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

[Jim Rossen <jimjimdr@xxxxxxxxx>]

Checking power tube bias when replacing the tubes in a fixed bias amp
is preferable but not absolutely essential.

Adjusting the bias consists of adjusting the negative voltage applied
to the tube control grid, thereby setting the range of current flowing
through the tube during operation.  The current is typically set to a
particular value when the amp is on but not presented with an input
signal and this is called the idle current.   Another method of
setting the bias involves analysis of a test signal run into the amp
using an oscilloscope.  Different examples of the same tube vary with
regard to the negative voltage needed to attain a particular idle
current.  If the negative bias voltage is too small (say -25 mV
instead of -35 mV), the tube may operate at an excessively high
current range-  this can lead to premature tube failure and possibly
tube failure in a manner that could damage the amp transformers and
other components.   This is the major risk of changing tubes without
checking the bias.

Another reason to check the bias is that bias adjustment affects the
sound of the amp.   A more distorted tone that is more feedback
resistant results if the power tubes are over biased and running at a
lower current.  This will also prolong tube life.  Manufacturers may
under bias amps for this reason.  Bassman RI and similar amps are
often biased so as to set the idle current  at 35 mil amps (mA) for
each tube.   An idle current of 15-25 mA may be favorable for
harmonica.

Some people are relaxed about bias adjustments, and feel that if an
amp sounds good and the tubes are not glowing red, there is no need.
Apparently, checking bias after a power tube change was not routine
during the golden age of fixed bias tube guitar amps.

Jim R



On Jun 8, 5:08 pm, rosco <roscoh...@xxxxxxxxx> wrote:
> Hey list,
> I use a lot of electronics but don't understand any of it. I know there is A
> LOT of knowledge represented on this list...so:
> My question is about biasing my Bassman RI. I want to change power tubes -do
> I absolutely need to re-bias when I do this? Does it need to be done by an
> amp tech? -they are few
> on the ground around here.
> Thanks in advance - and it's good to see the list so healthy these days!
>
> --
> Later,
> Roscowww.maybeaugust.comwww.myspace.com/maybeaugusthttp://www.facebook.com/home.php?#!/?ref=home