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Post subject: Pro Junior: Rebias EL84s??
Posted: Wed Jan 27, 2010 1:24 am
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I've had this little amp for 3 years and I LOVE it! :D

In case I will ever need new power tubes,
do EL84s need to be rebiased when installed???
Or can I simply replace the original power tubes with two new EL84s? :roll:


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Posted: Wed Jan 27, 2010 1:32 am
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Hey Thinline.

First of all welcome to the forum, just like Hotel California it's a lovely place!

Secondly yes you should re-bias your amp after any power tube change. Groove Tubes have that colour coding thing but to me its all bull5h1t plus GT's sound horrible.

Get yourself some JJ's, give her a re-bias and enjoy my friend.

CC

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Posted: Wed Jan 27, 2010 2:11 am
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Thanks CC!

The thing is, there are different opinions out there...
Can you explain why they need to be rebiased?

I'm quite happy with the Groove Tubes, but maybe I should try the JJs.


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Posted: Wed Jan 27, 2010 9:18 am
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Doesnt the pro jr have fixed bias like the blues jr?

If thats the case just by a matched pair of power valves and shove em in.

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Posted: Wed Jan 27, 2010 9:28 am
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You should still check the bias. I agree the GT coding is pure BS.
I also agree with the JJ's too. Just because the amp is a fixed bias doesn't mean you shouldn't check it or adjust it. It's just a matter of changing a resistor.


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Posted: Wed Jan 27, 2010 11:58 am
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Thinline10 wrote:
The thing is, there are different opinions out there...
Can you explain why they need to be rebiased?


Setting the bias means that a tech makes sure that the output tubes have the same amount of current running through them. It is a way of making tubes with inconsistent properties run consistently.

Today we can buy matched tubes to reduce the need to rebias an amp every time the output tubes are changed. Popping in a new set of matched tubes assumes that the pair coming out was matched and also consistent with the new pair before they were installed. What if they were not a matched set? What if the old tubes were running at different current levels? The bias should be checked when installing new tubes.


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Posted: Wed Jan 27, 2010 12:11 pm
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I thought the bias of a push pull design was set to give an equal ammount of push and pull. I.e whatever its pushing, its also pulling. Thats why marshall use a trim pot as a balance between sections rather than a couple of them as a variable resistors.

Enlighten me please wise ones.

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