quick question on bias readings

[2muchstuff]

I finnally got around to raising the bias on my amp and I noticed that some of the tubes are off by as much as 5 ma from eachother.Is this normal? if so do I just take the average of all four to get the reading im trying to achieve?

[hasserl]

This is common, even with matched tubes. Set the bias so the tube with the highest current is no more than 70% of max dissipation. From there you are free to set it lower, let your ears be the guide. Often amps sound just fine with the bias set to 60 - 65% of max dissipation, you don't HAVE to set it higher (internet gossip aside).

[donnyjaguar]

Sometimes the resistors feeding the screen grids are out of spec and this causes the imbalance. I'd be very disappointed if I put in a set of matched tubes and measured 5mA difference between them. After all, that's roughly equal to 10% tolerance which I'd expect of randomly selected tubes. In my experience, if a vendor offers sets of matched tubes, go for it. If you buy their unmatched tubes you're going to get some very unbalanced tubes methinks.

[2muchstuff]

thanks guys