New Matched tubes, Need to Bias?

[joswil44]

I just got a new Quad set of EL34's for my Ampeg VL-1002 Head and I figured I need to Bias the head but a friend said if the tubes are a Matched set you should have to Bias the head.

Is that true?

 

[joswil44]

I answered my own question with the VL-1002 on the Lee Jackson website it says you dont need to Bias the head when changing tubes, just flip the bias switch to either EL34 or 6550.

But I would still like to hear an answer on the Matched set Bias question in regards to other amps.

 

[jonnyc]

I don't think biasing has to do with whether the tubes are matched to each other, I think it has something to do with the "rating" of the tube vs. the amp. But I'm not an amp tech nor am I that knowledgable(just know what sounds good) so what do I know.

 

[joswil44]

I think your pretty close.

I tryed reading up some more and I believe it was how much current your applying to the tubes or something.

The matching of the tubes helps to get a proper sign wave or something of that nature.

 

[Thurgood]

Bias the amp. Matching addresses the uniformity of the tubes as a group. Bias relates to the operating current of the power section (loosely put I know). So say your amp is optimized for X current. You put in your tubes and bias to X current and athe theory and practice is that since the tubes are matched they will arguably all end up at X current together. THe old musketeers thing. All for one One for all. Peace. So go get that amp biased. (unless of course it is a fixed bias amp, in which case forget the above in reference to THAT amp.)

 

[dementedchord]

bias: this is a negative voltage used toshut the tubes off when not seeing a signal... typicaly the tubes are set to allow a few milli amps of current to pass through the tube... think of it as setting the idle on an engine... too much bias and the engine stalls... too little and the engine heats up and burns...
this has nothing to do with whether the tubes are matched.... matching is still a great idea... only a fool would change tube type and not rebias... although it's best to rebias everytime you retube IF you bias properly and then stay with the same tube rating then the likelyhood of problems is minimized if you dont want to everytime....

 

[Micter]

If you replace the power tubes you should bias the amp!

 

[ggunn]

If you replace the power tubes you should bias the amp!

If tha amp has an adjustable bias, then yes, it's a good idea.

If not, well, not. ;^)

 

[pikingrin]

If tha amp has an adjustable bias, then yes, it's a good idea.

If not, well, not. ;^)

Wow, okay, this entire thread has me a bit confused here. I was under the impression that if you stayed with the same type of tube, then all you would have to do is plug and play. For example, I want to change the stock tubes in my Blues Jr. to some better tubes... If I get the same tubes that are in there (different brand of course), then all I would have to do would take out old, replace with new and turn on, right? I thought biasing was only necessary when changing the tubes to something completely different...

 

[boingoman]

So go get that amp biased. (unless of course it is a fixed bias amp, in which case forget the above in reference to THAT amp.)

Wait a minute. All bias is adjustable. It's just a matter of how.

There are two kinds of biasing for tubes.

Fixed bias- the bias voltage is provided by an offshoot of the main power supply. This voltage is constant. Fixed, in other words.

Cathode or self-bias- a resistor between cathode and ground provides the bias voltage (sort of). This voltage is determined by the input signal, and it fluctuates. It is not fixed.

When people say "adjustable bias" they mean a fixed bias that can be adjusted with a pot. "Non-adjustable" means a fixed bias that uses a resistor instead of a pot. To change the bias on this type of amp, you have to swap resistors instead of merely turning a pot.

 

[ggunn]

Wow, okay, this entire thread has me a bit confused here. I was under the impression that if you stayed with the same type of tube, then all you would have to do is plug and play. For example, I want to change the stock tubes in my Blues Jr. to some better tubes... If I get the same tubes that are in there (different brand of course), then all I would have to do would take out old, replace with new and turn on, right? I thought biasing was only necessary when changing the tubes to something completely different...

Well, it's certainly more important if you change from, say, EL34's to 6550's (which are the same type but somewhat different), but most techs I have talked to about this say that it's not a bad idea on an adjustable bias amp to get the bias checked/adjusted when you change power tubes any time, with the possible exception of graded power tubes like Groove Tubes, which are hand picked and graded for their characteristics. There can be significant variability between individual tubes of the same type and brand.

 

[dementedchord]

If tha amp has an adjustable bias, then yes, it's a good idea.

If not, well, not. ;^)

flat fukin WRONG !!!

the only difference is the method of adjusting... if it's not adjustable by twisten a pot then you swap out resistors.... either way rebias your amps....