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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
oldhvymec
5,101 posts
 
What type of gear? ALL Cayin? Do they need bias? What does the owners manual say? You've had the sames gear 47 years? LOL Just a couple questions..

IF it requires the bias to be set, it's usually not to difficult. Some units the setting are on the inside, that makes it a little tougher. BE SAFE...

Regards

tablejockey
3,935 posts
 
In each case the tube was in the +/- tolerance range for satisfactory operating performance?

I wish I find the article I read years ago about "tube matching" by an engineer who determined it’s baloney-at least for audio equipment. Maybe a rocket ship to Mars it may be critical

Let’s be honest-who hears a 1ma MM reading? Perhaps it shows on test equipment, but your ears? .

That said, I still do everything in an owners manual.

oldhvymec
5,101 posts
 
The issue with balanced valves is just that "Balanced", left to right in volume and bottom to top, if it's a single, double, or triple.

Sometimes the left and right share the same valve, they need to be the same, bottom to top. Incorrect bias can cause the valve to distort to quick, (to high) or sound muddy and thin (to low).

If you're picky, and the amps are revealing enough, you'll hear the difference, no problem.. Gotta keep the pin pockets tight and clean and the unit tuned..

Regards
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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