Click to enlargeIs your B-52 MAD OVERHEATED?

This was an eye-opening repair project.

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 correctly?"

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 correct bias.

Reasonable current 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 did 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.

la7508 b-52
final notes on web page

Dan's Legendary Book "Inside Tube Amps" is now available on Amazon/Kindle

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Torres Engineering

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Yuba City CA 95993

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I am Partially retired due to illness, but getting better!

in business 48 years