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Bias Blues

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Hi Jeff,
I just read your article in the February 2007 issue of Premier Guitar about swapping out 6L6s with EL34s in a Crate amp and what goes into re-biasing. I wonder if you might be so good as to help answer another biasrelated question.

I own a Framus Cobra 100-watt head which has Electro-Harmonix 12AX7 preamp and EL34 output tubes. I read online that it ships from the factory with a “cold” bias setting of around 25mA and should be taken in to an amp tech to “heat it up” to around 33mA to minimize “crossover distortion and clean up some of the buzziness.”

I then took the amp to a repair shop. When I got it back, the tech said it was true that the bias was cold and didn’t have a “clean” wave on his oscilloscope. He ended up “warming” the amp up to around 32mA and said that the amp should now have a lot less distortion. That took me by surprise because the Cobra is supposed to be a high-gain amp, not low-gain. Ever since, the amp has had a good hard rock sound on channel 2, but not a really good metal sound on channels 2 or 3.

I understand that a clean sine wave on an oscilloscope indicates little or no distortion of the input signal, and you say in your article that you take into account the power output under load and the overall sound, in addition to the main bias current through the output tubes, when biasing amps to find the best balance. Is it possible that the guy who adjusted my amp didn’t take into account that I was looking for high-gain performance and instead cleaned up the amp as much as he could? If so, what kinds of things should I ask an amp tech in order to get better quality high-gain performance?

Thanks for any help,
Rene


Hi Rene,
First, let me thank you for reading the column. It’s fun to write and I hope informative to all who read it. Now, let’s see if I can shed some light on your particular situation. To distort or not to distort, that is only part of the question! We also need to know what kind of distortion and where it should take place. To start things off, consider this hypothetical question.

The information that you read online is:
A) probably correct
B) not a bad thing to do
C) maybe not right for you


Okay, it’s a trick question. The correct answer is “all of the above.”

The amp may come with a slightly cold bias and may need to be warmed up for players that wish to extract better clean, bluesy or classic rock tones, but that doesn’t seem to be your particular focus. Since you mentioned that it’s supposed to be a high-gain amp, it may have purposely been designed and set up with what is perceived by some as a cold bias. I’ll try to explain why, but first, I’d like to point out the difference between gain and distortion in very broad strokes.

Ask Amp Man Gain relates to the amount that a signal level is increased. Most of the time, most of the gain in an amplifier is developed in the first preamp stages. Too much gain in a preceding stage can cause a subsequent stage to distort. Early on in guitar amplifier design this was considered to be a bad thing – now we know better. Anyway, when a guitar amplifier is considered to be “high-gain,” the term refers to the amount of gain and distortion created in the preamp section of the amp. It really has nothing to do with the inherent distortion characteristics of the output stage. So when your tech said that the output stage would now have less distortion, he was not referring to, nor did he lower, the front-end gain or change the front-end characteristics of the amp. If you could have listened to the characteristics of the preamp section in the amp prior to and after the bias adjustment, they would have been virtually identical.

The effect you are now noticing is a result of the way the output stage is handling the preamp signal. While the 32mA measurement for the idle bias current is a pretty average setting, it shouldn’t be considered the correct or perfect setting – there is no such thing! It is a setting that would achieve less crossover distortion in the output stage and more faithfully reproduce the signal that was being sent to it by the preamp. This, as you have found, is a good characteristic when it comes to some uses of the amp but not for others. I’ve found that sometimes a colder bias on the output stage is better for players that focus mostly on very aggressive, high-gain styles of music. The additional crossover distortion developed in the output stage seems to add a little teeth to the sound. This may be the very reason that your amp ships with a cold bias. It is has been optimized for its target market – not all amps can be all things to all players.

If you want the characteristics of your amp to shine in the high-gain mode, have your tech readjust the amp back to its original bias setting. If, however, you can no longer live without the cool hard rock sound that the amp currently has, you could always add a stompbox for the over-the-top metal tones.

Now, go put the fangs back in your Cobra.






Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
jeffb@budda.com
www.budda.com
©2007 Jeff Bober
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