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A customer asked me this today and although I have a relatively fair knowledge and experience with tubes, I could not give him a descriptive answer on what might happen electrically given the number of variables and how each amplifier may respond to this anomalie.Can someone ( professionally )comment on this?
thanks, A-man
Follow Ups:
AM,Warning, un-professional response:
1.) Under-biasing the power tubes can make them run hot and shorten their life. Usually, you can tell by the tone of the amp, especially when cranked. Amps tend to distort and even clip too early in the volume range, when under-biased.
2.) Over-biasing power tubes tends to make them run too cold. The resultant tone is lifeless, sterile, with poor midrange response. If you really over-bias a power tube, you get this God awful crossover distortion---which you will never forget once you've heard it.
Since, over-biasing tends to run the power tubes too cold there is less danger of shortening their lives (all else being equal, in the amp), than if you under-bias them.
I know, biasing for dummies... But, it's a basic starting point.
To take some of Steve's starters:
1.) Under-biasing the power tubes can make them run hot and shorten their life.
Underbias, to adjust the grid more positive. Results in higher dissipation. Depending on the extent, dangerous is an understatement. To the extreme, a screen grid can break and short to another component in the tube. Usually disasterous. Watch inside of a tube as the bias is turned 'up' and current increases. Plate will at the end glow red, the bulb can overheat and structurally fail( soften, and burst inward ). There may appear orange spots in the screen grid, which get hotter and hotter as bias current is increased.and several other things I left out, all dependant on circuit design and operating points. A 550 volt Ultra-linear circuit will be more sensitive to underbias than most( usually, on most days, YMMV and a few other disclaimers)
2.) Over-biasing power tubes tends to make them run too cold. Less idle dissipation, longer life and as steve pointed out, usually worse sound. The lower demand on the Power Supply could cause it to exceed voltage ratings of some of its components. Note that a well designed one will be less sensitive to this that a poor one.
regards,
Douglas
From a strictly layman's view, I've read many times where severely overbiasing a tube will promote cathode poisoning, which will lead to much shorter, rather than longer tube life. If this is incorrect, let the knowledgeable folks correct me on this.
Music, my first True Love!
I think that that effect is reserved for valves that are actually cut off. If you can listen to music through it, it is likely turned on enough to avoiod cathode poisoning effects. Computer service valves had special cathode coatings to allow this to take place w/o adverse lifetime effects. How can you know how long a particular gate would be turned 'off' for?
regards,
Douglas
Thanks for the info Douglas. I should come spend some time with you. You ever do open house meetings for the old farts?
Music, my first True Love!
yep, from time to time. where are you? I am in geometric centre of PA.
regards,
Douglas
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