Re: [Harp-L] Re: small class A amps never get sag

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Class A amps do sag, small single ended amps (which can only be class A because 
there is no other end of the OT to take over on the OOP swing) will sound 
different with differing rectifiers. This is simply because differing rectifiers 
have different internal resistances, what ever current is required still must 
pass through the rectifier, if the current cannot rise (it usually can because 
one tube doesn't always draw the max current from the PT, two tubes can & create 
more sag) then the plate voltage will drop or "sag". There's no free ride.

The assertion that Class A amps don't sag is taken from Randall Aiken's paper on 
"Class A amps - the final word". This paper was not written concerning single 
ended, entry level guitar amps specifically (which are usually biased nearer to 
cut-off than saturation anyway, not ideal class A @ halfway between the 2). It 
is aimed at push-pull amps primarily & even states that the SF Champ that is, 
biased to class A is unusual (plus it has a negative feedback loop & a cathode 
bypass cap, which true class A does not require) Aiken says, "current in a true 
class A amp is constant, therefore there is no bias shift, unless the amp is 
driven into clipping, in which case all bets are off"....this is the bit that 
people fail to take on board, Aiken is referring to an amp operating in true 
class A (not  Class A biased closer to cut off, nor a class AB amp just not 
pushed) operating within its clean W RMS rating...not where SE amps used for 
harp usually operate.

Sorry, just had to get that off my chest. Another one of those myths repeated 
enough becomes the truth.

A good sounding SE/PP/Class A/Class AB amp may have a tube rectifier, a Weber CC 
or SS diodes only.

 



________________________________
From: Splash! <celtiac@xxxxxxxxx>
To: Harp- L <Harp-L@xxxxxxxxxx>
Sent: Tuesday, 14 December, 2010 13:46:24
Subject: [Harp-L] Re: small class A amps never get sag

I have an old Kalamazoo that had a rectifier go bad during a gig.

Actually, I didn't know what it was during the gig, it just started sounding
really smooth and buttery.  And then got a bit growly, and then failed.  It
took about 20 minutes to finally quit. The tones were amazingly great while
it was happening.

But not knowing what happened, I just swapped the entire set of tubes during
a break since I always keep a complete spare set of tubes in the back of
every amp I own, just for this reason.

The next day I tested each tube and discovered it was the 6X4 that failed.
But sounded SOOOO good while it was giving up it's dying breath.

Splash!
----- Original Message ----- From: "Rick Davis"
Yes, as we discussed earlier, small class A amps never get sag anyway,