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In Reply to: RE: PL Prologue autobias adjustment ...part 2 posted by rotarius on December 26, 2015 at 20:30:21
You have one tube that is much softer (lower current draw) than the others. This situation doesnt necessarily mean that this is bad but I would say you should look for another MATCHED quad preferably that is new because all tubes should be replaced at the same time if they are used. My experience with Chinese made equipment is spotty at best so it could be the equipment but my initial thought would be that one tube is to blame.
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The 6CA7 quad is new with 50 hours on it. The bias point is 62mA for all of them but I could not get over 340mV or 34mA with any of them. They sound fine though. The "adaptive" bias does not seem very adaptive to me.
Sorry I thought that only one tube was exhibiting this problem. Is this a new problem?
I think it's always been that way. I never checked it until I had the urge to try 6550/kt88 tubes and I thought I could set the bias higher basically to the Prologue 5 setting. So yes, the amp can work with a lot of other tube types but I don't see the point of trying them if those tubes are running on the cold side.
What is the stock tube for this unit?
Johns dad, stock tubes are Chinese EL34B rebranded.
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