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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 6:56 am
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"...I have blown 2 rounds of output tubes..." I won't judge you, maybe you needed the money, ho! JK! :lol:

What do you mean by "blown"? You''ll have to be more specific, but more importantly you have a five year warranty so you should probably use it. What exactly was wrong with your tubes?

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 7:18 am
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On top of Shimmilou advices, did you rebias after changing power valves?

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 7:32 am
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I doubt that the shop adjusted the fixed bias in the BJr (no bias pot), they probably just changed the tubes. Any decent store that is open on Sunday should have tubes, like GC. But, you really don't know if the tubes are bad or not, or what may have caused the tubes to go out. So, I would take it back and have them check to see just how hot the tubes are running (bias), and see if it is indeed the tubes again, and if it is the tubes, why are they going bad. :idea:

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 7:48 am
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Yes, if the tubes are running very hot it can lead to premature failure. The BJrs are known to run very hot from the factory (some more than others), unfortunately having biasing tools will not help as the BJr has no adjustment. You would have to either change a resistor value, or better yet install a bias pot, in order to change the setting. Of course if you do either, it will almost certainly void your warranty. If you don't care about the warranty, and are handy with a soldering iron and drill, it isn't extremely difficult to modify the amp to install an adjustable bias. I put one in my BJr with more than three years left on the warranty, but didn't need any warranty as the adjustable bias allowed me to run the tubes at a more reasonable bias, preempting many possible heat related issues, as well as making the amp much more versatile. :idea:

Here is a good site with lots of great info, from one of the leading authorities on all things BJr. :)

http://billmaudio.com/wp/

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 10:53 am
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I've been told the Blues Jr is fixed bias so you don't need to rebias when changing the EL84's, but then I keep hearing all this talk about people installing bias pots on their Blues Jr's. What am I not understanding here?? Why are they doing that if they don't need to rebias when changing tubes? I'm guessing people are doing it to just cool the bias in an effort to make their tubes last longer?

Also, unless you're a die hard JJ EL84 fan the GT EL84-R's that come stock in the blues Jr are not bad at all. I've grown to like them quite a bit! They're supposed to have unreal endurance and I recently found out my buddy has had them in his Marshall combo amp for 6 years straight and they still sound like they're in their prime.


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 11:08 am
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Rossfloss wrote:
I've been told the Blues Jr is fixed bias so you don't need to rebias when changing the EL84's, but then I keep hearing all this talk about people installing bias pots on their Blues Jr's. What am I not understanding here?? Why are they doing that if they don't need to rebias when changing tubes? I'm guessing people are doing it to just cool the bias in an effort to make their tubes last longer?

Also, unless you're a die hard JJ EL84 fan the GT EL84-R's that come stock in the blues Jr are not bad at all. I've grown to like them quite a bit! They're supposed to have unreal endurance and I recently found out my buddy has had them in his Marshall combo amp for 6 years straight and they still sound like they're in their prime.


You are misunderstanding fixed bias and adjustable bias. Fixed bias just means a negative voltage is applied to the grid to bias the tubes. Some fixed bias amps have pots to adjust the bias and some do not.

Cathode biased amps are what some call plug and play. You should be able to swap tubes without rebias as long as the tubes are balanced and not too extreme in their pulling of current cold or hot.


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 3:14 pm
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I'm wondering also, especially if it is the power tubes or a bad tech, or... Let us know what they find.

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 4:51 pm
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jazzgene wrote:
Rossfloss wrote:
I've been told the Blues Jr is fixed bias so you don't need to rebias when changing the EL84's, but then I keep hearing all this talk about people installing bias pots on their Blues Jr's. What am I not understanding here?? Why are they doing that if they don't need to rebias when changing tubes? I'm guessing people are doing it to just cool the bias in an effort to make their tubes last longer?

Also, unless you're a die hard JJ EL84 fan the GT EL84-R's that come stock in the blues Jr are not bad at all. I've grown to like them quite a bit! They're supposed to have unreal endurance and I recently found out my buddy has had them in his Marshall combo amp for 6 years straight and they still sound like they're in their prime.


You are misunderstanding fixed bias and adjustable bias. Fixed bias just means a negative voltage is applied to the grid to bias the tubes. Some fixed bias amps have pots to adjust the bias and some do not.

Cathode biased amps are what some call plug and play. You should be able to swap tubes without rebias as long as the tubes are balanced and not too extreme in their pulling of current cold or hot.


+1 you still need to check/adjust the bias in a fixed bias amp.

