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Date: Fri, 31 May 2002 17:43:36 -0700
From: "IronMan Mike Curtis"
Subject: Re: Preamp tube subs revisited (was: Bassman '59 RI)

On 31 May 2002 at 17:57, Markburne~ol.com wrote:

> If it were the case that lower preamp valves just gave you more sweep
> on the knob then surely substituting the, usually 1 meg, volume pot
> for say 500K would have the same effect.?

Nope. It would simply lower the impedance.

If it's in the amp, there are many considerations for the value of
the pot. A major one is the design input impedance. Using the
proper value pot gives the best match, and the highest power
transfer.

If you're using a crystal mic, a lower Z gain pot (input) will load
the element, resulting in thinner tone. I also find that CM's and
CR's also like higher impedances, and beef up noticeably.

Swapping preamp tubes not only affects gain, but also places the
amplifier curve differently, due to the different biasing each tube
requires. Normal design procedure is to bias the tube so that
quiescence occurs as near as practical to the center of the piecewise
linear portion of the curve. Replacing the preamp with a lower gain
tube most likely results in a biasing more toward the lower limit,
which will generate more distortion at lower levels.

> Experimentation is the key

Yup. A lot of this is just "mojo".

- -IronMan Mike Curtis - harmonica, guitar, organ bass, vocals
Hear the #1 hit PLAY THEM BLUES, Top 10 on mp3.com Jump Blues
for the past YEAR! http://www.mp3.com.IronmanCurtis
Order IronMans CD "Doin' It All Myself~br>http://www.bumpNgrindrecords.com