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First question here. Please be kind if I've messed up somehow :)

I'm hoping someone can explain the theoretical electronics (rather than just trial & error) that connect the interplay between (a) the guitar volume knob, (b) the sensitivity differences between amp inputs, with low sensitivity = -6dB and (c) amp volume, particularly in terms of the point at which a tube amp starts to break up.

It may be worth noting that I'm particularly interested in the combo of a Fender Strat and a Fender Custom Deluxe Reverb reissue and that the latter only has one volume knob per channel (not a gain and volume pairing).

Let's say for the purpose of discussion that the amp breaks up when the amp volume is on 4, the guitar volume on 10 and the full sensitivity input is being used.

Scenario 1: If I use the lower sensitivity (-6dB) and keep everything else the same, will it require higher volume setting on the amp before it breaks up (and why)? If so, will the actual dB-generated volume be just the same or will the tube break up at a quieter point? BTW: I note from the first question listed below that there will be tone differences as well as volume differences.

Scenario 2: Similarly, if the guitar "volume" is lowered to 5, what difference does this make to the amp volume setting and overall actual volume necessary for break up to occur (and why)?

Thanks.

In an attempt to find an answer already posted, I've looked at these two questions:

Difference between "hi" and "low" on amps?

How can I overdrive my tube amp at low volume?

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    If you don’t have a master volume and won’t use an attenuator, you cannot make the amp break up at a lower volume no matter what you do with the volume knobs or inputs. The only other things you can do are change the bias point on the amp, change the power tubes, and/or change the speaker load impedance and sensitivity. This is why both master volume knobs and power attenuators exist. Non-master volume designs are just super loud when they break up. Jan 6 at 3:25
  • Thanks, Todd. I'm still interested in understanding the mechanics better even if the solutions are limited. I may just settle on just using the clean tone and the high headroom and avoid the break-up point entirely, but I'm hoping greater insight into the interplay between the various controls will be helpful.
    – Nick
    Jan 6 at 5:32

1 Answer 1

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I’m not sure what you mean by the amp breaking up, but I guess you mean reaching saturation?

I’ll try to give you a quick run down of how such amplifiers work. Essentially amplifying is a bit of a problem: You have some sort of electric signal (time dependent) and increase the voltage and the power, usually by some time independent voltage.

An early way of getting this done were triode vacuum tubes. This uses an effect where a light bulb will emit electrons in use. This is paired with a grid, which will electrostatically absorb some of the electrons depending on voltage. And suddenly you have a quite crude amplifier (later this was superseded by the much superior technology of transistors).

Now clearly this technology is not perfect. The tube can only emit electrons, not take them in (so it is asymmetric), and if you put in a higher voltage the output will not be equally much higher (non linear).

This means that this amplification will change the waveform (distortion). In a HiFi-amplifier you’d use multiple stages with negative feedback loops to make this more linear, in a guitar amplifier this is what you want!

Now, the problem is that this distortion depends on the amplitude, so a small signal will distort less than a big one. The way to amend this is to attenuate this before the actual amplifier (gain). This can then adapt the signal so you can drive the amp less hard. Due to compression turning up the gain will partially increase volume, partially color.

Now, let’s say we have an active instrument (already with a built in amp) the signal might be too strong. Then we might attenuate the signal. This is what the two inputs are for.

After going through the (pre)amp and optionally an effects section the signal will go through a power amp, which drives the speaker.

Usually you do have another knob here, which is the volume knob.

Technically gain specifies the factor by which a signal is amplified. In these contexts this is is usually a pot that sends part of the signal to ground. These can sit in different positions in the signal chain, and this will have different effects on the sound, and different vendors use different names.

But generally you have some something like this:

Input → attenuate (gain) → preamp → attenuate (volume) → power amp → speaker

You’d use the attenuated input if the attenuation of gain all the way down is not sufficient for a clean signal.

(Sometimes you’d get an additional amp before the preamp to drive the preamp harder, often called drive).

The guitar volume knob will act the same as a gain knob, just at a different section in the signal chain: It cuts away some of the signal.

As far as I understand the Volume knob on your amp acts as ”gain“, so there is no way to attenuate the signal after the preamp section.

Now if we ignore frequency dependent stuff (the guitar volume knob’s effect will depend on frequency) each knob will cut away a certain factor of the signal. And assuming that the signal path to the preamp is sufficiently linear you’d expect a total (logarithmic) gain at the preamp of

g_guitar + g_input + g_volume

Now the problem is that the positions on the knobs are only labels. What matters is the resistance curve of the potentiometer under the knob. Since perceived loudness is logarithmic pots for audio tend to be so called ”logarithmic“ pots, thats is, the resistance curve is (in theory) exponential of the angle. While no pot is perfect in this case this would mean the log gain of guitar and volume knob are proportional to their angle. So you can probably say ”An adjustment of the guitar knob by x degrees is like adjusting the amp knob by y degrees, and using the attenuated input is like adjusting the amp knob by z degrees.

But the exact values will depend on the individual pots. Now in practice the guitar knob also has a frequency dependent effect and will be affected by every non-linearity in the signal path. The attenuated input gain will be affected by any non-linearity between the attenuation and the preamp. So these will affect the tone in slightly different ways.

So now addressing your scenarios:

  1. If you use the attenuated input a smaller signal will reach the preamp, so you get less distortion. To get more or less the same distortion you will need to reduce the attenuation of the volume knob. The distortion of the amp more or less depends on the volume that comes in, so it should distort more or less at the same loudness (although as I said this does also depend on other non-linearities in the path, and where exactly the voltage dividers sit).
  2. Same as 1, but the change of signal directly at the guitar affects more stuff, so you will hear a noticeable change of tone (loss of high frequencies), especially on a passive instrument.

Generally also one thing to keep in mind is that the longer a signal path is the more noise there will be. So if you have a strong signal and attenuate at the end you will have much less noise than otherwise. This means that using the guitar knob is nice for quick, interactive control, but you should try to keep it on the open side. Choose the input so that you can use as much of the range the amp gives you (if your distortion is unusable at 5 half of that knob is unusable, but it gets harder to fine tune as your scale is now 0 to 5). Then choose your amp gain so that you get the most distortion you want with guitar pot fully open. Then you can fine adjust on the guitar.

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  • Belated thanks for this, @Lazy. It's been super helpful. I've had to read it a few times and reframe some of the paradigms that I've had in my head. For example, I'd never really understood that guitar knobs are pot(iometers)s that divert some of the signal to ground, such that 10 allows the whole signal to go through. It's super clear to me now why using the non-attenuated input with the guitar volume on 10 is the most likely to induce distortion. I've still not mapped in my head how the channel gain sits within the preamp flow, but I'm looking at the amp circuit diagram and giving it a go :)
    – Nick
    Jan 9 at 4:04
  • *potentiometers :)
    – Nick
    Jan 10 at 20:48

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