Can bias too high current cause cracking noise?

[Alan0354]

I just read a post about the amp having loud cracking noise and OP found out the bias is too hot. He readjust the bias and claimed the problem went away.

My guess is 6V6 that supposed to be 22mA, but it was bias to 33mA instead. I just don't believe it's the bias current that cause the cracking. What do you think.

[soundguruman]

The cracking noise ( cross over distortion in audio) usually happens when the bias is too cold.
The bias too hot will make the plate turn red.
But bias too hot will also damage the tubes, or cause unusual noises, so it is entirely possible that this post was correct.
Yes, overheating will sometimes cause unexpected noises in the tube for sure, or it will destroy the tube, etc...

[Chuck H]

+1
SGM is correct. Cracking noises could certainly be a consequence of tubes working too hard. Just because there's no electronic reason for it doesn't mean there may not be a physical reason.

Going 100mph in your car doesn't kill you either. But even if everything about your car is rated to accept that speed, if you hear a cracking noise it's best to slow down.

[überfuzz]

I've seen a lot of Fender amp with high bias. The heat has often made me nervous, hot PT etc. However, I don't know if I ever come across a tube going south as a result of this high baising. Have you ruled out all other stuff that might be going on in you amp..?

[Alan0354]

I just read the thread and he claimed that fixed the problem. 6V6 normally biased at about 25mA, going to about 33mA is not the end of the world!! I am just puzzled and put it here to hear what's you opinion.

So you guys encountered this before?

[Jazz P Bass]

If I had an amp that crackled & then didn't after lowering the bias, I would be looking at the tubes themselves.
Or the OPT.

Running the amp flat out is a lot more stressfull than bias is.

[Chuck H]

Certainly! But running the amp flat out with a hot bias is worse still. The bias point not only affects the idle current but also amount of time the tubes will be in cutoff vs. saturation. If the tubes don't spent enough time in cutoff to limit their net power when clipping they can more easily over dissipate.

[Alan0354]

So it is possible to have cracking sound when biased too hot, but nobody seen it yet? That's all I want to verify as you guys are all very experience amp people.

[Chuck H]

I've heard amps make cracking sounds many times with no electronic reason for it that can be derived from a tube chart. Determining exactly why that amp made cracking sounds with a hotter than normal bias may well be impossible. Trying to determine the cause based on one slim description of a symptom and one noted adjustment that abated it is futile. Nothing about what happened exactly or why exactly it happened can be definitively concluded with the information provided. It's not even certain that the hotter bias was the reason for the cracking noise. Only that a cooler bias stopped it. With so many interactive operating parameters it's short sighted to make that assumption. Example:

Observation - "I touched a stove element and it burned me"

Conclusion - "Stove elements burn me if I touch them"

WRONG! WRONG! WRONG!

Touching a stove element doesn't burn anything. It's heat that burns things. And stove elements are quite safe to handle most of the time.

[Alan0354]

Yes, I know the analogy, but I just want to know whether people encountered this before.

I believe it's either something intermittent in the bias circuit or a defect tube. If everything is normal, hot bias will not make cracking sound.

[Tom Phillips]

...If everything is normal, hot bias will not make cracking sound.

I agree. Especially given the fact that you are comparing 25mA vs 32mA. As always, the operating point change could toss something else over the line. However, there is no specific or common observation that this slight bias difference would directly cause a crackling sound.

[soundguruman]

So it is possible to have cracking sound when biased too hot, but nobody seen it yet? That's all I want to verify as you guys are all very experience amp people.

Biased too hot
could be crackling, hum, popping, or the tube just shorts out and blows a fuse...depends on "how" hot it gets.

A bad speaker cable can make the output tubes turn red too. Might fool you into thinking that the bias is wrong.
So, verify that the output speaker load is correct first, before taking corrective actions.

[Alan0354]

Yes, I know other things can happen. I just want to concentrate that if everything else works normal, tube is good, 1.5 times the recommended bias will not cause cracking. Something else must be wrong.

[soundguruman]

1.5X recommended bias would probably turn the plate red, short the tube out.
You can be sure if the bias is that far off, that there will be the possibility of noises, as the internal grids meet meltdown velocity, rendering the entire unit fubar...

However for a brief moment, it will sound great in a blaze of glory... then upchuck.

[überfuzz]

...Something else must be wrong.

If energy and time is right anything can happen. /Heisenberg