Explain to me.......

[daz]

........what things will affect how much compression a OD tone has. I have gone thru OD tones with the amps i've built that were very compressed and smooth all the way to bright and hard with seemingly no compression. Yet i am talking about sounds with the same amount of drive. I always used to want LESS compression in my OD tones, (probably when i used some low end production amps in the past) but now i'm experiencing too much of that. Be careful what you ask for ! Anyways, what are some things that take you from one end of the spectrum to the other with gain being the same? Particularly from the PI thru the output.

[BackwardsBoB]

Here's my take, and I haven't verified it experimentally. That pretty much makes it a guess. Feedback appreciated:

If you look at tube curves, and superimpose a load line, you see a fairly constant voltage gain around an operating point near the middle of the line. As you move close to cutoff, the lines for the different grid voltages get closer together. There's less gain. Eventually, you reach cutoff, and gain goes to zero, because the electrons can't turn around and flow from the plate to the cathode. As you move toward saturation, the lines get farther apart, so there's more gain, until the grid goes positive and starts eating electrons. Gain goes to zero, the waveform gets a flat-top, and the bias voltage may get pulled negative, and stay there until the input signal starts to fall down, reducing the output current on the falling edge of the input, and perhaps for a while thereafter.

For single tubes,this makes saturation sound more like a diode clip (a single diode, not one in each polarity), with sharp compression and lots of odd harmonics, and makes cutoff sound a bit smoother, especially if the tube isn't driven far into cutoff. Backing off the volume a bit, so that the tube doesn't quite reach cutoff or saturation, yields compression of positive input peaks, and expansion of negative input peaks, which adds the octave harmonic of the waveform.

Unless you're managing to hit both saturation and cutoff, only one side of the waveform will be limited, either sharply or smoothly. The other half of the waveform is free to vary in amplitude with picking dynamics. The tube inverts the signal, and you have the option of treating the other half of the waveform in the same way in the next stage. So in a pre-amp, by varying the bias point on the load lines and inter-stage attenuation, in two stages, you can get:

1. clean through both stages.
2. saturation or cutoff distortion on half the waveform, distorting the signal but leaving you dynamics.
3. any combination of saturation and/or cutoff on both halves of the waveform, resulting in "high gain" tone with long sustain, and differences in picking attack only changing harmonic content.

You need a triode stage on the input to get the signal strong enough to overdrive the second stage, so in three triodes, you have lots of options, when you include inter-stage filtering and attenuation, the response of the grid bias to saturation due to the value of the grid bias resistor, grid stop resistor and coupling capacitor, and the operating point of the tubes. Plate resistance changes with grid voltage too, so heavy plate loads will introduce addtional non-linearity.

If you clip one side of the waveform with one tube, and the other side with the next stage, and the grid bias resistor and coupling capacitor for the second stage aren't very large, loud parts of the signal will retain more dynamics, as the grid circuit for the second stage attempts to center the asymmetric signal from the previous stage. Attack will still be compressed heavily.

Single ended output tubes behave much like a single triode. A cathode biased tube with an un-bypassed cathode resistor will be more linear, with reduced gain, and harder to overdrive than a fixed bias tube, or a cathode biased tube with a bypass cap.

For push-pull configurations, the signal has to go through a phase inverter to get the two drive phases. With a long-tailed pair, the tube the input signal drives can be driven to saturation and/or cutoff, and the other tube can follow this behavior on the same half-cycle, since it's input, driven into its cathode, is a copy of the first tube's output. It may also be set to clip by itself on the other half-cycle. For example, if both tubes are run near cutoff, the "input" tube would cut off when overdriven, and this would similarly clip the negative going output of the "following" tube, and then when the input tube is conducting heavily and accurately passing the other half of the waveform, the following tube might be driven into cutoff itself. Same thing can happen if you drive the input tube into saturation, with the output passing the clip with the addition of any cathode degeneration effects, and possibly reaching saturation itself on the other half of the waveform. Single-tube phase inverters will introduce distortion on the same half of a waveform, and can clip both halves of the waveform, but you'd need to drive them really hard.

Class AB output stages always clip the push side of the pair once you turn them up to fun, but this doesn't sound like what guitarists call "distortion" negative feedback reduces the effect, as does the stored energy in the output transformer. If you drive the tubes hard enough, perhaps through matching gain stages following the PI, or reduced screen/plate voltage, you can get them to saturate, and this happens balanced, on both halves of the waveform, for hard compression and odd-harmonic generation, unless the PI has an imbalance.

So all through the amp, you have opportunities to clip one or both sides of the waveform. It's possible to leave one half of the waveform fairly unmolested, and severly clip the other half, generating lots of distortion while retaining dynamics, or clip both halves of the waveform with varying amounts of distortion for that high-gain "sustains forever" range of sounds, and you can take advantage of the different sounds of saturation and cutoff when you do your clipping.

Then there's power supply sag, the preamp cathode bypass caps, PI supply voltage, ultra-linear screen circuits, mixed quads, unbalanced output push-pull pairs, and a host of other factors.


Did I get it right? I really want to hack up a Carvin X-series amp and put in pots for cathode resistors and inter-stage attenuators to check out the drive channel pre-amp stage theory with my ears and a scope. I don't care much for the drive channel anyway. I'm pretty happy with the output stage, populated with either 6L6s or EL34s, and I want to keep the clean channel, with sag and 6V6 options.

Here's one thing I'm wondering about. Let's say you take a clean waveform, and run it through a triode, approaching cutoff, without getting there. You get even harmonics, right? Now what if you do it again in the next (inverted)stage? Do you get more even harmonics, or does the resulting increased symmetry of the waveform reduce them?

[BackwardsBoB]

Did I scare everybody away?

Did I scare everybody away? Sorry about that.

[Paul P]

Did I scare everybody away?

Not me

I find reading your posts enlightening.

Paul P

(just tagging along behind daz picking up the pieces dropped by people
like BackwardsBob)