[Harp-L] Re: changing tubes, biasing on Bassman RI

[HTownFess <Spschndr@xxxxxxx>]

Rosco, if you are simply replacing the original main tubes with the
Groove Tubes that Fender uses, then the originals may have a color
code, and if you replace with a new pair with the same color code,
there's no need to check bias.  Check with a Fender dealer on that
one; Fender was trying to make it simple and has had that color-code
program for a few years now.

If you have a standard Bassman RI, then it will have a bias resistor
instead of a bias pot, and you really ought to have a tech install a
bias pot, which does not cost much if the tech knows what they're
doing (something like $2 in parts and the circuit board does not have
to come out of the amp).  It is worth doing, and if you are using
another brand of main tube than stock, it will be necessary to bias
the new mains.

If you have a Bassman LTD RI, it will already have a bias pot. You can
adjust that pot yourself with an insulated screwdriver if you exercise
reasonable caution about not touching anything else in there--as
noted, there is lethal live voltage/current in there and you have to
set bias with the amp on--but without a bias measuring setup, you
won't know for sure what's going on and you run the initial risk that
Jim mentions.   You do *not* have to "hook leads" into the amp to
measure bias with reasonable accuracy if you use a biasing adaptor
like a Weber Bias Rite that fits between a main tube and its socket.
Minimal bias measuring setup that way is probably a single Weber Bias
Rite adaptor and a digital multimeter, and that will probably cost you
at least $50-60 to get.  Are you sure there aren't any guitar players
in your area who do their own biasing?

If a tech is going to bias the amp, it is best if you are there and
play the amp and listen to what it does across the usable bias range.
Warm, cool, and in between can do different things and you should
decide for yourself about that.

Stephen Schneider