Why have volume control in tone stack?

[Paul Lebow]

In the process of tweaking an old amp with single volume and tone control, I realized the volume control setting has a big impact on the tone control and can actually make it almost ineffective at full volume. Why not use the guitar volume instead to control preamp distortion and, instead have an interstage control between the preamp and inverter?

[Bergheim]

I think a better question is why the tone control is arranged in such a way that its effectiveness is dependent on the volume control setting.
Moving it downstream in the signal chain, for example after the next gain stage or even as a Vox cut control (only works in output stages that don't have NFB) makes it more effective, especially for reducing overdrive fizz.

Why not use the guitar volume instead to control preamp distortion and, instead have an interstage control between the preamp and inverter?

Convenience, mostly. A more common name for the "interstage control" is master volume No problem in having both.

What are your tweaking goals?

[nuke]

Schematic?

If it is an "old" amp, it was probably not intended to provide distortion at all.

Every amp has some set of design goals it was intended to meet, and different levels of skill and diligence to meet those goals. Some are intended to be cost-no-object in getting performance, others were intended with cost the only object.

[B Ingram]

In the process of tweaking an old amp with single volume and tone control ... Why not use the guitar volume instead to control preamp distortion and, instead have an interstage control between the preamp and inverter?

The Ampeg M-15 you're talking about didn't have any "preamp distortion." The power tubes would be the first thing to distort in those amps, per "Good Audio Electronic Design" at the time it was created.

The Volume is not "in the Tone Stack" either, because there is no "tone stack" per se. That amp was designed to be: Input --> Gain Stage --> Volume --> Phase-Inverter/Power-Section. They tacked on a Tone control as a convenience feature, the same way Fender took its 1950s Champ (with only a Volume) and tacked on a Tone control to create their 1950s Princeton (which was a "better model" that had Volume and Tone).

- The Tone control added is able to interact with the Volume control to darken the sound, or to brighten the sound. It achieved a good Performance/Cost balance at the time.
- It might seem goofy to you today, but there were amps of that time whose Tone control could only make the amp darker, not brighter.

The Design Goal for the M-15 was "Make the pickup signal strong enough to drive the Power Section, for clean power to the speaker." But some pickups are stronger/weaker than others, so the preamp is given a little more Amplification than it needs, and given a Volume control to "turn down strong pickups" and "turn up weak pickups." Most Instruments have their own on-board Volume control, and the player was expected to mostly control loudness at their instrument (which was gonna be closer anyway while they're playing on-stage).

[B Ingram]

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