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Post subject: Amp biasing
Posted: Thu Nov 22, 2012 6:24 pm
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I have a Blues Junior about 4 years old and have recently swapped out the power tubes, can anyone tell me if this amp needs to be biased?


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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 6:38 pm
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Welcome......Can i also ask........Do you do whatever a spider can??? :P

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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 6:51 pm
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Spidey, amp's bias should be checked, when you replace the output tubes. Too cold bias will hurt the tone. Too hot bias (esp "red-plating") can damage the tubes and amp.

Have a good one!


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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 7:17 pm
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Spiderman wrote:
I have a Blues Junior about 4 years old and have recently swapped out the power tubes, can anyone tell me if this amp needs to be biased?


Note that the BJr has fixed bias, but the bias is not adjustible unless a bias pot is added (or you change resistor values). BillM sells kits to add a bias pot with instructions on how to bias the amp:

http://billmaudio.com/wp/?page_id=1175

http://billmaudio.com/wp/?page_id=1155

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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 7:55 pm
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Thanks for the info you guys very helpful to me. If I put the same power tubes in as before when my tone was good will the bias still need to be checked? I mean buy the same ones that were in it when I first got the amp, or is there that much variation potential anyway?


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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 8:25 pm
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I would check the bias, everytime I replace the output tubes. Just because it's the same manufacturer, same era made, even same lot number --- doesn't mean the idle current draw is the same.

In GENERAL, old school US or Western Euro made NOS output tubes were built to a tighter tolerance. And they tended to be more forgiving to electrical variants. Today's new tubes tend to range all over the place. UNLESS you buy from a dealer that carefully tests and selects them under amp-like conditions. And separates them by idle bias mA numbers.

Then, if you know your old tube's original mA --- you can select a pair with numbers close to the old tubes' original numbers. You can't measure the old tube's idle bias, now. They've worn out and are not conducting current, like when they were new.

Good luck! And welcome to the Forum. :mrgreen:


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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 9:45 pm
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BMW2002Ti wrote:
I would check the bias, everytime I replace the output tubes. Just because it's the same manufacturer, same era made, even same lot number --- doesn't mean the idle current draw is the same.

In GENERAL, old school US or Western Euro made NOS output tubes were built to a tighter tolerance. And they tended to be more forgiving to electrical variants. Today's new tubes tend to range all over the place. UNLESS you buy from a dealer that carefully tests and selects them under amp-like conditions. And separates them by idle bias mA numbers.

Then, if you know your old tube's original mA --- you can select a pair with numbers close to the old tubes' original numbers. You can't measure the old tube's idle bias, now. They've worn out and are not conducting current, like when they were new.

Good luck! And welcome to the Forum. :mrgreen:


Only problem is, there is no way to easily check the bias of a BJr without either modification to the amp or using a bias probe.

http://support.fender.com/schematics/gu ... ematic.pdf

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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 10:02 pm
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Again thanx guys and ya I kinda figured that second part out after reading more on my specific amp and modding it sounds like my next step. What is a bias probe and does that just give you a reading that you can't change anyway, like just testing the bias?


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Post subject: Re: Amp biasing
Posted: Thu Nov 22, 2012 10:28 pm
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Spiderman wrote:
Again thanx guys and ya I kinda figured that second part out after reading more on my specific amp and modding it sounds like my next step. What is a bias probe and does that just give you a reading that you can't change anyway, like just testing the bias?


A bias probe is a set of sockets that installs between the power tube sockets and the power tubes themselves. It is used with a digital voltmeter to measure cathode current and plate voltage so that the proper bias point can be determined. Since the BJr uses EL84 power tubes, you would need a probe like this:

http://www.amp-head.com/product_info.ph ... ucts_id=71

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Post subject: Re: Amp biasing
Posted: Fri Nov 23, 2012 2:02 pm
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Bill, i think i should say that this is about a BDRI not a blues jr. big difference. 6l6 vs el84. the word BLUES seems to be the variable here.


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Post subject: Re: Amp biasing
Posted: Fri Nov 23, 2012 3:10 pm
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blackstrat71 wrote:
Bill, i think i should say that this is about a BDRI not a blues jr. big difference. 6l6 vs el84. the word BLUES seems to be the variable here.


Ah, no.

Spiderman wrote:
I have a Blues Junior about 4 years old and have recently swapped out the power tubes, can anyone tell me if this amp needs to be biased?

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Post subject: Re: Amp biasing
Posted: Fri Nov 23, 2012 8:33 pm
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With most amps with two output tubes, you can check the Plate current of each output tube fairly easily without a bias probe. With amp off and caps discharged, measure the resistance of one or both halves of the OT primary (red to brown, and/or red to blue), in the BJr it should be approx 100 ohms on both halves. Once you know the resistance of the OT primary halves, turn the amp on and measure the DC voltage across one or both halves of the OT primary, and Ohm's Law will give you the Plate current, I = E/R.

In the event of unbalanced OT primary halves, such as is the case with the HRDlx, just match each half of the OT primary with the corresponding output tube to get the current for that particular tube.

For the BJr, the bias is quite hot from the factory, and if your original output tubes didn't red-Plate, and they lasted 4 years, you'll likely be fine if you just replace them with the same type and color/number rating.

I will third the recommendation to add an adjustable bias to the BJr, although many use the amp as-is and it works fine.

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Post subject: Re: Amp biasing
Posted: Sat Nov 24, 2012 6:49 am
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sorry guys, i apploigize i thought i was on bluesy's amp mod thread. my bad. too much turkey and booze i guess.


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Post subject: Re: Amp biasing
Posted: Sat Nov 24, 2012 8:25 am
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hey spidey~
listen to shimm


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Post subject: Re: Amp biasing
Posted: Sat Nov 24, 2012 10:52 am
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blackstrat71 wrote:
sorry guys, i apploigize i thought i was on bluesy's amp mod thread. my bad. too much turkey and booze i guess.


:lol: :lol: :lol:

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