Swapping Power Tubes W/out Bias

[trower]

What Are The Dangers Of Swapping Out Power Tubes In An Amp On The Fly'?.... Is It O.k To Do A Ruff Comparison On Quads Of El34 Type Tubes For A Few Minutes In Say.. A Marshall And Then Lug It Over To A Tech To Bias?

[brad347]

I do this sometimes... if using quality tubes they are often fairly close, not always. Listen for crossover distortion (a sign that the tubes are biased too 'cold' and won't give you an accurate assessment of what they sound like) and especially watch for signs that the tubes are getting too hot, if the plates start glowing, especially a deep red, then you're in trouble and you should kill power to the tubes immediately. Try to keep it at sane volumes as well.

Putting in new tubes without bias there is always a danger of putting premature wear on the tubes.

[donnyjaguar]

I'd be interested to know if anyone has every destroyed their amplifier by doing this. I've never managed to melt down a transformer myself - although I have killed tubes by pumping too much idle current through them.

[brad347]

I think you could conceivably kill tubes but not transformers... except MAYBE if you had the wrong fuse inserted... others correct me if i'm wrong.

[devbro]

What does "crossover distortion" sound like?

[brad347]

crossover distortion occurs in a class a/b amp when one tube stops "pushing" before the other starts "pulling" if you know what I mean... it basically sounds like a terrible, unmusical distortion that happens at lower volumes than distortion should... you will recognize it by how terrible it sounds. That's basically the simplest way I can put it.

To add to what john phillips said, some amps and tube types are more likely to be biased at the upper limit than others. For example, lots of amps run 6V6s pretty "hot."

[Blue Strat]

The bigger risk is in not doing a fair comparison between the tubes you're comparing. Listening to one set biased way cold and the other dead on (or hot) is not even a valid comparison, hence a waste of time and potentially dangerous to the tubes.

Here's an idea, get a bias probe. You should have one anyway. No more trips to the tech for things you can do yourself.

[brad347]

i need to get one of those bias probes!

[Old Tele man]

re: "...What does "crossover distortion" sound like?"

...a mechanical analogy: it's the "bumpy spot" in the middle of a teeter-toter ride caused by "worn-out bearings" when one side "hands-off" to the other side going up and down.

[trower]

The bigger risk is in not doing a fair comparison between the tubes you're comparing. Listening to one set biased way cold and the other dead on (or hot) is not even a valid comparison, hence a waste of time and potentially dangerous to the tubes.

Here's an idea, get a bias probe. You should have one anyway. No more trips to the tech for things you can do yourself.

YEA, A most valid point, Would you believe I just got an e-mail from a fellow gearpager offering to loan me his Bias probe! I'v said it before and I'll say it again..THIS PLACE IS THE BEST! best people! best advice!!