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Post subject: BJR III EL84's questions
Posted: Sat Sep 19, 2015 2:58 pm
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Received my amp today w/the cannabis rex speaker, and so far so good. Now I don't plan on making any tube changes right away, but can't help but be curious about the GT's in there that have a '3' rating, which I gather is 'soft' and distorts quicker. Noticed the manual says you can go down to a '2' or up to a '4' rating without a re-bias. Now since I mainly uses fuzz and od pedals, I may somewhere down the line want to try some JJ's or Tungsol EL84's. So question is, what level hardness on the JJ's or Tungsols would I get to be in step with what GT '4' medium distortion rating gives? Or is it better to just change V1 to 5751 for later distortion purposes, and leave the power tubes as is? Thanks for any info.


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Post subject: Re: BJR III EL84's questions
Posted: Thu Sep 24, 2015 2:29 pm
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wt24 wrote:
Received my amp today w/the cannabis rex speaker, and so far so good. Now I don't plan on making any tube changes right away, but can't help but be curious about the GT's in there that have a '3' rating, which I gather is 'soft' and distorts quicker. Noticed the manual says you can go down to a '2' or up to a '4' rating without a re-bias. Now since I mainly uses fuzz and od pedals, I may somewhere down the line want to try some JJ's or Tungsol EL84's. So
question is, what level hardness on the JJ's or Tungsols would I get to be in step with what GT '4' medium distortion rating gives? Or is it better to just change V1 to 5751 for later distortion purposes, and leave the power tubes as is? Thanks for any info.


Anyone?


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Post subject: Re: BJR III EL84's questions
Posted: Thu Sep 24, 2015 5:16 pm
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My 2c.

GT's rating system applies to GT branded tubes. GT rates the tubes so that you don't have to worry about biasing as long as you stay within the same rating class. Other branded tubes definition of "early" to "medium" and "late" distortion will differ. It is best to have the amp biased when replacing power tubes, unless you stick with GT brand and same rating.

If you visit the GT website, they explain which if their tubes are rebranded JJ's, Sovteks, Chinese, etc. That way you can get the JJ version or the tungsol version but with GT's rating system.

On another note, maybe give the 5751 a shot. At least you don't have to re bias, and it's always a good idea to have a spare set of tubes for troubleshooting.

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Post subject: Re: BJR III EL84's questions
Posted: Fri Sep 25, 2015 8:04 am
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socal323 wrote:
My 2c.

GT's rating system applies to GT branded tubes. GT rates the tubes so that you don't have to worry about biasing as long as you stay within the same rating class. Other branded tubes definition of "early" to "medium" and "late" distortion will differ. It is best to have the amp biased when replacing power tubes, unless you stick with GT brand and same rating.

If you visit the GT website, they explain which if their tubes are rebranded JJ's, Sovteks, Chinese, etc. That way you can get the JJ version or the tungsol version but with GT's rating system.

On another note, maybe give the 5751 a shot. At least you don't have to re bias, and it's always a good idea to have a spare set of tubes for troubleshooting.


Thanks for the lone reply. So how does anyone, including a qualified tech, rebias the amp without there being a bias pot inside to adjust?


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Post subject: Re: BJR III EL84's questions
Posted: Fri Sep 25, 2015 9:58 am
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E-mail Eurotubes with that question, they will give you advice free. They sell JJ's exclusively I think, but they have steered me very well over the years.

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Post subject: Re: BJR III EL84's questions
Posted: Fri Sep 25, 2015 5:58 pm
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wt24 wrote:
socal323 wrote:
My 2c.

GT's rating system applies to GT branded tubes. GT rates the tubes so that you don't have to worry about biasing as long as you stay within the same rating class. Other branded tubes definition of "early" to "medium" and "late" distortion will differ. It is best to have the amp biased when replacing power tubes, unless you stick with GT brand and same rating.

If you visit the GT website, they explain which if their tubes are rebranded JJ's, Sovteks, Chinese, etc. That way you can get the JJ version or the tungsol version but with GT's rating system.

On another note, maybe give the 5751 a shot. At least you don't have to re bias, and it's always a good idea to have a spare set of tubes for troubleshooting.


Thanks for the lone reply. So how does anyone, including a qualified tech, rebias the amp without there being a bias pot inside to adjust?


I haven't had a chance to look at the schematic but If the amp utilizes cathode biasing then it's just a matter of swapping in a new set of tubes. The amp will self bias. If it's fixed bias and there's not pot to be found, a qualified tech would have to replace the tubes, check values to see how the new tubes are biased and if needed replace the bias resistor with the appropriate value to set it to the correct bias.

Hope this helped.

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Post subject: Re: BJR III EL84's questions
Posted: Sat Sep 26, 2015 1:05 pm
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The stock BJr is fixed bias, with no bias pot. A resistor in the bias supply can be changed to adjust the bias, and even better would be to change that resistor to a pot so the bias can be easily adjusted. When changing output tubes in a fixed bias amp, the bias should be checked/adjusted. Changing preamp and PI tubes requires no bias adjust.

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Post subject: Re: BJR III EL84's questions
Posted: Wed Sep 30, 2015 6:41 am
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Power tubes in Blues Junior's run very hot at around 40-45mA you can a get cooler grade of tubes from Eurotubes but I found they still are up there in current, I changed out a resistor in my BJ III to get my bias to the normal 70% idle level which is roughly around 25mA, I've done quite a few mods to my amp to have it run as clean and loud as I can get it since I only use pedals for my dirt.

I would highly recommend the Bias mod if you want longer power tube life and less power tube break up!


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Post subject: Re: BJR III EL84's questions
Posted: Thu Oct 01, 2015 2:53 am
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J.LaGrassa wrote:
...I would highly recommend the Bias mod...


Absolutely! It isn't extremely difficult, and well worth it.

I put a bias pot in mine:
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Post subject: Re: BJR III EL84's questions
Posted: Wed Nov 04, 2015 4:42 pm
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Just to further muddy the waters, Groove Tubes is now a wholly owned subsidiary of Fender. So you could drop them a line and ask them which GT's would work in your BJ.


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