Modified Mesa Boogie 50/50

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Kiwifpms158

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Hi,

I've made the acquisition of a Mesa Boogie Studio Preamp and a Mesa Boogie 50/50.

Channel A of 50/50 delivers no sound below 5. The sound remains very low and linear between 5 and 10.

So I've decided to open the amp in order to check the tubes. I've noticed 2 strange things :
- The 4 power tubes are Groove Tubes GT-EL34B. I've read everything that it is recommended use Mesa tubes because of bias.
- The amps has been modified (see pictures). A pot has been wired to the 47uF capacitor.

IMG_20141004_211455_zpsf1ab02e6.jpg

IMG_20141004_211441_zpsfd9e1021.jpg


Does anyone have an idea on the purpose of that pot ? Is it a bias setting ? How can I solve channel A issue ?

Thanks !
 
That pot is a bias adjust (judging from where to it is wired)....
Concerning the issue with Ch.A....did you try swapping the tubes (12AX7)? The one closest to the Inputs (right-most tube when looking from front/above) its the Input and the other 2 are the PIs....
 
Thanks for your answer.

I also think it is a bias pot. The problem is that it is set to the minimum ==> I guess the previous owner turned it and now it isn't properly set (I don't believe the minimum to be the proper setting for the EL34B...). What's the risk for my amp if it isn't properly set ?

I'd like to swap the tubes but as a newbie I don't know the sequence... Which one do you think is the right one ?

- Fan / AABB / input
- Fan / BBAA / input
- Fan / ABAB / input
- Fan / BABA / input
- Fan / ABBA / input
- Fan / BAAB / input

A=Channel A
B=Channel B

Thanks for your help !
 
If it was mine, I would get a matched quad 6L6GC, take out that ominous pot and see how I like the Fifty/Fifty as it is/was.
I liked mine as it sounded original ;)

YMMV......
 
Thanks.

I tried to swap the tubes. I tried different sequences... But the sound of channel A remains crappy and low :? so it's not a tube issue...
 
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