#1
Three Question Quiz: Two short answer and one Open Ended.
1. When you are biasing, which direction is dangerous to go in? I understand that if you bias too "cold" the amp will sound distorted, but it isnt harmful, but if you bias too "hot" the amp's tubes could die within seconds. Which direction is which? Raising the bias would be getting hotter or what?
2. On certain amps that have a bias test point, such as the Peaveys or Bugeras, I believe the test point reading when you insert a multimeter is inaccurate and it is better to use a bias probe on each tube. Is there any method to calculate or make up for the error in the test point?
3. In theory, could you bias an amp pretty "cold", so that it sounds lousy but will not be damaged, and then slowly raise/lower (refer back to question 1) the bias until the sound cleans up and the tubes aren't too hot, and you will have properly biased your amp?

Go! and thank you.
#2
1. Biasing upwards is hotter.

2. Your premise is wrong. Why would the test point be inaccurate? It's the same thing as the bias probe, just not, you know, a pain in the ass.
There are certainly methods for calculating error but if you're so concerned, replace the test point resistor with a 1% resistor instead of the 10% that's probably in there. It doesn't matter anyway, but if it will make you feel better...sure. Go for it.

3. Yes, but it's considered bad form to bias by ear only since you could A) be way low, and not be getting the best tone you could be, even if it's acceptable or B) have different tubes and be running them too hot since your ears are very bad at detecting current.
#3
Well first off, what amp are you biasing? We could give you a better answer fi we knew this.

The bias voltage is a negative voltage, with respect to ground. So if you put the probes in the proper testpoins on your amp you should see something like -42 volts. The more negative, the less gain. The less negative, the more gain. Making it less negative is more dangerous to your tubes.
#4
Quote by Watterboy
Three Question Quiz: Two short answer and one Open Ended.
1. When you are biasing, which direction is dangerous to go in? I understand that if you bias too "cold" the amp will sound distorted, but it isnt harmful, but if you bias too "hot" the amp's tubes could die within seconds. Which direction is which? Raising the bias would be getting hotter or what?
2. On certain amps that have a bias test point, such as the Peaveys or Bugeras, I believe the test point reading when you insert a multimeter is inaccurate and it is better to use a bias probe on each tube. Is there any method to calculate or make up for the error in the test point?
3. In theory, could you bias an amp pretty "cold", so that it sounds lousy but will not be damaged, and then slowly raise/lower (refer back to question 1) the bias until the sound cleans up and the tubes aren't too hot, and you will have properly biased your amp?

Go! and thank you.


1) You got it spot on, bias too hot and you're dissipating too much heat and will probably fry the tubes, bias too cold and you get crossover distortion. Most people set the idle current to 70% of the tube's maximum dissipation.
2) I don't see how having test points is any less accurate than using a test probe either way you're using a multimeter and either measuring the plate current directly or measuring the voltage across a resistor going from the cathode to ground, I guess some probes allow you to measure plate voltage directly so you can get the optimal range to set your bias, usually amp manuals will just tell you to bias to _ _ mA, which is really not a particularly accurate method. Test points don't allow you to measure plate voltage.
3) Some people say they can bias by ear, but I prefer to check by actually measuring. I don't have enough money to risk blowing a set of tubes just because I was too lazy to hook up a multimeter and read a number.
#5
Roc; Well done. Thank you for all of your excellent answers. I'll keep all that info in mind. End-citizen; I'm biasing a bugera 333. And just to get things straight, since biasing upwards means I'm getting hotter and is more dangerous, that would also mean I'm approaching closer to 0 or going past that, since that would be getting less negative? And al; Thank you also for the input. I guess I stated my question wrong. It isnt that it is inaccurate but it doesnt measure plate voltage (although I believe I can get it from external sources). So what I'm confused about is if I know approximately what I am supposed to bias to for my tubes, how do I know if I'm correctly biasing to that amount of V through the test point?
Last edited by Watterboy at Jan 4, 2009,
#6
one other question.. if you are biased too hot, is there any quick way of knowing you are before your tubes burn out? Namely, will they immediately glow very brightly or give off an odor?
#7
they might glow very britghly, you may have very little headroom, you may smell burning, you may even see smoke.
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#8
Plate voltage can differ from amp to amp, but generally tube amp makes give you a conservative range so you're always biased slightly colder than what most people consider... "optimal," for lack of a better word. Basically you want to bias as hot as you can while staying in a safe operating range. And yes, when you're measuring the bias voltage, the less negative, the hotter bias is going to be.

You can tell when you're biased too hot if the tubes start to redplate (the gray plates in the center will start to glow orange), but it's not a good indicator because if your tubes are redplating almost immediately they're biased WAY too hot. And once tubes start redplating, they're pretty much on their way to being shot. New production tubes aren't so robust. It's a slippery slope once redplating starts.

If you want to know approximately what to set your bias to, you need to measure your plate voltage and you need to know the maximum plate dissipation of the tubes you're biasing. Adjust the bias so that the idle current*plate voltage does not exceed 70% of the maximum plate dissipation.
Last edited by al112987 at Jan 4, 2009,
#9
alright so... if the value i need to bias it to through the bias test point is the idle current then: idle current = (.7 x maximum plate dissipation) / (plate voltage)

the plate voltage for the amp is about 480 v.
I dont know what the max plate dissip. is for the tubes, but they are 6L6 stock tubes in the 333. Any idea what it might be?
#11
Neato so its an approximation but.. my current should be: current = (.7 x 30watts) / (.48 mV) = 43.75 watts / mV + or - 4.375 for error.

So basically, when biasing through the bias test, I should aim for about 44 w/mv then?
#13
Okay, sounds fantastic then! Al you get an A. The highest grade in the class. *pats on head*