Re: [Harp-L] Feedback

["Joel B. Chappell" <joel.b.chappell@xxxxxxxxxxxxxxx>]

To: 'MARK BURNESS'; 'Greg Heumann'; harp-l@xxxxxxxxxx 
Subject: RE:[Harp-L] Feedback

Gents-

<Very, very few amps used today for harp are Class A (in fact very few <designed in the last 50yrs), the vast majority of push pull amps are AB, <idling 6L6 power tubes anywhere from 4W each to 20W+ per tube. Greg was <using the term "saturate" in the generic sense of pushing them hard enough <to get them to audibly drive. 

I was referring to older guitar amplifiers that seem to be the rage today. To run AB1 or 2 correctly usually requires a good stiff fixed bias supply which runs up cost $$ so the thinking back in the day was to run cathode bias in class A. This is probably mostly true of home entertainment [hi-fi/stereo] amps rather than guitar amps.
 
<Class A push pull amps dissipate the same, regardless of signal, only <within their clean RMS power rating. When pushed beyond that this does not <apply.

Correct, once again my home entertainment career prevails â.
 
<Whenever you push a push pull amp hard the bias effectively shifts towards <the colder end of the spectrum towards cut-off and away from saturation in <the technical sense, regarding tube plate current. As is evidenced by the <presence of crossover notch distortion under heavy drive.

Correct again and pentode gain curves being what they are you are now off the linear portion of the plate load line.
 
<As regards harp amps, primarily because of the limitations of acoustic <feedback, it's usually a tricky enough feat to get the amp in question to <make it's maximum rated power output, irrespective of whether it's clean <RMS rating or not. With a Strat on the other hand you might easily see the <typical stage amp make half as much again under drive.

Well now this is where the speaker system comes into play. You can get 100 dBA @ 75â out of a 15â speaker that is horn loaded or in a tuned port enclosure [JBL and the like] with about 45 -50 Watts RMS out of the amplifiers, where you would need 200-250 Watts RMS, or more, to get infinite baffled speakers [Bose and the like] to provide the same ambient levels.

<What does all this really mean? Not a great deal, people tend to primarily <choose amps because of the sound they make and manufacturers W RMS ratings <are usually determined without a speaker attached (often a critical aspect <of a harp friendly design), plus loudness of an amp is measured in dB, you <might have a discrepancy of 2 to 4 times the volume (dB) for amps of the <same wattage (clean RMS). So take all the numbers with a pinch of salt & <trust your ears ;-)

I prefer to use an audio meter set to human ear response curves in the actual venue. I have a Wave-Tek sweep generator and the combo is great for finding hot spots. On the bench I have a pair of non-reactive 125 Watt, 8 Ohm loads for preliminary testing on the limited repair work that I do.

One last question. Why do harp musicians want amps that âcolorâ the response of their instrument? I can understand the guitar folks looking for heavy metal distortion, but as a number of folks have pointed out blues harps donât usually approach those volumes and get that dirty. I have a Fender Passport system that is fairly flat and I have played my harps about 6-inches away from a SM57 mounted in its stand and it sounds great, plus I see Buddy Green on TV and other participants in this forum on YouTube and they donât have a mic jammed right in front of their harp and they sound great.

Regards,
Joel