Mesa Boogie F-50.. what is this?

[Andree88]

Hello everyone,
I've just bought a second hand Mesa/Boogie F-50. This amp is amazing but I saw one thing when I've disassembled it:



It seems that the first owner cut the "tracks" (I've draw red lines to let you see the cuts) and put 2 resistors (the two in the red circle).
Can you tell me why? What happens with the 2 resistor instead of the normal track?


I've saw that inside is different from the images I see on web.. I've put a circle to the differences:



Can you tell me what is the difference?

Few more questions:

1) Why the treble pot seems almost no-working in CH2? It makes Channel 2 too "trebly"
2) Is it possible to mod the FX Loop from parallel to serial?
3) Is it possible to mod the FX Loop to have 100% effect and not only 90% (I see that my volume pedal didn't cut all the sound when toe up)?
4) Is it possible to mod to have adjustable BIAS with a pot (I've 2 JJ 6L6GC now)?
5) Is it possible to mod the contour mode? Channel 1 is perfect, Channel 2 is great, Channel 2+Contour is too "midcut" for me.

Thanks all sorry for my bad English, I'm Italian

[loudthud]

Looks like 10 Ohm resistors were added in the cathode current path to monitor bias current. The traces on the PCB were cut to allow current to flow through the resistors. Mesa does not like adjustable bias and they want you to buy their tubes.

[Andree88]

Looks like 10 Ohm resistors were added in the cathode current path to monitor bias current. The traces on the PCB were cut to allow current to flow through the resistors. Mesa does not like adjustable bias and they want you to buy their tubes.

So what changes in terms of sound with the resistors in the path? I've put JJ6l6GC instead of the two TAD the first owner had..
Thanks!

[loudthud]

Should be no perceivable change other than the warmer bias after adjustment. They just allow you to monitor the current without any fancy contraption like a "bias probe".

[Andree88]

Thanks, so it's just to check current but the sound it's the same like if the amp doesn't has the two resistors?

Other question.. where is the resistor that I have to change with a pot for make my F50 to adjustable bias?
Thanks again

[Jazz P Bass]

http://music-electronics-forum.com/t29189/

The resistor in question is directly under V4.

[Andree88]

http://music-electronics-forum.com/t29189/

The resistor in question is directly under V4.

Yeah, thanks, I saw the post.. but under V4 there are 5 resistors (2 orizontals, 3 verticals) and I don't know which one to change I needed a picture to know exactly which resistor is! Thanks man

[Jazz P Bass]

Have you measured the resistance of the 5?

[Andree88]

I've discovered 2 more capacitors (in first post picture) and some resistors changed near V3 (I think for use 12AT7 instead of 12AX7.. but can I still use 12AX7 in V3?).. Who can help me knowing what it's changed in my amp? The first owner said he bought it directly from Mesa and never done mods.. bah?

[Andree88]

I have discovered something about my F-50. It has a lot of changes. And the first owner said that he doesn't know nothing.. bah?!

I hope you can help me, it makes me mad and I have no techs near to me...

Here's the original (I got it in web)




The differences:

1) They added 2 black capacitors are connected to the original blue capacitors, why? Maybe are blue faulty capacitors?





2) They change the 270 ohm, 5%, 9w resistor with 2 with same value.. another faulty resistor?


Original



Changed




3) They changed this capacitor with.. this? What is?

Original



Changed





4) They changed 4 resistors near V3. It's for use the 12AT7 instead of 12AX7 in V3 Fx Loop? I don't know the color of original resistors. Can I still use a 12AX7 in V3?




5) They put a bias cathode change.. (see first post) can I put back Mesa original tubes or I need to remove the 2 resistors?


Here's the original Schematic of F-50

https://drive.google.com/file/d/0B01...BJakJaNlk/view

Thanks!

[Jazz P Bass]

Sell the amp!

[Andree88]

Sell the amp!

Nope, I just bought it 10 days ago! It's awesome but I want to know the changes..

[Pedro Vecino]

Pic 1: Original filter capacitors are not so old (2005). Most likely to work perfectly.
The added pair is surely coupled with the intention of a more tense bass response. Specially in the second channel with high gain setting (sounds quite soft).
Pic 2: Could be. It maintains the same value. No problem.
Pic3: It seems two capacitors inside (series probably). You will have to extract to see its value. I do not know the reference of the component in the circuit, but removing it you will see.
Pic 4: All four components maintain original values. 220K/330K/1K5 and 0.22uF (original is tantalum capacitor). No problem
5. If they are 10 Ohms resistors (or 1 Ohm), you can leave them in place. You can measure through them the current flowing through the tubes.

Finally: it gives me the feeling that the changes are purely experimental. Itīs relatively easy to recover the amp to its original form.

[Andree88]

Pic 1: Original filter capacitors are not so old (2005). Most likely to work perfectly.
The added pair is surely coupled with the intention of a more tense bass response. Specially in the second channel with high gain setting (sounds quite soft).
Pic 2: Could be. It maintains the same value. No problem.
Pic3: It seems two capacitors inside (series probably). You will have to extract to see its value. I do not know the reference of the component in the circuit, but removing it you will see.
Pic 4: All four components maintain original values. 220K/330K/1K5 and 0.22uF (original is tantalum capacitor). No problem
5. If they are 10 Ohms resistors (or 1 Ohm), you can leave them in place. You can measure through them the current flowing through the tubes.

Finally: it gives me the feeling that the changes are purely experimental. Itīs relatively easy to recover the amp to its original form.

Thanks, you are saving me

So if I put the Mesa's 6L6 back I don't need to change nothing? Just plug in?

For 5, I only read the colors of resistors so I think they are 10 ohms.. It's like a bias measure spot? If it is, then I'll leave it in.

[Pedro Vecino]

The original bias adjust is very conservative. You should have no problem.