Can I bias my 6505+ / 5150-II?

[taper420]

Not can it be biased.... can I bias it? I'm kinda new to this whole thing. It has bias ports that are bias voltage, not mA. And it has a bias pot. I've done research on biasing and I can't find any details about biasing this specific amp, or any amp that has a bias port that reads "bias voltage".

From what I hear.... these amps are biased very cold stock so that you can just throw in any tube, without having to rebias. So maybe I don't have to bias ( I just put a new set of matched power tubes in and it does sound very good, better than before, with a nice blue glow to the tubes) Also I hear that the bias pot does not have a very large sweep, so even at the highest setting, it's too low to avoid crossover distortion.

So what I'd like to do is bias the amp to the best setting using the controls I have available (no bias mod... and yes I know all about it). So even though I may not get it to the optimum setting... it will still be as close to it as I can get it with a stock unit.
So how do I use a multimeter and the bias ports and pot to get a good setting? And which way do I turn the bias pot to increase the mA to the tubes (make it warmer)?

My schematic:
http://www.schematicheaven.com/newam...ey_5150-II.pdf

[Steve Conner]

Your link is dead.

I've heard of amps where the bias jack measures the grid voltage on the power tubes, which isn't very useful :-( I think the 6505+ may be one of these. You can't measure the actual bias current without busting the amp open and clipping a meter to the guts.

If you don't want to do that, you can bias by the ear, eye and nose method. It's a little ghetto, but it kind of works. Just crank the bias hotter until the tone stops improving, or until the plates start glowing, or until you smell burning: whichever happens first. Then back it off a little.

Oh, and if you don't know which way to turn the bias pot, that blue glow is your friend. It's caused by the plate current, so the direction that makes it brighter is hotter bias.

Finally, a lot of people seem to think that the 5150's stock bias setting is just right for its high-gain metal tone. If you don't like it, maybe you want a different amp.

[taper420]

I happen to really like the tone of my amp. A lot. In fact it sounds even better now that I took out the stock tubes and put in a matched quad.

I just keep running into a lot of people who keep saying... oh you changed your power tubes.... you better get it biased.

But then there's people who say the amp is biased cold so you don't have to get it rebiased.

I like the sound as is, so if my amp is not in danger, I'd like to leave it just how it is.


I updated the link above.

[drewl]

Like most Mesa Boogies, I've found almost all the 5150's I've worked on to be biased very cold.
Most of "the sound" is derived from the high gain preamp, but if you run the master high and the gain low biasing the amp hotter/correctly will greatly improve the sound.

[Enzo]

All the previous posts are telling the story. In the 5150 series of amps, PV develops the majority of the tone in the preamp. The power amp is biased cool. There is an adjustment, but it is of limited range so that amp owners can't set the thing wildly wrong. You won't get the amp in trouble by setting it by ear. If you don't notice much difference, then set it in the middle.

The factory spec is to set the bias for -55 volts. This is a cool setting, but it is what the amp is designed for.