What makes different tube amp heads different?

Question

I want to make my orange dual terror have that chimey/glassy sound that an VOX AC 30 has, i know amp heads have circuit boards and tubes, but my question is, what makes a tube amp sound different then another one of a different brand? Is it the tubes? Could a change of tubes make one amp head sound like different one? What exactly is it? Thanks

Answer 1

We can divide any complete all-tube guitar amp system into three major sections, each of which has important effects on the final sound: The preamp, the power amp, and the speaker cabinet.

All tube preamps almost always exclusively use dual-triode tubes that are either 12AX7 tubes or very similar (e.g., 12AT7). The main aspects of a preamp that affect the sound include:

• Input stage circuit design
• Gain stage circuit design
• Tone stack circuit design

All three of those are essentially fixed for a given channel in all but a very few amps. You can swap out tubes for compatible tubes, with largely subtle differences in final sound. So you might have channels with different circuit designs in the same amp, but generally if you want a different preamp sound, you have to buy a different amp.

Aspects of power amps that affect the sound include:

• Choice of power tube(s) (e.g., EL-34, 6L6, EL-84, 6V6)
• Power amp class (A and AB being the most popular by far)
• Power amp bias point(s)
• Output transformer design

In amps with adjustable bias voltages, the power tubes can be swapped for a few other compatible tubes and of course the bias can be adjusted. There are a few amps where the amp class is selectable. Output transformers are essentially fixed and are popular targets for high end mods.

For speaker cabinets:

• Impedance matching with the power amp, or lack thereof
• Choice, number, and arrangement of driver(s)
• Cabinet design (e.g. open-backed, close-backed, ported)

Drivers can be and often are switched out. Cabinets are not really changeable but different cabinets can be used in different situations, and it's often possible to run more than one cabinet at a time to blend different sounds.

Answer 2

Of course there are many categories of both solid state and circuits that can affect tone and response, not to mention the many variations in speakers and cabinets. But the main difference between tubes and the majority of solid state amplification is the non linear transfer curve of a tube, vs. the nearly straight line linearity of most solid state devices, such as transistors and FETs. Consider this example of one tube circuit's transfer curve.

Without getting too deep into complex theory, notice that the signal in this case has been "biased" (or positioned) at nearly the straightest point along this curve. But imagine a situation where the signal is intentionally made so large, that it will be affected by the lower and upper parts of the transfer curve, where there is indeed much more curve! The effect would be to compress the lower and higher parts of the input waveform.

Taking this further, in real tube circuits the curves near the top (saturation) and bottom (cutoff) are seldom this identical. So a typical methodology is to use two tubes in a combination called "push-pull", where the top half of the signal is amplified by one tube, and the lower part is reversed and amplified by another. After this, the second half output is reversed again, and the two halves are combined in a transformer. This would then result in identical shaping / compressing of both halves of the waveform.

Though this kind of instantaneous compression is really a form of distortion, it is in fact a very musically useful kind of distortion! It is very rich in odd harmonics, which indirectly cause each note played to sound somewhat like it has a built in major chord! The sound is also often associated with "warmth". Another useful effect is that at low volume levels, the dynamics of the guitar are NOT compressed, offering the sonic appearance of wider dynamic range and sensitivity, and that glassy clarity we all like to hear, while louder playing is slightly subdued volume wise, along with a very musically pleasing "crunch" that is subtly added as the signal level increases.

It is not impossible to simulate this effect in a solid state circuit, but it almost always requires complex modern DSP techniques, in which the sound is rapidly sampled and processed in real time by a fast CPU. It is more difficult however to simulate this effect with ANALOG solid state circuits. And certainly, while the NATURAL "distortion" created by tubes and their characteristic transfer curves has always been the foundational hallmark of classic rock guitar sound, the natural distortion of solid state amplifiers is almost always less useful, and often ugly.

Answer 3

It is everything about the amp. This is why artists have various amp and cab combinations in order to get all the tones they want.

While you can use the same tubes as your favourite amp, you'd also need to have the same bias conditions, the same circuitry etc so do your research carefully.

It is definitely worth trying a change of tubes if you want a different tone, but be aware it may not give you the same sound as your favourite amp (in fact it may be nothing like it.)