Amp bias question

[Diego Rugiero]

Hi guys, I was checking the current bias in my ENGL Powerball and I'd find something strange (al least for me): Three tubes reads 28 mA and one tube reads 32 mA. I'd buy this tubes as a matched quad six months ago (Winged C tubes)

I'm testing using the Eurotubes bias probe.

I know 4 mA is an acceptable difference, but I can't understand why only one tube is mismatched and higher that the others. The four tubes were matched good when I'd put them.

 

[Diego Rugiero]

If anyone is interested, I checked this with an audio tech and it's not a big deal. He said that 5 mA (and even 10 mA in some cases) is an acceptable difference between power amp tubes.

 

[53Crëw]

I know 4 mA is an acceptable difference, but I can't understand why only one tube is mismatched and higher that the others. The four tubes were matched good when I'd put them.

As tubes "burn in", they can drift a little. That's why many of the good tube suppliers will burn them in first, then measure them to match them up in a matched set. Even if they are a burned in, and matched as a set, they can still drift a bit over time. 4 mA should not be a big deal. Generally, you can set the bias based on the hottest running tube to be sure you're in a safe operating range for the set.

It may also be that they had three tubes that matched perfectly, and then just grabbed the next closest tube to complete the set. They don't always have four that are exactly perfectly matched.

You should be good.

Cheers.

 

[Diego Rugiero]

Thanks for the info!