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Questions and Answers
What is BiasFrom mgarvi--(at)--anix.com Mon Jul 8 09:16:33 CDT 1996
From: mgarvi--(at)--anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Okay, I'm confused now. (Re: biasing)
Date: 8 Jul 1996 01:39:37 -0400
In <4rmqv7$hl--(at)--ews.asu.edu> deafe--(at)--map2.asu.edu writes:
>
>My SF Bassman has, I'm assuming, fixed bias. I assume this because I
>didn't see anything like minipots, but I did see some resistors.
Hi Hal,
Just wanted to correct a couple misconceptions about biasing terms.
There are two basic topologies for bias circuits: fixed bias and
cathode bias (or self-bias). But it doesn't relate to whether they
are adjustable or not.
The bias is simply the voltage measured from the tube's control grid
(G1) to the cathode. In almost all power tubes, the cathode should be
more positive than the control grid. This keeps electron flow within
the tube under control. If the bias goes too far positive (well...
toward ground), the tube will start to conduct too much current, the
plates will show orange spots, and the tube may be damaged.
The exact voltage will vary depending on the type of power tube,
and even seemingly identical tubes can vary quite a bit. That's
why it's nice to be able to adjust the bias voltage.
Considering that it is the RELATIVE voltage that matters, there are
two ways to make the cathode more positive:
1) LOWER the control grid's voltage ('fixed bias') --or--
2) RAISE the cathode's voltage ('cathode bias')
Fixed bias: A 'fixed' negative voltage is applied to G1 (first grid
or control grid). This is usually derived from a separate tap on
the power transformer. The term 'fixed' is misleading, since it
has nothing to do with whether it is adjustable or not. Most high
powered amps are fixed bias.
Cathode bias: G1 (control grid) is kept at ground level (0 volts),
but the *cathode* is elevated by placing a resistor in series with
it. The resistor is usually from 200 to 500 ohms or so. As the
tube starts to conduct, it will pull current thru the cathode resistor
and the 'cathode side' of the resistor will start to go more positive.
Naturally, as the cathode goes positive, it makes the cathode-to-G1
voltage look like it going more negative. In this arrangement, the
tube reaches a point where the voltages and currents reach a balance.
Hence the term 'self-bias'. Due to the self-corrective action of
the cathode bias circuit, there is much less need to tweak bias.
The tube sorta does it itself.
Cathode-bias (self-bias) is also the way that 12ax7's in preamp
circuits are biased. Though the voltages and currents are much
different from those in a preamp tube, the concepts are the same.
That's what those 1.5k resistors are on the 12ax7 cathodes.
Another somewhat confusing thing: The bias voltage is controlling
the amount of current going thru the tube. It's the current that
is important: the bias voltage is just the means to control it.
Due to variances in tubes, it is tough to set optimal operating
points by simply setting the bias voltage to -48 volts or whatever.
The better way to do this is to monitor the *current* while you
adjust the bias voltage.
Also, the mentions of current and voltage above all relate to
quiescent (zero-signal) conditions. They determine how the
tube is operating when there is no AC signal applied. The
signal will drive the operating points up and down from the
zero-signal bias point.
>Does "rebiasing" this amp require actually swapping out resistors? Like
>desoldering, removing, replacing, and resoldering? Or is there some
>"magic" of which I am not aware (or minipots I can't see)?
Sounds like your amp doesn't have bias pots. Unfortunately, you'd have
to change resistors or mount your own pots. Trace the cathode pin to
see if there's a high-wattage, low-value resistor there. That would
explain the lack of bias pots (it would mean it's 'cathode bias').
>Please forgive the newbie-ish question, and don't worry, I'm not gonna go
>stickin' my hand in the thing unless I know EXACTLY what's going on.
Well, you're no fun, are you?
MGarvin
From mgarvi--(at)--anix.com Mon Jul 8 09:16:48 CDT 1996
From: mgarvi--(at)--anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Okay, I'm confused now. (Re: biasing)
Date: 8 Jul 1996 02:49:30 -0400
Just read thru my post and thought I should clarify a couple things.
>MGarvin:
>The bias is simply the voltage measured from the tube's control grid
>(G1) to the cathode. In almost all power tubes, the cathode should be
>more positive than the control grid. This keeps electron flow within
>the tube under control. If the bias goes too far positive (well...
>toward ground), the tube will start to conduct too much current, the
>plates will show orange spots, and the tube may be damaged.
The bias is usually measured from the control grid to the cathode,
so it will actually be a negative voltage. If that voltage is not
negative enough, then the tube can overheat.
>>Does "rebiasing" this amp require actually swapping out resistors? Like
>>desoldering, removing, replacing, and resoldering? Or is there some
>>"magic" of which I am not aware (or minipots I can't see)?
>MGarvin:
>Sounds like your amp doesn't have bias pots. Unfortunately, you'd have
>to change resistors or mount your own pots. Trace the cathode pin to
>see if there's a high-wattage, low-value resistor there. That would
>explain the lack of bias pots (it would mean it's 'cathode bias').
Now that I think of it, you *did* mention that you have a Bassman amp,
right? That amp might have a pot mounted thru the chassis, toward the
front of the amp on the right side. That pot should be visible, though.
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. Post if
you need to follow up on this.
MGarvin
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