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 5:24 pm
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tele_head wrote:
Thanks guys for your feedback on understanding bias with this amp. I'm still scratching my head on why I would seemingly continue to burn through power tubes.??


You shouldn't unless there is something really wrong with your amp. Turn down the lights and look at the power tubes. Are they glowing bright red?


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 7:33 pm
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Fixed bias simply means that there is no user adjustment for the bias, it is a set or fixed value. Fixed bias amps can be either Cathode-biased or grid-biased with no adjustable pot. Cathode-biased amps are considered to be "self adjusting bias" because a higher current draw by the tube will lower the Plate voltage keeping the idle wattage at relatively the same value for all tubes. Grid biased amps will allow each set of tubes to draw a different amount of idle current at the same bias setting, hence the need for an adjustment pot to set the idle to a proper level for the particular tube set used (PITA to change out a bias resistor whenever you try different tubes, a pot is much easier to adjust). The BJr is a fixed, grid-biased amp. And I agree, the bias should be set properly for each set of output tubes, whatever configuration that the amp has. :idea:

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 8:33 pm
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shimmilou wrote:
Fixed bias simply means that there is no user adjustment for the bias, it is a set or fixed value. Fixed bias amps can be either Cathode-biased or grid-biased with no adjustable pot. Cathode-biased amps are considered to be "self adjusting bias" because a higher current draw by the tube will lower the Plate voltage keeping the idle wattage at relatively the same value for all tubes. Grid biased amps will allow each set of tubes to draw a different amount of idle current at the same bias setting, hence the need for an adjustment pot to set the idle to a proper level for the particular tube set used (PITA to change out a bias resistor whenever you try different tubes, a pot is much easier to adjust). The BJr is a fixed, grid-biased amp. And I agree, the bias should be set properly for each set of output tubes, whatever configuration that the amp has. :idea:


Please don't take this the wrong way but in tube amp parlance, fixed bias equals negative grid bias. Some fixed bias amps do have provisions for moving this negative voltage up or down. Other fixed bias amps have a resistor that sets the bias and can't be adjusted without changing this resistor value. Basically, fixed bias means a tube is biased using DC voltage, which is a negative voltage applied to the grid of the tube with repect to the cathode. In a cathode biased amp, a resistor is placed between cathode and ground and the grid of the tube is referenced to ground. The voltage drop across the cathode resistor will make the cathode more positive in respect to the grid. Adjustment trimmers have nothing to do with the the bias methods being called fixed or cathode bias.


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 8:55 pm
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jazzgene wrote:

Please don't take this the wrong way but in tube amp parlance, fixed bias equals negative grid bias. Some fixed bias amps do have provisions for moving this negative voltage up or down. Other fixed bias amps have a resistor that sets the bias and can't be adjusted without changing this resistor value. Basically, fixed bias means a tube is biased using DC voltage, which is a negative voltage applied to the grid of the tube with repect to the cathode. In a cathode biased amp, a resistor is placed between cathode and ground and the grid of the tube is referenced to ground. The voltage drop across the cathode resistor will make the cathode more positive in respect to the grid. Adjustment trimmers have nothing to do with the the bias methods being called fixed or cathode bias.


This is correct. I was straightened out on this on another thread. Hit Aiken Amps site and he explains it.

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Last edited by rjake on Sun May 22, 2011 8:56 pm, edited 1 time in total.

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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Sun May 22, 2011 8:55 pm
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:lol: OK, you are correct, I'll stick to calling it a grid-biased or Cathode biased then. I should have said that I call it a fixed bias if there is no adjustment pot. :idea:

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Post subject: Re: Blues Jr iii consistently blowing power tubes
Posted: Sun May 22, 2011 10:58 pm
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jazzgene wrote:
You are misunderstanding fixed bias and adjustable bias. Fixed bias just means a negative voltage is applied to the grid to bias the tubes. Some fixed bias amps have pots to adjust the bias and some do not.

Cathode biased amps are what some call plug and play. You should be able to swap tubes without rebias as long as the tubes are balanced and not too extreme in their pulling of current cold or hot.



Thanks man!


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Post subject: Re: Blues Jr iii consistently blowing power tubes.
Posted: Mon May 23, 2011 3:25 am
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tele head ,

You must check and reajust you bias BEFORE using your knew power tubes. Sure it's wrong, driving tubes very very hot.

Blues Junior do not have adjustment potentiometer so I'm pretty shure the shop you bring your amp are not able to do bias adjustment . This need to replace a/ or some resistors.


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