Friday, May 17, 2013

Peavey 6505+ Biasing

A Peavey 6505+ arrived here needing a general going over. It was first determined that the power tubes needed replacing as there was a cathode current spread of about 17 ma, lowest to highest. I also tested the allegedly new preamp tubes and found one to be very weak.

Once I got my preliminary work done I powered the amp up on the bench and noticed it was very low on volume, and that there was a lot of crackling and popping when tapping the very flimsy preamp tube board.

Some cleaning and swapping was in order, but what cured the issue was replacing V5 with a new old stock GE preamp tube.

Setting the bias is relatively easy on this amp because there is an external control, but the range was sufficient to only get me to 17 ma-not enough by a landslide.

The fix is removing the 12k 1/4 w resistor on the bias control board and replacing it with a 6.8k resistor. Here you see the results with plate voltage at 439 and cathode current at about 35ma.

Although super high gain amps are not my cup of tea, this one seems to be reasonably well constructed for the genre, but the preamp tube board is a definite weak point. When installing preamp tubes from the outside the board can be felt flexing and viewed from underneath there's no doubt that this is not a good feature-there is not nearly enough support. In the attached pix you can see the back side of the preamp tube board, the bias control board that the range resistor gets changed on, V5 which was the source of all that trouble, and the finished result.


When doing my research I found that a lot of people on the chat boards treat the bias control pot as something of a dial-a-torque sort of arrangement. Turn the knob, get the tone-that sort of thing. I attached a warning label and taped over the control.

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