Re: [Harp-L] Feedback

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

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. 


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.

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.

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.

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 ;-)



________________________________
From: Joel B. Chappell <joel.b.chappell@xxxxxxxxxxxxxxx>
To: Greg Heumann <greg@xxxxxxxxxxx>; harp-l@xxxxxxxxxx
Sent: Monday, 2 May, 2011 18:39:04
Subject: RE: [Harp-L] Feedback

Usually the steel core in the speaker output transformer saturates before
the output tubes totally saturate. This "squares-up" the waveform. Most old
amps run a pair of 6L6s at about 25 to 30 Watts in Class A bias conditions.
Overdriving to saturation may force them into Class AB1 operation where they
draw a little grid current but they are all done at 60 Watts. Class A amps
dissipate the same power regardless of drive and usually fall off due to
core losses when overdriven, though the voice coils in the speaker my
overheat and burn out with the distorted square wave signals.

Joel B. Chappell, Senior Electrical Engineer
21 Billings Street
Milford, NH 03055

Rick, I simply mean driving the tube to or beyond the level at which it
makes its maximum power. 

/Greg

> Greg, can you please define "Power Tube Saturation" as it applies to harp
> amps such as the Bassman types?
> 
> Thanks.
> 
> -- 
> -Rick Davis
> The Blues Harp Amps Blog
> http://www.bluesharpamps.blogspot.com/