. Apparently FETs drift wildly with temperature, and there are huge variations between devices of the same type.
My guess that FETs were never meant to be used in the linear region, so no manufacturer ever bothered to optimize them for this purpose.

Furthermore, while I agree that most guitar sounds are wildly clipped, this does not mean that subtle nonlinearlities don't play a role too. I dont see why the power amp, which usually has a lot of clean headroom these days, could play such a big role in shaping the sound. Think about it this way: If you look at the waveform of a cello, a singer or whatever, it will probably look more like a distorted square wave than a clean sine. Still if you play it back throug an old tube radio or a modern SS HiFi, you will hear a difference. By your logic this difference should be swamped by the THD already present in the original signal.

I think Steve is a heretic and should burn at the stake!

[Hermanni]

...continue on to TRIODES?

Yes please