Spot check on biasing a 6505+

  • Thread starter Thread starter GJgo
  • Start date Start date
GJgo

GJgo

Well-known member
Hey guys, I picked up an older USA made Peavey 6505+ in pretty good shape. Going through it, one thing I wanted to do is to spot check the bias since these have the voltage tap on the back and the exposed bias pot.

The 6L6 that it came with are matched well but testing REALLY cold in my tester. They're even colder than Mesa's "RED".

The amp is running ~483 plate volts at 123 wall V.

As it came the previous owner had set the bias pot to where it was measuring 55v at the bias taps, which from what I've read is as intended. (I know the taps are polarity reversed as not to confuse people.) At this setting the tubes are running 18mA.

When I dime the bias pot the tubes get up to 19 mA, and the bias taps move down (up really) to 53.5v.

Even though from what I know this design, like the Mesas, gets its gain from the preamp and the power tubes don't need to be especially hot, 18-19mA still seems too low.

Any advice on this one? Paging @glpg80?

Thanks guys.
 
The USA made 6505+ need a resistor changed, I think it’s R124 from memory but you’d need to share a screenshot of a schematic so I can confirm on here. The change let’s the bias adjustment actually have room to get the tubes out of crossover distortion which Eddie liked at the time. If you google 5150 II bias mod then you could also find what to do from decades ago. Mine is modified. The board isn’t hard to reach at all and it’s not challenging to do. You’d want to bias it at 32mA idle on the hottest tube. You don’t want to use the coldest tube because too warm of a bias on all of the other tubes makes the amp feel greasy under your fingers and lose a lot of rhythm riff percussiveness these amps are known for.

Also helps to lift the power board and change all of the screen grid resistors to 1K 5W ceramic fire proof. Really helps extend tube life once you’re out of crossover and they start getting used properly. I noticed a big difference tonally and in a good way once the screen grids were upgraded to Marshall values.
 
Last edited:
You must log in or register to reply here.
Back
Top