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Carvin X100B Repair (done)


MiKro

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13 hours ago, Djstatix said:

I can see the board and resistor.  That potentiometer, where did you get it?  Looks like r126 is the insertion point.

It is a trimpot that I had. Yes the insertion point was where R126 would be. On the schematic for the 1986 version may be different? Also not sure what tubes you use? if 6l6 then my values may not work as I was using EL 34 as my power tubes. It is a Round Trimming Potentiometer 1/2watt most likely I got it from Jameco or digikey.

One more thing the Bias value shown on most of the schematics is wrong. Since this is a fixed biasing scheme on the cathode side, you are doing all 4 tubes at once not each individual tube. like many amps. Also most Carvin amps were biased towards the cold side .

mk

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10 hours ago, MiKro said:

It is a trimpot that I had. Yes the insertion point was where R126 would be. On the schematic for the 1986 version may be different? Also not sure what tubes you use? if 6l6 then my values may not work as I was using EL 34 as my power tubes. It is a Round Trimming Potentiometer 1/2watt most likely I got it from Jameco or digikey.

One more thing the Bias value shown on most of the schematics is wrong. Since this is a fixed biasing scheme on the cathode side, you are doing all 4 tubes at once not each individual tube. like many amps. Also most Carvin amps were biased towards the cold side .

mk

Awesome!  Thanks for the help.  I have the 6L6’s installed.  I’ve seen where people installed the Pot on R127 but R126 makes more sense looking at schematic.

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1 hour ago, Djstatix said:

Awesome!  Thanks for the help.  I have the 6L6’s installed.  I’ve seen where people installed the Pot on R127 but R126 makes more sense looking at schematic.

I had a long discussion about that very thing with Rob R as you said many put it at R127 and I was not positive. Rob R thought I was on the right track as well. LOL

mk

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17 hours ago, MiKro said:

I had a long discussion about that very thing with Rob R as you said many put it at R127 and I was not positive. Rob R thought I was on the right track as well. LOL

mk

Your board looks like a 4C mine is a 4B.  Comparing to schematics.

any idea why r127 has two resistors in series?  C2 has a leg lift with a resistor in series too.

 

416DCC12-B5BF-425C-8666-8F1711845343.jpeg

75F5F23E-CEDC-4110-B0E0-ACB595444CE8.jpeg

Edited by Djstatix
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2 hours ago, Djstatix said:

Your board looks like a 4C mine is a 4B.  Comparing to schematics.

any idea why r127 has two resistors in series?  C2 has a leg lift with a resistor in series too.

 

416DCC12-B5BF-425C-8666-8F1711845343.jpeg

75F5F23E-CEDC-4110-B0E0-ACB595444CE8.jpeg

No clue unless someone has biased it before? Was it set up originally for EL34 power tubes and changed to 6L6? That may be the reason as well. Also not knowing off of the top of my head what the resistance should be on the schematic, maybe they put them in series to get the correct resistance as they did not have the right one?

I am only guessing now though.

mk

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  • 2 weeks later...

Maybe, I have no idea.  It has 6l6’s installed.  I replaced it with the factory 91k resistor.  Thought about putting a variable 100k potentiometer there but for now the 91k.  This amp had several owners I believe before me.  I’ve done the few easy gain mods too.  Re doing all the tolex for the amp and I’ve found a carvin cab to refinish as well.  I’ll post the pics when I’m done.

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  • 1 month later...

Hi, thanks for detailing this so well! I want to add a bias to mine X60B, but there is so much contradictory information out there and in my limited knowledge, the way you did it makes the most sense to me. But I was wondering, did the values you use work? And if you don't mind, could you please explain or show a photo of the back of the little bias board you made? I'm just not sure what lugs on the pot go where and did you ground the 47k resistor to the back of the pot? Thank you!!!

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3 hours ago, Wardo said:

Hi, thanks for detailing this so well! I want to add a bias to mine X60B, but there is so much contradictory information out there and in my limited knowledge, the way you did it makes the most sense to me. But I was wondering, did the values you use work? And if you don't mind, could you please explain or show a photo of the back of the little bias board you made? I'm just not sure what lugs on the pot go where and did you ground the 47k resistor to the back of the pot? Thank you!!!

I would need to see the schematic for your Amp, as far as the bias board just look at the schematic that is posted along with it. I do not have a picture of the back side.

mk

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14 hours ago, MiKro said:

I would need to see the schematic for your Amp, as far as the bias board just look at the schematic that is posted along with it. I do not have a picture of the back side.

mk

Thanks mk for replying. I can't even find a schematic for the poweramp section. Preamp is the same. Did you ground the resistor to the back of the pot? I only see two wires in the little board you made.

Thanks again!

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  • 4 months later...
On 9/9/2022 at 8:20 AM, Wardo said:

Thanks mk for replying. I can't even find a schematic for the poweramp section. Preamp is the same. Did you ground the resistor to the back of the pot? I only see two wires in the little board you made.

Thanks again!

No it is floating just like the resistor it replaced. One of the resistor ends terminates to ground originally. The pot makes a variable resistor, the fixed resistor is in case the pot fails then it fails hot which is what you want.

mk.

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