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Post subject: Bias Question
Posted: Tue Jul 30, 2019 11:19 am
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Let me start by saying I am a complete novice when it comes to electronics so if my questions appear stupid it is because I do not know any better!

I have a Fender ’65 Deluxe Reverb Reissue and a few months ago I starting using a TAD attenuator with it so that I can get that great blackface breakup without damaging anyone’s hearing.

Then I got to thinking about biasing. I typically bias my output tubes at about 65% at idle but if I crank the amp when playing to, say, 7 on the volume then I guess the tube will be operating somewhat closer to the 14w that each tube is rated at. I have no idea what the percentage might be but for the sake of my question let’s say it is 85%.

So my question is; if I bias my amp hotter at idle, say, 75%, will I get the same tube performance, 85%, with the volume set lower than 7?

I often read that if tubes are biased too hot then they will have a shorter life. But will those same tubes also have a shortened life if I bias them at 65% and run the amp on a higher volume? Is there any difference between setting a "normal" bias and running the volume high then lowering the sound level with an attenuator compared to a hot bias and a lower volume setting?

Does that make sense? I hope someone will be able to enlighten me.


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Post subject: Re: Bias Question
Posted: Tue Jul 30, 2019 1:42 pm
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You don't need to bias at 75 % a Fender Amp. It is too hot IMO
Do ear test with guitar at less than 60 % or 50% bias.

Your question; running a amp with hot bias at low volume, are tubes will last same as running a amp a high volume with colder bias ?

Short answer ; IMO I think so.


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Post subject: Re: Bias Question
Posted: Sat Aug 03, 2019 2:16 am
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I now consider this matter closed.

I biased my tubes to approximately 80% just to see what they sounded like. I did not like what I heard so I won't be doing that again!


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Post subject: Re: Bias Question
Posted: Sat Aug 03, 2019 2:43 am
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65Blackface wrote:
I now consider this matter closed.

I biased my tubes to approximately 80% just to see what they sounded like. I did not like what I heard so I won't be doing that again!



At 80 % you will burn your tubes if you play loud.

On all Fender amps I fix, ( a lots ) sound is not better with more than 60 % bias, not to say 50 %


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Post subject: Re: Bias Question
Posted: Sun Aug 11, 2019 1:01 am
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Idle bias doesn't change with volume. If you set the bias at 65%, that's what it will be with volume all the way down, or all the way up.

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Post subject: Re: Bias Question
Posted: Sun Aug 11, 2019 1:34 am
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shimmilou wrote:
Idle bias doesn't change with volume. If you set the bias at 65%, that's what it will be with volume all the way down, or all the way up.



Bias will stay same but.....
if bias is at 70 % at idlle, at full volume tube may go to 124 %*
If bias is set a 50 % tubes may go to 90 % *at full volume.

* digit may not be right, it is only to give you an idea.

That is why we must set bias at 0 volume , no sound.
In other words if you set bias at 70 % when playing guitar loud.
With no sound, the right bias may read 10 % * :lol:


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Post subject: Re: Bias Question
Posted: Sun Aug 11, 2019 11:10 am
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Remember, bias is DC, but amp operation is AC. :wink:

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