I'm replacing the power tubes on my peavey valveking, It's fixed bias, can I just stick the tubes in?

It's kinda jacked up, but fixed bias and self-bias mean almost contradictory things.

When a tube is self-biased (ie cathode biased, like a preamp tube) then there is no way to adjust it... no need, even, the tube basically adjusts itself.

When a tube is fixed bias, though, it means there's a pot somewhere that has "fixed" the amount of bias for the tube.

The benefit of this is that if you can match the tube current and voltage requirements, then you're good to go - just drop in and that's that. This is the direction that Mesa has gone, for instance - you are guaranteed to get the correct spec of tube if you buy a Mesa tube for your Mesa amp.

The drawback... if you can't match the tube current or voltage you run the very real risk of running your tubes too hot, which can lead to tube failure. I love the phrase catastrophic tube failure... (catastrophic tube failure!), but you get the idea. Power, sizzle, poof.

If you have a set of matched power tubes from the same vendor, same type of tube, then I would personally feel comfortable just slapping them in, with the caveat that I would let them burn in for a few hours, checking them to make sure they weren't glowing funny. I acknowledge that this isn't ideal - enough variation does occur that this could lead the tubes to be running a little too hot or a little too cold, but I use my eyes and ears (and nose!) to tell me that stuff.

However, if I'm not using the same manufacturer and/or type of tube, then yeah, I'm going to take it to an amp tech and have it biased for those power tubes.

To my mind it's worth spending a hundred bucks or whatever it is on an amp tech if it means keeping my 1000$+ amp from blowing up on me.

Now, I've got an Epiphone Valve Jr, and you can bet that I won't give two sh... uhhh... dangs.... about whether it's biased right or not.... I'll be teaching myself how to bias with it as my helpless victim!

Really, though, it's best to err on the side of caution. There is lethal voltage in those filter caps, so changing the bias is not something to do willy-nilly....

Saul

Yes, if the bias is fixed you can just put them directly in. But ONLY if the tube's are matched up correctly based on their plate current, otherwise it can damage your amp or even send you a potentially lethal electrcic shock. You should be able to find the specs in the manual or on the amp itself.

For more information try this site:

http://thetubestore.com/powertubeinfo.html#q4

Peavey Valve King