Herbert bias question.....

mraajr

mraajr

New member
My Herbert is currently loaded with Penta Labs KT88s. It seems that after awhile of good loud play one of the tubes' plates (anode?) is glowing. It is not red-plating by any means but is definitely starting to glow whereas the rest do not. Right now it is reading 30ma. Is this too low? What should it be at for KT88s?
 
The single tube is reading 30mA or the pair?

For a pair thats way too low, a single KT 88 should be biased around 40mA-45mA.
So 80-90mA the pair.
 
That is the pair. As a matter of fact all 3 pairs read right at about 30ma. I will bring it up. Would that cause the tubes to run hotter?
 
Well after finally setting my multimeter right :confused: One of the pairs was reading at about 150ma :LOL: :LOL: I took them all back down to 90ma per the website but I guess if Peter says I should go down more.....
 
Well Peter speaks from his experience. Newer production tubes should be run pairwise at 60mA then, if the heat isn't a big deal (if you don't crank it too much), you can go higher as suggested.
 
I have a feeling that if it can handle the heat of 150ma.....it can take 90ma without breaking a sweat. But I may back it down another 10ma-20ma just to be certain. I think I would rather listen to Peter than myself in this case.
 
higher as suggested = from 60/pair to 90/pair.

I woldn't drive todays tubes beyond 65% of max. plate dissipation.

Your plate voltage is 480V if you want to calculate yourself btw.
 
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