Wednesday, October 9, 2013

Biasing a Peavey 5150

The Peavey 5150 is another in a series of high gain amps beloved of metalheads and I rarely see them, because the owners are mostly not about tone. This once came in because all it would produce was a loud groan when powered up.

A little investigation revealed that one of the brand new preamp tubes was shorted so that got replaced in short order and I got sound but not muscle. A little more investigation revealed that the number 4 power tube had no screen voltage. So out came the tube board and all four screen resistors got replaced with some 220s rather than 100 ohm units, because that's what I had, and if 470s were good enough for Leo then......... It's worth noting that a plug lies right on top of  #1 so you can't get at it with a multimeter probe but nevermind. This is a good time to check everything on the back side of the tube board, including the two flyback diodes.

Although this amp is fixed bias, it's nonadjustable in the stock configuration, the level being set by R68 which used to occupy the space in the picture. It's mounted parallel to the adjacent electrolytic capacitor.  And they're usually biased way too cold.

A short digression.

In a guitar or bass amp "bias" refers to bias voltage, which is a negative voltage applied to the grid. This sets the operating level for the tube. Bias voltage can be obtained in two ways. In a fixed bias arrangement, the negative voltage is supplied by an external source such as a dedicated transformer winding. Even if there's no adjustment feature it is still fixed bias, and if it's adjustable like we did here it's still fixed bias.  The other ways is cathode bias, which uses a cathode resistor to accomplish a similar purpose by raising the cathode above ground potential. We don't have that here, and it is less efficient, more touch sensitive, and usually used in lower power amps.

To improve function it is necessary to incorporate an adjustment pot. I had a Bourns 10k pot that you see here. On one leg I attach a suitable resistor-in this place an 8.2k 1/2 w. number and cut a piece of spaghetti tubing to insulate the lead.. I take the center pin of the pot and wrap it around the unoccupied other pin and secure it with a dab of Mr. Kester's best solder.  Then I wrap about an inch or so of wire around the pin with some spaghetti covering and solder it in place.

To finish up the job, I set the resistance with a multimeter to about 15k as a good starting place, and then wrap the end of the two leads around either side of the capacitor and solder them in place.

Then I stick the Bourns pot down with a dab of superglue. Don't go crazy, you might need to remove it some day.

The next thing to do is fire the amp up with any good bias probe installed and flip the standby switch to on. If things start going sour shut down and do a rethink. But if you've got it right you should see about 25ma per tube.

Then it's a simple matter of measuring plate voltage and using the Weber bias calculator to find the right level you want. If they're new tubes always leave a little room for drift. At 481v on the plates I set it to about 34 ma on the hottest tube.

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