Help Please Guitar Project Mess

Trout · 14th September 2005, 03:53 PM

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.

Schematic

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
Gene

EC8010 · 14th September 2005, 05:10 PM

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.

tubelab.com · 14th September 2005, 07:19 PM

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.

tschrama · 14th September 2005, 07:30 PM

dito on tubelab.com. 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...

edit..

voltage seem at bit low for a marshall type circuit, but it should work.. What are the DC voltages on the tubes ?

Enzo · 15th September 2005, 12:19 AM

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.

Trout · 15th September 2005, 12:36 AM

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.
Gene

aletheian · 15th September 2005, 03:31 AM

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.

kevinkr · 15th September 2005, 03:45 AM

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.

Kevin

14th September 2005, 03:53 PM	#1
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	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. Schematic 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 Gene

14th September 2005, 05:10 PM	#2
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

14th September 2005, 07:19 PM	#3
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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14th September 2005, 07:30 PM	#4
tschrama diyAudio Member Join Date: Jul 2001 Location: The Netherlands	dito on tubelab.com. 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... edit.. voltage seem at bit low for a marshall type circuit, but it should work.. What are the DC voltages on the tubes ?

15th September 2005, 12:19 AM	#5
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	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.

15th September 2005, 12:36 AM	#6
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	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. Gene

15th September 2005, 03:31 AM	#7
aletheian diyAudio Member Join Date: Apr 2004 Location: Philly	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.

15th September 2005, 03:45 AM	#8
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	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. Kevin

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