Re: Greetings from Belgium

>     I don't know for sure, but I'd guess that a amp that uses two 6L6 
> tubes, like yours, won't even work if you pull an output tube.

When an amp uses more than a single tube in the output (i.e. a pair of 
6V6's, 6L6's, etc.), they're usually in "push-pull" configuration.  One 
tube amplifies the positive side of the waveform, and the other the 
negative.  If you pull one of these, you get a distorted sound.  This 
distortion is not the same as you get by overdriving the amp, although it 
will overdrive sooner because you now have roughly half power.

The "other" configuration for 2 tubes is parallel, but this is far less 
common.  They have to be biased class A (idle current @ 50% power), which 
is much less efficient, and wastes electricity.  Class A also runs 
hotter.  I don't know of any musical instrument amps that use parallel 
class A finals.

With 4 tubes, you have parallel tubes in a class B (or AB) push pull 
configuration.  If you remove 2 tubes on the same side, you again get a 
half wave "distorted" signal.  If you remove one tube from each side, you 
get an undistorted signal at half power.

A brief explanation of bias (class A, class B, etc.):

A sound is comprised of pressure and vacuum.  In your harp, when you blow 
or draw, the reed vibrates, creating variations in air pressure that we 
can express as "higher than normal" and "lower than normal" air pressure 
(measured at a finite point).

When we put this into a microphone, etc., it converts the sound into 
electron flow.  Depending on the phase of the microphone, it will convert 
pressure into positive electron flow (what we call a "negative" charge, 
believe it or not!), and vacuum into negative electron flow.  (or vice 

A class A amplifier is set up to amplify BOTH of these.

A class B amp is set up to amplify only ONE of these.  It is "cut off" 
during the "other" half of the signal.

Sometimes your amplifier may blow up, and just produce sound when you put 
a loud signal into it.  This is pobably because it's biased into class 
C.  Class C amplifiers are not used for audio, but are common in AM 
broadcast radio.

With push-pull amps, they can distort if run at "pure" class B.  This is 
"crossover distortion", caused by nonlinearity at very low signal levels, 
and the fact that it's hard to get a perfect class B bias.  To minimize 
this, we often run in class AB, where each side amplifies a little of the 
other side.  

Class AB1 tells us that positive grid current never flows.  Class AB2
means that, during some portion of the waveform, positive grid current
flows.  If this means nothng to you, ignore it - it's not important to
this discussion. 

-- mike curtis

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