Bias Circuit Question

Cjsinla

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I was looking at the way that Mojotone implemented a way to adjust the bias on their Vibrolux kit. It seems to me that a similar mod could be done to my tweed Super build as well.

Vibrolux
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I mean that I should be able to replace that 56k resistor with a pot and smaller resistor, right?

Super
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ErictheRed

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Yes, except that pots can physically move with a little vibration and whatnot over time. So you'll want to keep checking it, I guess, before you fire it up.
 

Cjsinla

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Yes, except that pots can physically move with a little vibration and whatnot over time. So you'll want to keep checking it, I guess, before you fire it up.
I have an extra bias adjustment pot that came with my Marshall clone that I could use, that should hold tight against the vibration I would think. I also have another regular sized pot, the type that I used in the Vibrolux. It’s not as loose as a regular pot I’m just not sure if I have enough room to mount it. The small Marshall type pot might fit on the board.

C90230E9-6E71-4B9B-8D32-F9D45B85101E.jpeg
 

sonar

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You might be able to mount a mini pot right on the board. Common mod with Princeton Reverb’s, but more real estate on the PR bias board.
 

Cjsinla

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You might be able to mount a mini pot right on the board. Common mod with Princeton Reverb’s, but more real estate on the PR bias board.
I do have one. As soon as the new set of tubes comes in the mail I’ll take the amp apart and take a look.
 

sonar

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I do have one. As soon as the new set of tubes comes in the mail I’ll take the amp apart and take a look.
Cool!

Post pics of your solution if it isn't much of a bother. It makes the forum a bit more interesting.
 

Cjsinla

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I started looking inside the amp in preparation to adding a couple of 1 ohm resistors to help with biasing. The Weber grounding scheme has only pin 8 grounded. I assume that I can connect pins 8 and 1 to ground via the 1 ohm resistor.

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I also have an extra bias adjustment pot left over from my 2204 build. It looks as if it is supposed to be PCB mounted. I plan to use it despite that. Do you think that I need to ground the unused terminal?

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@V2 @Soul Tramp
 

Cjsinla

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Am I right to assume that the Super amp is fixed bias even though it has no bias adjustment pot? If so, this amp reads at 30.4 and 32.8 mA’s on the new power tubes I just put in. I was looking at the Robinette bias calculator discussion about fixed bias vs cathode bias. If this amp is fixed bias it is right where it’s supposed to be. It’s running too cold for it to be cathode biased.
 

V2

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I'm a bit too tired to read through the thread at the moment, but feel free to message me to discuss.
 

Pappy58

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Am I right to assume that the Super amp is fixed bias even though it has no bias adjustment pot? If so, this amp reads at 30.4 and 32.8 mA’s on the new power tubes I just put in. I was looking at the Robinette bias calculator discussion about fixed bias vs cathode bias. If this amp is fixed bias it is right where it’s supposed to be. It’s running too cold for it to be cathode biased.
Yes, the Super is fixed bias, and if your new tubes fell right in I wouldn't touch it. I consider that mojo-vibrolux mod a hack. I had to to disconnect it and put in a fixed resister because it was making the trem act all wonky and the voltages were all over the place.
 

Cjsinla

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Yes, the Super is fixed bias, and if your new tubes fell right in I wouldn't touch it. I consider that mojo-vibrolux mod a hack. I had to to disconnect it and put in a fixed resister because it was making the trem act all wonky and the voltages were all over the place.
I’m sorry to hear that you had to put in a fixed resistor. Mine was working so well I was hoping to do the same thing with my Super.
 




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