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Old 14th September 2005, 03:53 PM   #1
Trout is offline Trout  United States
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Default Help Please Guitar Project Mess

Hi Guys,

I need some opinion/help/slapped around a bit/

I Have been trying to get this preamp to work attached to a known power amp I have built a few times in the past.

All I get is well, Um, Nothing but a mild Hum from speaker.
I basically get no signal and suspect its actually a design or drawing mistake.

Maybe you guys can find it as I have been just guessing without much success.


To Me I think Pin 8 of the second 12AX7 is wired incorrectly to the tone controls. But I am just note sure.
Any Ideas Would Be Great
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Old 14th September 2005, 05:10 PM   #2
EC8010 is offline EC8010  United Kingdom
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Well, the tone controls look weird, but then I'm not familiar with guitar amplifiers, maybe that's just the way they do things. I'd like to see the second section of V1 have its very own grid leak resistor, but other than that I don't see any reason why this circuit shouldn't work. Rather large grid-stoppers on V2, mind.
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Old 14th September 2005, 07:19 PM   #3
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This schematic came from a late 60's early 70's Marshall amp. Probably one of the super lead series. All of the component values and connections are correct. I have built these and they work. If yours doesn't work, I would suspect a wiring error or a sick tube.
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Old 14th September 2005, 07:30 PM   #4
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dito on Pull tube 1, check whether tube 2 works by tapping the tube of or touch the grid with an INSULATED tool.. same with tube 1 in place...


voltage seem at bit low for a marshall type circuit, but it should work.. What are the DC voltages on the tubes ?
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Old 15th September 2005, 12:19 AM   #5
Enzo is offline Enzo  United States
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I don't like the way the two input channels are wired together at the input jacks, but that is a minor thing. This circuit should work fine. The tone stack is wfed by the cathode follower, a common Marshall approach. This should work, I would be assuming you miswired something.
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Old 15th September 2005, 12:36 AM   #6
Trout is offline Trout  United States
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I have been walking back threw the entire build again, Yet, All the Tubes check Fine, I tryd swapping them out several times with NOS tubes only to get the same results.

I think you all are correct, Clearly I must have a brain glitch going and missed something.

I have built 6 projects so far this summer, This is the only one that has failed to actually produce sound of any kind, Grrrr Figures, Its so pretty to.

I'll update on this in a day or 2, I do have a suspected componet now.
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Old 15th September 2005, 03:31 AM   #7
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That's a pretty standard Plexi circuit, I could draw it from memory. The input jacks are tough to wire up on those, if you are using the marshall jacks, other than that, I would suspect a bad tube or wiring problem.
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Old 15th September 2005, 03:45 AM   #8
kevinkr is offline kevinkr  United States
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I'd start by disconnecting the input jacks and driving the bright and clean pre-amplifiers separately. If you wired them incorrectly you may have a perpetual short at the input.

When you plug into bright input you are supposed to be able to mix both together I guess, if you plug into clean the bright path is disabled.

If you wired as shown the thing will work. The tone stack is very standard marshall practice and similar to what fender used as well.

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