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From: "Don D."
Date: Thu, 01 Oct 1998 16:36:06 -0700
Subject: Re: amp rebiasing and "starved plate" operation

> From: John Thaden
> Date: Thu, 01 Oct 1998 11:53:32 -0500
> Subject: amp rebiasing and "starved-plate" operation
>
> In another thread, Don D. wrote
>
> >On a stock RI Bassman, there is no way to
> >easily rebias the amp, and Fender usually biases the amps "cold" to save
> >on tube wear and to cut down on warranty replacement on the cheap,
> >imported tubes they use. This will tend to make the amp sound "thin" or
> >"lifeless." If you replace the rectifier and power tubes with good US
> >NOS tubes and rebias the amp a bit hotter, you'll find you get a big
> >improvement in tone.
>
> I'm not sure I know what Don means by 'cold', but I've read that some amp
> designs run the output tubes with less plate voltage than spec, because
> they then distort at lower volumes and have a round tone. Also, that the
> single 12AX7's in some of the tube distortion pedals are set up this way.
> My question is, does 'bias' refer to the plate voltage? If so, wouldn't
> lower (colder?) bias give a nice tone at lower volumes?
>
>
> John Thaden -Gil Franklin Band
> Little Rock, Arkansas, USA

The subject of biasing amps is very a very complex one at best and I'll
be the first to admit I don't know all the electronic theory behind the
subject. Mike Curtis, where are you? And a discussion of the matter
could get very long also, but what I was refering to is current or
amperage. Most amp techs would agree that 6L6 tubes sound best in most
amps when the bias current is set to 35-40 milliamps. I was recently
involved in a discussion with someone who had the bias checked on his
stock RI Bassman and found that the bias was running at around 20ma.
Sure, it's fine and the tubes will last longer, but you'll find that
most amps will sound fatter and richer when the idle current is a bit
higher in the 35-40 ma range. Of course you can over do it and if the
plates on the tubes start glowing red, you know you have too much
current, this situation would most certainly shorten tube life
considerably.
As far as your question about voltage and it's relationship to current
in biasing, I'll leave that question to the electronics experts on the
list. I do know that extremely low biasing current will cause a tube to
"clip", but this kind of clipping creates a "buzzy" distortion, not the
harmonically rich compression that most harp players think of when you
overdrive an amp. I don't know if this is the same thing you're
refering to, but most of the tube distortion pedals I've heard have a
very harsh sound to them.
Hopefully some of the more knowledgeable electronics wizards on the
list can help clarify your questions.

Don D.