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To bias or not to bias

After making the switch from solid state to tubes and now having three tube systems, I spend a fair amount of time biasing my amps to obtain peak performance as recommended by many enthusiasts.

As I was doing this one day I thought back to my youth when many people had tube equipment (Yes, I am that old) and it dawned on me that I don't remember anyone biasing their equipment back then. I had a college roommate that had McIntosh tube equipment and he never biased his rig. It always sounded fantastic. I remember my Dad having Scott tube amps and tube radios, playing them for years and other than replacing a bad tube every now and then, never biasing his gear. Again, they sounded great.

So that led me to thinking, why do we pay so much attention to biasing today? Is it because the equipment requires it or is it audiophile tweaking run amok?

Your thoughts on the matter would be much appreciated.
catama
6 responses Add your response
atmasphere's avatar
atmasphere
11,794 posts
 
If you are adjusting the bias more than once every 6 months I would regard that as 'tweaky':) Seriously, the tubes should be able to hold up that long without adjustment.
thorman
1,262 posts
 
I just do my Thor Monos once a week ( Takes 30 seconds each side )never had a problem and maybe that is one good reason.......
apachef1
164 posts
 
Bigger tube amps require biasing more so than smaller amps
When you have 8-16 power tubes per amp getting that many numbered tubes matched will be a challenge. Even if you have a matched group of 8 or 10 power tubes, larger power tubes tend to drift and one tube will be pulling more of the load. Biasing allows you to make everyone play nice on the power tube grid.
Examples would be ARC ref 300 and 600 amps
my mcintosh mc-225 just requires a quad of closely matched 7591s and she is good to go
eldartford
4,262 posts
 
Many tube amps used a self-biasing tube circuit (not the same thing as auto-bias). A self-biasing tube circuit has no means of adjusting bias. This eliminates the need for a bias voltage power supply. Eico offered one amp kit in both self-biasing and set bias versions. The self biasing amp was less expensive and delivered less power. Otherwize the specs were similar.
dougdeacon
3,338 posts
 
It depends on the quality of the amp, the tubes and the stability of the incoming voltage.

My amp hardly ever needs rebiasing, even after a tube change. I check it every 6 months or so if I have nothing better to do, but it hasn't really changed in at least two years. It helps that it has robust and stable power supplies, uses current production power tubes that I always buy matched to the existing tubes and that my power conditioner provides stable voltage. All of these reduce tube drift, thus reducing the need for rebiasing or rebalancing.

That said, changing the bias does make a very audible difference. Bias adjustment provides a trade-off between harmonic extension and depth vs. tube life. But whatever number I settle on tends to stay put for many months.

YMMV of course, depending on all the above...
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