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TUBE BIAS, socket to me!

BIAS: (I'm starting from zero understanding) 

I have never measured/adjusted bias in the 3 tube amps, 3 tube receivers, and 2 tube preamps I have acquired over 47 years. I just switched my current Cayin from 6550's to KT88's. Adjust bias? Adjusters inside, scary electrocution warnings. I could pay someone else to do it, i.e. Steve at VAS 1 hr away in NJ, soooo, 

What really counts? (personally I don't care about either heat or life, but would like to understand)

Heat?
Life?
Output stays Matched when adjusted?
Acoustic Performance?
_________?

Over the years, fronts off, bottoms off, I hose em down with contact cleaner/lubricant, compressed air, all controls and switchers, any adjusters, swish full spin back and forth. Kill any spiders, look for, replace the rare burnt resistor. 
Then leave any adjusters (whatever they are) in the middle position, button it back up.

Two tube testers, my big hickock always agrees with small portable one, test strength, shorts, matched strength old and newly purchased. Large collection of NOS, used. Often used test essentially same strength as new ones.

When they go, it's usually a short.
elliottbnewcombjr

Showing 3 responses by millercarbon

millercarbon's avatar
millercarbon
12,531 posts
 
Einstein's Theory of Audio Relativity:
Sound quality equals double blind BS times the vintage age squared.
millercarbon's avatar
millercarbon
12,531 posts
 
thanks for providing that understanding, big help just knowing 'why' bias exists.

Is it fair to say, if it sounds great, don't mess with it, or why bother as some suggest?

Thanks and yes, if it ain't broke don't fix it. 

Technically, the bias setting really does make a difference. In practice however it is like an oil change, you will always swear it makes the car run better. 

By the way, the same concept of bias is used in tape recording. The little particles in the layer we want to magnetize to record on, they don't respond very well if all we do is feed them the audio frequency signal. They are kinda slow when asked to change from zero state. But if we excite them with a bias current, in this case 60kHz or higher, this puts them in a state where they respond much better to the music signal. Very similar or analogous to tube bias.  

Mine one time somehow got way out of spec, something like 2 or 3 times what it should be, and so I was anticipating this nice improvement after spending the few minutes to lug out the meter, get down on the floor twiddling and testing to dial them all in nice and perfect. And then... nuttin. Nada. Oh well. At least the car runs better now.
millercarbon's avatar
millercarbon
12,531 posts
 
What really counts? (personally I don't care about either heat or life, but would like to understand)

Heat?
Life?
Output stays Matched when adjusted?
Acoustic Performance?
_________?

First off, why do we do it at all? In very general layman's terms its to improve signal response and sound quality. Okay, but how?

What happens when we send a signal through a cold tube? Not much, right? Tube has to warm up first. Because a tube is a kind of valve that controls the flow of electrons we have to have some electrons to start with, of which there are damn few when its cold. 

But even after we warm it up we find every time we send a signal the first little bit that comes out isn't as good as later on. Like it has to go through warm-up all over again only this time on a very fine small scale. But the details in music are all very fine small scale so even this little bit matters.

What we do to solve this is keep the tube up in its optimal response zone by adding a small steady bias current. How small? There is no one answer. Its a judgment call, and a trade off. More bias, more current, more heat, more power, more wear, less tube life. In a nutshell.


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