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Old 10th November 2007, 02:14 AM   #1
rickl is offline rickl  United States
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Default black face champ tubes

This is a father/son project on the cheap. I picked up a Squire amp and converting it into a Black Face Champ clone using a 6SL7 and a 6v6GT.

Digging through my parts bin I can't find a 6SL7. Can I use a 6SC7? I know the 6SC7 is similar with a shared cathode but will it work?

Here is the schematic I'm using.

If a 6sc7 will not work, I'll use a 6sj7 and build a 5C1 but the boy wants tone controls.

Thanks.

Attached is a pic of the underside (5y3 -> 6sc7 -> 6v6).

rick
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Old 10th November 2007, 06:43 PM   #2
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Hi,
I had a friend in high school that had a tweed "champ" and it used a 12ax7 connected to a 6v6. The only Fenders that I am aware of that used octal preamp tubes, were the very early tweed amps. I am pretty sure that the "blackface" amps all used 9 pin preamp tubes. Do you have a 12ax7 tube in your parts box?
Since the 6sc7 has a common cathode it will make your circuit act differently than the 6sl7.
Daniel
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Old 10th November 2007, 08:33 PM   #3
Sheldon is offline Sheldon  United States
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The two sections are run under fairly similar conditions, so the common cathode may not be an insurmountable problem. The easiest thing to try would be to just split the difference on the cathode resistor: (1.5k + 1k)/2 = 1.25k. As the current will be for two plates, half of that value, or about 620R, and double the bypass value.

Sheldon
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Old 10th November 2007, 11:48 PM   #4
rickl is offline rickl  United States
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Thanks for the replies.

For the cathode resistor value on the 6sc7, I thought it would be *twice* the value not half since twice the current will be going through.

The plan was to use something around 2k2 and measure the results.

620R seems low.

Thanks again.

rick
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Old 11th November 2007, 12:08 AM   #5
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The resistance needs to be half so the voltage across it is the same with twice the current going through it, hence the bias point is the same. Don;t forget also to use a higher cathode bypass cap...like 47uF for the same frequency response.

edit: this is assuming the 6sc7 has similar curves...i didn't check
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Old 11th November 2007, 12:24 AM   #6
rickl is offline rickl  United States
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Got it. Ohms law. Thanks. Why I doubt Sheldon...

rick
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