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Biasing Peavey Ultra 112...noob question.

6/7/2004 5:23 PM
Biasing Peavey Ultra 112...noob question.
Hey I'm new to tube amps, but I need to replace the tubes on my Peavey Ultra 112, and want to know about biasing! I plan to replace them with the same types currently inside, 6L6's, and would like to bias it myself. However, I have read that this amp has a fixed bias...  
so should I  
a) replace the tubes, without biasing the amp  
b) measure current, and replace resistor with a value i think should work  
c) replace the resistor with a pot  
or d) take it in a let a tech charge me 40-60 bux for the job ?  
I have a background with elecronics, and effects building, but haven't yet tackled amps, so i know very little about them.  
Thanks so much
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6/7/2004 5:16 PM

Just to be technical, even the adjustable bias supplies are considered "fixed."  
The Peavey amps are designed to not need adjustment, they get their tone mostly from the preamp. You can plug in the new tubes and play.  
Lots of guys prefer their amps biased right up to the verge of red plating to get the "most" out of an amp. Modifying the amp for adjustable bias is not complex, but if you don't know what you are doing, it may be best to leave it to a tech. Yes, my shop would charge about $60 for the job for example.  
Replacing the lower resistor with a pot is the essence of it, though I recommend a resistor in series with the pot to limit how low it can be adjusted. That is instead of using say a 25k pot, I would use a 15k resistor and a 10k pot in series. Or something like that. Without looking at the print I don't know, but some amps have a very low upper resistor, like 470 ohms, so no matter what you do with the lower resistor you will never get good voltage division, so we replace the upper resistor with something like 3k.  
One could replace the lower resistor with a different value, but then next time you change tubes, you have to do it all over again. If you are modding it in the first place, why not make it adjustable.  
Try the new tubes in the stock amp first, then decide if you need to change anything. It may sound just fine to you.
6/7/2004 5:39 PM

Thanks so much!  
I have previously recieved many different opinions about the nessecity of biasing amps, but never anything about _my_ amp.  
I think the stock sound should be good. I've (surprisingly) never opened my amp up, since i bought it used. I seriously doubt it was modded, and sound(ed) great! I even plan on buying the same cheap Ruby tubes that are in it, that have recently become very noisy :).  
Thanks again!

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