Tube Biasing

G-String

G-String

New member
I have asked this question about replacing tubes in my Fender Deville. Can't seem to get anyone knowledgable on the subject, and I know someone in this forum can explain: First, Why do you need to bias tubes? Second, what is it. (what does it do for the tubes or amp). And finally, How do you do it? I know from talking to other people that you should'nt do it yourself if you don't know what you are doing for risk of damaging the equipment. But I still would like to know. Thanks........
 
The bias current is the power that runs the tube. (I'm explaining this very simplisticly because I have a headache and don't want to think to hard) Every tube has a voltage that it works best at. When you are adjusting tha bias, you are making sure that the tube is seeing the right voltage for it to work as intended. Depending on the difference between the old set of tubes and the new set, the adjustment might not be necessary, but you have to look to find out. Normally, you do this with a meter and there is an adjustment pot inside the amp. The trick is knowing what the bias voltage is supposed to be. Some amps are fixed bias, so there is no adjustment.
 
well...

Most Fenders- and probably yours- are biased through a cathode resistor. THe current passing throught it creates an effective negative bias voltage that translates to the correct bias current. Fenders are usually biased colder than marshalls and such and therefore usually have less problems.

If someone can confirm this than you really don't have much to worry about as you wouldn't be able to bias your amp without modification anyway....

Cathode Biasing is more forgiving than "fixed" bias, so rebiasing usually isn;t necessary, unless your amp is having problems...
 
O.K. Thanks. I have heard that you should bias every time you change tubes, and I have also heard that you did'nt have to bias every time. I've changed the tubes in my Fender Deluxe and it sounds great to me-I guess I want it to be the best that it can be, and I'm pretty selfish when it comes to my tone.
My Deville has Sovtek tubes in it and they are the cheaper ones. (5881WXT's). I bought it like that. I don't like 'Em. I want to go with some 6L6's that have A better bellike tone. Thanks A lot for the info.
 
Some Fenders have adjustable bias built in, in the manual for mine it gives the settings and you simply plug in the digital voltmeter and turn the screw left or right till you reach the desired setting. Nearly idiot proof. Then you do the same for the output balance.
It also says "If you are not sure leave it alone"
 
As there are lethal voltages present inside the back of a tube amp, even when unplugged, it is definitely best to leave it to a professional if you are un-trained. A poorly biased amp could result in premature failure of your new tubes or worse, damage to the power transformer. Since both are more expensive than a professionally done bias (a quick and inexpensive operation) I would, again, recommend a professional. Good luck...
 
Biasing an amp refers to the POWER tubes only,. and has nothing whatever to do with the smaller Preamp tubes,....

It is basically an adjustment for the voltage in the power tubes,and is VERY important for several reasons, one is for the life of the tubes,... too hot, and they burn right up,.... too cold, and the amp sounds like shit,....too far out of sync,...(unbalanced tubes), and there is all kinds of noise issues,... hums, crackles, static, etc,... and is why it is a neccessity to have fairly balanced power tubes,..... (usually in sets of two at a time, altho there are some matched quads if you need them,....)they work as a pair in amps that use two or more power tubes,...

It is usually not needed when using Fender amps, as they are built for all levels of skill, aand as such are set up so anyone can trade power tubes with no ill effects to the amp.....

This is not true of all amps,... some of the 'botique' types are EXTREMELY sensetive to changing power tubes, and will not react the way you want,.. and can even be damaged by tubes that are too different from the old ones you are replacing.

If you are sort of confused by it all, take your amp to a skilled person to do this for you,....


better safe than sorry would be a key phrase to remember ....


By the way,... I do have some used bottle shaped RCA 6L6G tubes if you need them,.... much cheaper than new ones,... and they sound extremely nice compared to the cheap stuff they build now....

Email me for details....

Steve
 
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