[Harp-L] Watts Up With That?

[Rick Davis <bluesharpamps@xxxxxxxxx>]

I’ve been thinkin’ about watts lately… the wattage ratings that harp amp
makers claim for their amps. There is no standard method of measuring amp
power being used at all. Sometimes it seems the numbers are related more to
marketing than to real engineering.

There is a rule of thumb used by some amp makers that goes like this: a
certain kind of power tube has the potential to produce X watts of power.
So, if they use 6L6 tubes they claim 20 to 25 watts per tube; if they use
6V6 they claim 9 to 12 watts per tube.

But the amp circuit design has a LOT to do with the amp getting to that
potential. For example, a cathode biased amp has a tough time getting beyond
25 clean watts in a 2x6L6 amps, while a fixed biased amp may get all the way
to 50 watts. (For now, don’t worry about the technical mumbo jumbo. Just
agree with me that amp watt ratings can be fuzzy.)

You might notice I wrote the phrase “clean watts” in the previous paragraph.
Why would a Chicago Style blues harp player want a clean amp? Well, you
don’t, but the amount of power an amp can generate before clipping (the amp
starts running out of power) is an important measure of its performance.
Hi-fi buffs will recognize this spec from their favorite stereo: 100 watts
RMS per channel @ 8 ohms with .1% of total harmonic distortion.

That last part about percent of distortion is the missing piece in harp amp
power ratings. Tube amps are capable of producing power beyond their clean
power rating, and the distortion in tube amps can be a lovely sound, while
in solid state and digital equipment it can be very harsh sounding.

So then… To what point do we drive a tube amp when testing for power output?
Should we dime the amp all the way to get ultimate peak power? There are
several problems with that: Tone sometimes degrades considerably at that
level, and nobody ever plays that loud anyway because they get feedback
before getting there. (My 5-watt 1970 Fender Champ is excused from both of
these rules.)

What power rating will make sense to amp shoppers? How can we make the
system more honest and meaningful? My proposition is this: All harp amps
makers should publish a clean RMS rating as well as their best estimation of
real usable power, NOT maximum theoretical power.

The clean signal should be derived by driving the amp into an appropriate
dummy speaker load and measuring on a scope the electromotive power output
in volts. Crank the amp until the sine wave just begins to visibly deform,
back it off to clean, and use Ohms Law to calculate watts at that exact
point.

All amp makers should publish this spec, and all consumers should demand it.
If you take your amp to a tech and it does not produce the level of clean
power specified by the manufacturer, you should return it for repair or
refund.


-- 
-Rick Davis
The Blues Harp Amps Blog
http://www.bluesharpamps.blogspot.com/