5150 Bias mod for cleaner poweramp tone ?

[faisal]

Quick question which is causing much confusion for me, lots of varied stuff on the net so i thought i'd ask here.
Will adjusting the Bias on a Peavey 5150 to get it out of crossover distortion, clean up the poweramp section considerably ? I run some external preamps into the Return of the 5150 and am trying to get better clean tones from those preamps.
It seems this is a mod which could offer improvement and clarity.

BTW, can i get some explanation on what exactly crossover distortion is ? And is it possible to get a guide on the 5150 Bias Mod which my tech can perform for me. I live abroad so can't send my amp to any of the celebrity modders in the USA. I must attempt this locally, but wont do it myself.
Any help would be highly appreciated.
Thanks
Faisal

 

[GuitarGoat]

If you can get it out of cold biasing it will be out of crossover distortion. I can't remember off the top of my head the true definition of crossover distortion but that's what happens when you cold bias your tubes. It clips in a not so pleasing way. If I remember correctly, cold biasing will cause your tubes to last longer, but you'll get crossover distortion if they are biased too cold. Bias them too hot and they won't last as long but have a more pleasing distortion. Or if they're center biased, they won't distort too much at all because of the biasing, just if they're overdriven.

 

[Tawlks]

Yes.

5150s are biased very cold, in the region of crossover distortion, bias mod it, up the bias and take it out of that region, it will improve the cleans quite noticably.

I'm a bit surprised you'd mention that, I always thought the Peavey's clean tone issues were the fault of the preamp, rather than the power amp. Interesting.

In another thread you were thinking about a Mesa poweramp? Honestly, if you own a Rockmaster preamp by Peavey, it may be worth using that with a mesa poweramp. I didn't read your thread completely so sorry if I missed the point.

 

[faisal]

Hey guys, thanks for the replies.
yes, till yesterday i was contemplating a Mesa Poweramp, but i feel i need to put a stop to this gear lust for a while. I love the 5150 driven tones, i like my Marshall JMP-1 and the Rockmaster. between these 3 i am pretty much covered for a while. If the Bias Mod can get my cleans to be better, i'll probably be really happy with my rig till i have money saved .

The poweramp is a major player in tone and especially so in the 5150. Both my other preamps have a certain 5150 character when running into its poweramp. I read that the bias mod will help the cleans a lot and therefore will affect the tone of the other pre's as well.

Thanks guys!

 

[Shask]

Crossover distortion is the distortion caused when the signal crosses from power tube pair to power tube pair.

The powertubes work sort of like a teeter-totter where they work back and forth, and the small transition from side to side is where crossover distortion can occur if they are biased really cold.


That being said, it is hard for me to say the amp will "clean up" when using external preamps. It all depends on how you hear things I remember when I did mine years ago, the cleans got better, and the amp overall just sounds cleaner and more defined. BUT, I also think that people make the bias mod out to be WAY more than it actually is. people act like it completely transforms the amp, and it doesnt. But, I will say it is a nice improvement overall, even though it is a smaller improvement.

It doesnt hurt to try though! It is only $5 in parts and 15 minutes to do it.