Repair semantics: Headroom vs. Clean power output

[Rhodesplyr]

I'm an organ/keyboard player, not a guitar player, so I occasionally have some trouble understanding how guitar players describe what they're hearing.

For example, I just repaired a silverface Deluxe Reverb, and I carefully bench-tested it. It puts out a healthy 22 Watts clean before clipping with a sine wave input. B+ caps and cathode bypasses are new. Plate and cathode resistors are within spec. Tubes are good. It's biased correctly. Everything is textbook. However, the amp's owner says that this SF DR doesn't have "as much headroom" as his other silverface Deluxe Reverb. I've never seen the other DR, so I have no idea what mods may have been done to it. It may have had the PI blackfaced. I'll find out soon.

I grant that he's trying to tell me something real, so what do guitar players mean when they talk about "headroom"? And where should it point me in terms of figuring out how to address a concern about it as a technician when I'm pretty sure that an amp is objectively working as designed and putting out its rated clean power?

I should add that, yes, I do have an electric guitar to test amps, but I don't play well enough in any particular style to know if the amp is zeroed in on a particular style of playing. On organ, I know how to determine if it sounds right for jazz, rock, gospel, etc...

[mozz]

Doesn't have as much headroom means, it starts to distort sooner. It could just be the tolerances of the resistors, the pots, the transformers, the tubes.

[Dave H]

what do guitar players mean when they talk about "headroom"?

"Doesn't have as much headroom" to a guitarist usually means that it starts to distort at a lower number setting on the vol pot dial not that it starts to distort at a lower sound pressure level as you would expect it to mean. If both DRs are putting out 22W clean but one starts to distort at '5' and the other at '7' on the vol pot a guitarist would say the one which distorts at '5' has lower headroom.

[J M Fahey]

Which is a popular definition but inaccurate.

*Real* headroom from an acoustic or band playing point of view means that he can comfortably rise volume loud enough to match the drummer (which is a basically fixed volume sound source) yet keep signal "clean" (although a scope might say otherwise) , while "inadequate headroom" might mean he must play very carefully to stay in the "clean" zone, and no headroom meaning that the only way to be heard at all is to rise volume into very noticeable distortion (think Tiny Terror and such).
The actual number on the volume scale means little or nothing.

All those repacing 12AX7 with 12AT7 (very small change) or 12AU7 to improve headroom are missing the point if amp isn´t actually loud enough.

[xtian]

If both amps put out 22 watts clean before distorting, then just loosen the volume knob, rotate it so it says whatever number he's expecting, then tighten it back down.

Guitar amps are 5% electronics and 95% psychology.

[Dave H]

Which is a popular definition but inaccurate.

Not to Nigel Tufnel it isn't

[pdf64]

Yes headroom is a horrible word, it might mean any number of things.
Folks use it when they're trying to sound 'technical' and then may get annoyed when it's not understood / misunderstood.
Back before the internet etc they might just tend to say that the amp distorts before it gets loud enough.
Nowadays they read that the Fender magic happens at '4' on the volume knob (no matter what the strength of the instrument PUs are), and if their amp happens to be too clean, too overdriven or not loud enough at '4', there's obviously a 'headroom' issue.
Because it's a technical word that has meaning in other contexts, they think there must be a technical problem which a competent tech should be able to identify and fix.
In this context, I see headroom as being the relationship between the max available gain, the taper of the controls, and the sound pressure level produced, the latter being a product of the power output and cab sensitivity.

Most likely the DRs have different sensitivity speakers, or differing control tapers.
I think later SFs had all tone controls 30% audio, which makes a big difference on the bass (previously 10% audio).
Thus with the same control settings, the earlier amp would have a lower gain over the all important low end, so would overdrive much less readily than the later amp.
So the earlier amp may be seen to have more headroom, though some may see it the other way around.

If you can't get the 2 amps together, perhaps suggest to him that he plug each amp chassis into the cab of the other, to see if it's just the speakers.

[J M Fahey]

Not to Nigel Tufnel it isn't

Hey, **Nigel** himself is inaccurate

And his Bass Player is even more inaccurate while displaying certain .... ummm .... "physical endowments"

[glebert]

Some things that I think get ignored on the headroom discussion is that (IMHO) distortion will manifest itself differently for different levels of pick attack, and will have a certain frequency dependency. It can also depend on what was being played a few milliseconds earlier (e.g. power supply sag). Also speakers can greatly affect the distortion of an amp, could be that his other speaker is less prone to breakup, is more efficient so is louder in the clean area, etc.