Why Not Bias By Scope

From kee–(at)–den.com Sat Jan 18 21:54:16 CST 1997
From: kee–(at)–den.com (R.G. Keen)
Newsgroups: alt.guitar.amps
Subject: Re: `67 DELUXE REVERB BIAS Intructions – PLEASE!
Date: 19 Jan 1997 00:10:19 GMT

Steve Nielsen (nielsen–(at)–cmail.orst.edu) wrote:
: On 14 Jan 1997 11:50:32 GMT, tremolu–(at)–ol.com (Tremolux) wrote:
: >Ahhh, the inconsistent, non-repeatable bullshit GT method. Oh well, it’s
: >your amp.
: Hold on now Tremo’, ol’ buddy! Gimme some reasons.
: Why is it inconsistent and non-repeatable and male-bovine-fecal?
Well, I’m no Tremolux, but, picking out things from the biasing section
of the tube amp faq, I’d say that it’s like this:

When you bias to give yourself a noticeable notch, you bias it into
nearly class B, as the notch is the artifact of the gain changing as one
tube turns off. As you increase the overlap to remove this notch, you
move it toward class A, and the overlap hides the turn off artifacts by
moving them well into the other tube’s conduction cycle. The
inconsistency and non-repeatability come from the fact that to the human
eye, the amount the waveform changes per unit change in bias voltage or
current drops dramatically as you move through AB. (If your signal isn’t
very large, you stay in A – that is, the signal never gets big enough to
make one tube turn off.)

If you were to plot visual waveform change versus standing current, it
changes a lot as you move from B, gets steadily flatter as you move
toward A. This is one of those situations where eyeballing is is very
difficult to make consistent. It’s not because it’s impossible, or that
you’re necessarily inept, it’s that the changes get hard to perceive, and
people who do this tend to start listening to it to hear differences and
drift gradually towards class A, which sounds sweeter.

In the tube amp FAQ, I mention that you can do this, and achieve
consistent signal quality in biasing, but you need to use a distortion
analyzer to measure the quantity that is actually changing (the crossover
distortion) and not an oscilloscope where you’re trying to make your eyes
and brain pick out the distortion in bad circumstances.