Adjusting BIAS

barmesan · 6th January 2007, 12:53 PM

I have VAC 80/80 KT88 amp. After adjusting bias so that plate current is exactly same, I can hear some hum from speakers. If I adjust bias by ear I can get that hum to disappear. But plate currents differ quite much then. 63 mA and 65 mA for other pair and 52 mA and 65 mA for another pair.

Which one is better? To adjust bias by ear to get minimum hum or to adjust by meter to get identical plate currents?

My amp is working in triode mode and is biased towards class A to get best sound quality.

Sincerely Yours, Sixtus Oest

Merlinb · 10th January 2007, 11:07 AM

When you discover something like that it indicates that having the valve off-balance is introducing a small hum signal that is cancelling the hum out. If you can't find the true source of the hum and beat it directly, I say go with your ears

ray_moth · 10th January 2007, 12:18 PM

I am not familiar with the amp. One possibility is that you are measuring voltage across cathode resistors that are not accurately matched. Another is that the primary winding of your OPT is not tapped exactly in the center. Whatever the problem, it's usually best to balance by ear,

6th January 2007, 12:53 PM	#1
barmesan diyAudio Member Join Date: Dec 2005 Location: Helsinki	Adjustig BIAS I have VAC 80/80 KT88 amp. After adjusting bias so that plate current is exactly same, I can hear some hum from speakers. If I adjust bias by ear I can get that hum to disappear. But plate currents differ quite much then. 63 mA and 65 mA for other pair and 52 mA and 65 mA for another pair. Which one is better? To adjust bias by ear to get minimum hum or to adjust by meter to get identical plate currents? My amp is working in triode mode and is biased towards class A to get best sound quality. Sincerely Yours, Sixtus Oest

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10th January 2007, 11:07 AM	#2
Merlinb diyAudio Member Join Date: May 2006 Location: York	When you discover something like that it indicates that having the valve off-balance is introducing a small hum signal that is cancelling the hum out. If you can't find the true source of the hum and beat it directly, I say go with your ears

10th January 2007, 12:18 PM	#3
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I am not familiar with the amp. One possibility is that you are measuring voltage across cathode resistors that are not accurately matched. Another is that the primary winding of your OPT is not tapped exactly in the center. Whatever the problem, it's usually best to balance by ear,