Board in - Please check my work!!

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Megachunk
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Board in - Please check my work!!

Post by Megachunk » Sun Nov 13, 2005 11:44 pm

Hi All,

Thanks for all your help on this thing, it's been invaluable and I wouldn't have made it this far w/o all of you.

The board is in wired up and I tested voltages. Some seems fine, others were a bit wacky, but I figured i wasn't getting a good reading on my DMM. I'll post those tomorrow. ANYWAY, got tubes in and fired up. Bias was very low. Here's what I found out:

There was a big hum, which makes me believe its a grounding thing. I probably have my jacks jacked, becasue now I can olny fit 2 jacks in with the new board. (any diagrams on wiring 2 jacks, and where to run ground, one for I and one for II - they can both be hi gain) V2 is a bit microphonic, but pins 1 and 7 are tied together (is this right?) When I turn up the prescence, it hums louder. I did plug my guit into it, and the guitar was grounding when I touched it. and a very quiet, feeble guitar sound came out. I do need to tie pin 6's together to the board, but they seem to be connected indirectly through the 1/2 power switch.

Anyway, when you dudes have a moment, can you look at my pix and tell me if anything looks funkified? I mean in bad way.
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Flames1950
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Post by Flames1950 » Mon Nov 14, 2005 12:07 am

On the second 12AX7 tube socket, where it says pins 1 and 7 tied together, wasn't there a 100K resistor from pin 6 to pin 1 on the JCM800? You need that resistor there for the board you're using.......usually the resistor goes from 6 to 1, then the resistor lead bends back around and goes to pin 7.
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Post by Flames1950 » Mon Nov 14, 2005 12:09 am

You'll also need a jumper from the bias pot to the 47K resistor above it, unless it's under the board and we can't see it. Jumper the leg of the pot closest to that resistor to the resistor turret.
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Post by Megachunk » Mon Nov 14, 2005 12:11 am

There was no resistor there to begin with. Maybe the tech who installed the 1/2 power switch took it away. I'll put one there tomorrow. Should I still tie those pin 6's together on the output tubes together?

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Flames1950
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Post by Flames1950 » Mon Nov 14, 2005 12:11 am

How did you end up doing the inputs with only two jacks? It's obvious why you had to do it that way, man there is no room to put anything in that amp on the fron side!! :shock: :shock:
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Post by Megachunk » Mon Nov 14, 2005 12:12 am

Flames1950 wrote:You'll also need a jumper from the bias pot to the 47K resistor above it, unless it's under the board and we can't see it. Jumper the leg of the pot closest to that resistor to the resistor turret.
YES! It's under the board. As are a few other connex.

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Flames1950
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Post by Flames1950 » Mon Nov 14, 2005 12:13 am

Megachunk wrote:There was no resistor there to begin with. Maybe the tech who installed the 1/2 power switch took it away. I'll put one there tomorrow. Should I still tie those pin 6's together on the output tubes together?
The output sockets need to be left with pin 6 wiring intact. I'm looking at the second preamp tube socket only. The JCM800 may have had that resistor on the PCB instead.
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Post by Megachunk » Mon Nov 14, 2005 12:13 am

Flames1950 wrote:How did you end up doing the inputs with only two jacks? It's obvious why you had to do it that way, man there is no room to put anything in that amp on the fron side!! :shock: :shock:
Yeah, it's a long skinny chassis! I'm thnking the jacks are a big part of the prob. There nothing running to ground on them right now.

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Post by Megachunk » Mon Nov 14, 2005 10:54 am

Got it working! I had a misplaced wire on the pots, grounding the jacks and doing that V2 resistor must've done the trick! For whatever reason, my bass pot is doing very little. It's hooked up right... Is there a cap I can add to beef up the bottom end? I do play out a 4-10 cab. All other pots are fine. It's a tiny bit hummy, but I'm sure there's some grounding I can do to help eliminate it. Can anyone help me with biasing it? I just need to know where the ground lead goes. Is it center tap? Not sure which it is. I test on pin 3 of the output tubes, correct?

BTW, even now it blows the PCB out of the water! What crunch!

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