This was an eye-opening repair
A new B-52 amp was
brought into the shop because it "didn't sound right", and he was
right, it sounded pretty bad, mushy with unpleasant distortion.
The amp was also
running REALLY, REALLY hot - the new set of Winged C 6L6 power tubes
had their paint burned off after only a few minutes of operation. Tubes
running this hot can fail at any moment taking the entire amp or a
transformer with them.
The first thing to
test was the always controversial issue - "is the amp biased
It was obvious from
the red-hot tubes that it was not, the amp has a bias control but it
did not have anywhere near the range needed to get the tubes into
draw for 6L6 tubes in a high powered amp like this is 28 to 38
milliamps. this amp had about 120 milliamps per tube and was just
cooking them into failure and poor tone.
The existing fairly
tricky and clever bridge rectifier bias system did not have anywhere
near the capacity needed to get this amp into the right bias range, it
not provide enough negative voltage even at it's max.
We decided to just
disconnect the existing bias system from the
power tubes and install a new fool-proof Torres Design supply
taken from the high voltage secondary. The existing bridge rectifier
provided voltages for other solid state functions within the amp, so it
was left intact.
This also allowed us
to install better quality capacitors, diodes and resistors in the bias
circuit for the power tubes.
with the new bias
circuit installed and setup the amp could be biased into the correct
range for the power tubes without overheating and without the chance of
tube failure that was so prevelant before.
Tone with CORRECT bias
was massive, huge, tight and solid with all the "mush" gone and really
great distortion and crunch. Correct bias also provides a LOT
more power from the amp!!
Overall, a great
design-project getting the max performance and versatility
from the B-52 amp. Now it can use any 6L6 tube, KT66 6550 or even the
KT77 giving you a whole lot more amp for the money.