Marshall Super Lead Project -Any Help Appreciated- - diyAudio
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Old 29th February 2012, 07:51 AM   #1
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Default Marshall Super Lead Project -Any Help Appreciated-

This is kind of a two part-er. I am the owner of a Marshall Super Lead, (will check year in the morning however I wanna say it's a 76). A week or two ago after returning home from a short tour I plugged in my amp and guitar and was shocked to find that I was getting next to no volume out of the amp, only upon turning the Master volume (yes, it has a master volume as it has been modded) up was I able to get any sound out of the speakers and what I did get was very quiet and very fuzzy. My first thought, bad guitar jack/cable/cab, so I changed all of those and got the same result, my second thought... Tubes. So I ordered new Tubes (both balanced power and pre) from Tube Depot and installed them. Before I attempted to bias the tubes I plugged my guitar in just to see if having the new tubes in had changed the result, still quiet and fuzzy. I have looked at the board and other components looking for broken solder points or burnt/damaged components without much luck finding anything. So at this point I am casting this public inquiry to see if anyone has any suggestions for what I should try to do. I am a 2nd year computer engineering student and am no stranger to basic circuit concepts, however I don't have much experience troubleshooting and am hoping to get some help.

I will post images of the amp as soon as I am able, but if you have any ideas based on my description, please don't hesitate to comment. Thanks in advance.

(b.t.w. the "second" part of this question is about wanting to modify this amp to give it channel switching capabilities, however I would really like to get this old tank up and running again before I worry about taking it to the next level)
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Old 29th February 2012, 08:38 AM   #2
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By FAR the most common cause of faults in valve amplifiers is the anode load resistors in the preamp stages, these commonly go high and O/C.

Check the anode voltages on the triodes (pins1 and 6), you'll probably find one that's very low. While you're doing, it's as well to check all of them, as if you change just the current faulty one, one of the others will probably fail next week!.
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Old 1st March 2012, 09:16 PM   #3
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Default Digging Deeper

Are the triodes the bases into which the pre-amp tubes are plugged in? In addition, I am removing the main board just to give the other side an inspection and to re-solder any sloppy or bad joints. I have disconnected all but those wires that are running to the capacitors, the contacts where they are soldered are coated with a red dye of some sort. Is there a proper way to remove this dye (my first inclination was to just burn right through them and then clean up after, then a little voice in my head told me that that was a stupid idea). Also I am pretty convinced that the caps are drained (let em sit for a week unplugged), I checked the voltages and they are down in the LOW mA range (something like 0.0010), but once again this amp repair stuff is a little larger than the circuits I usually deal with so I want to be safe and ask, is there any way to safely and assuredly discharge these caps?
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Old 1st March 2012, 11:11 PM   #4
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In addition, and simply to illustrate what a newb I am to amplifier circuits, I just found out that in England "valve" apparently is synonymous with "tube" here in the United States. So in order to clarify, I have changed all the "tubes" or "valves" in the pre-amp section already.
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Old 1st March 2012, 11:27 PM   #5
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Also just realized that "triode" is synonymous with "tube" or "valve"
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Old 2nd March 2012, 12:07 AM   #6
Enzo is offline Enzo  United States
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A tube or valve is the same thing.

A triode is one form of tube. Tubes can also be tetrodes or pentodes or diodes, and a number of other things too.

a 12AX7 is a dual triode type of tube - two separate triodes live within the same glass tube.

Tubes plug into a "tube socket." The male pins on the bottom of the tube fit into female connectors in the holes of the socket.

And please be careful, the voltages inside a tube amp can KILL YOU.
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