From mgarvi–(at)–anix.com Fri Jul 12 21:18:34 CDT 1996
Article: 18748 of alt.guitar.amps
Path: geraldo.cc.utexas.edu!cs.utexas.edu!swrinde!newsfeed.internetmci.com!panix!not-for-mail
From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Okay, I’m confused now. (Re: biasing)
Date: 12 Jul 1996 22:02:00 -0400
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A couple people have asked about my comments on bias pots.
Also, the quote levels in this thread have been a bit confusing,
so I’ll try to clarify my previous post.

>>> Mark Garvin (mgarvi–(at)–anix.com) wrote:
>>> The pot on many Fenders is not actually bias pot, but a ‘hum-balance’
>>> control. It controls bias for only one of the output tubes. It’s
>>> easy to change it so the pot is a true bias control, though. […]

[note that I said *many*, not *all*]

> Bill Spencer writes:
>I’m not sure you would want to … see below.

>> deafe–(at)–map2.asu.edu wrote:
>> serve a valid purpose…

> Bill?
> [‘inane condesention snipped’
^^^^^^^^^^^^^^^^^^^^^^^^^^
Huh? I thought Hal’s questions were fine.

>Bill writes:
>These Bassmans uses a circuit not seen in other amps. There is negative
>feedback around the output tubes from the transformer *primary* to thier
>inputs. At the secondary, the power supply ripple is cancelled out,
>mostly, by the two tubes being out of phase. At the primary there is
>ripple (about 5 volts).

I don’t remember that the model number of Hal’s Bassman was ever stated.
I thought he just said it was a silverface. Most Bassman amps have
normal feedback loops, though the phase is reversed on some.

The Fender vintage (for all models) that is usually regarded as being
the overall best-sounding design is the ’63. That’s the one that most
techs are looking at when they do ‘blackface mods’. The control in
the ’63 amps is a true bias control, hooked to both push-pull sides.
[Also note that Fender ‘corrected’ the ’63 design, which operated the
output tubes outside of tube mfg’s ‘grid resistance path to gnd’ spec]

The bias circuit in some Fenders is indeed wired so that both tubes
can be controlled, but in opposite direction. Fender often refers
to this as ‘hum-balance’ but it should not be confused with the hum-
balance trimmer that’s sometimes found on the filament center-tap.
This circuit cannot correct crossover notch problems in a pair which
is matched but happens to run colder than normal.

But Fender *did* make lots of amps with one side set with a resistor
divider, and the other side adjustable. I’m no amp historian (not
as interested in ‘when’ as in ‘what’), but I believe these were
made from ’65 to ’68. I’ve seen a ton of ’em. I believe all of the
enigmatic combined-fixed/cathode-bias amps are wired this way. Again,
I’ve heard Fender refer to that control as a ‘hum-balance’ rather than
bias, and from what I’m told, it was factory-set for lowest hum level.

Both control schemes (bias vs balance) have their merits. For real
tweakers, separate bias controls for each output tube will cover both
goals. However, given the choice of just one, I’d pick the ’63-style
bias and try to use reasonably well-matched output tubes.

Incidentally, I believe the merits of attaining perfect output tube
matching in guitar amps are overstated (aside from the hum questions
stated above). The usual goal is cancellation of even harmonic
distortion in output stages. Why do that?

MGarvin