Tube Variance

[Bluesbreaker]

I have two back up sets of KT66 tubes. Both are early 1960's and one pair are Gold Lions and the other are IEC/Mullard. Both sound great, by the way. I use the general rule of thumb and bias to the 70% range which is around 44mA. I do not have the proper scope equipment to bias the amp so I use the Bias King tool. I am reading numbers as opposed to waves.

The Gold Lions are biased to 44mA and when I put the Mullards in the bias jumps to 72mA. I adjust the bias down to the 44mA range and they sound fantastic. For curiosity I put the Gold Lions back in and the milliamps drop to 22mA. Now, it this a true variance between tubes or are the Gold Lions telling me they are near death.

Both sets of tubes biased properly sound incredibly good. Just curious about this rather large variance in the milliamp gap.

[demonufo]

All normal. Different tubes can have very different parameters which will in turn make them bias totally differently. Which is why you should bias every time you change tubes. Even same brands can be very inconsistent. I've had different batches of Mullard re-issues (Reflektor plant) that have behaved VERY differently to each other in the same amp. Around 25mA different, in fact, and one of the sets would not bias in range at all without altering the circuit.

[Bluesbreaker]

Thanks for the info.