Bassman RI biasing

There is a Bassman reissue mod by Gerald Weber about rebiasing
when you change from a solid state to a tube rectifier, just
one or two issues back in Vintage Guitar.  It is simple (for
those who can identify a resistor and do a bit of soldering).
Just replace the bias resistor with a 500K ohm trimpot (i.e.
tiny little adjustable resistor).  Then you can just twiddle
this and get what ever bias you like, and then just leave it.
He says you need to do this because changing from the solid
state rectifier to a tube version lowers the plate voltage on
the power tubes causing them to idle at a lower current which
you can recover by this adjustment.  I have done all this on
my Bassman RI and yes its true, I measured the voltages and
the idle current (trainwreck transformer shunt method).  And
I tried a 5AR4 and a 5U4GB.  Now I did this quickly  while my
wife was out with our three kids under 6yr. so I could put the
volume up to 3.5 without freaking them out.  And the preliminary
conclusion was that I kinded liked the bias as is, i.e. without
doing the mod., closer to 10-15 milliamps.  When I increased the
idle current the amp started to sing with more distortion (in a
good way) but the trade off was a mushy bottom, namely you can't
punch a 2 draw as well, and it was more likely to feedback.
I'm not sure which tube rectifier I liked better, I need to get
the kids out of the house again for some more volume 3.5 time.
Anyhow, the mod is easy to try.  Incidently, Webber says in his
book that 5AR4's are often really 5Y3's that "designer companies"
relable.  The 5Y3, he says, is a good studio rectifier
because it sags more easily, but is not robust and will burn out
soon if used on a gig basis, but then buy a couple if you like it.
He tells you how to distinguish a 5Y3 from a 5AR4 and yep, the
5AR4 that I have from Mesa is really a 5Y3, so in fact I have not
tried a 5AR4. Okay, one more comment, since I rarely post and I get
a lot of help from harp-l.  I play  a JT-30 thru the Bassman RI and
I like it a lot.  I more often play a JT-30 thru a 65 Princeton
Reverb and I love it! That's why I've been tweeking the Bassman RI
to make it a backup amp that I really like.

Mark Reuter

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