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"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
But basically the point is that unless there's anything wrong, don't change the valves.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Where do your suspicions lie, guys?
Have a look in the back and see if all the preamp valves are glowing properly - there should be two spots of orange light at both the top and bottom of each valve. If not, that valve is either faulty or not making a good contact with the socket - wiggle it and see if it comes on. But even if all of them appear to be lit correctly it still could be one, since that's only the filament glow not the signal voltage. To definitely confirm or eliminate a vale problem, you need one spare 12AX7 - replace each of them in turn with it and see if/when the problem stops.
If it's not a valve it could be a sticking or faulty channel relay, which is going to be a much bigger pain to fix even for a tech. Less likely but still possible is a preamp plate resistor, faulty volume pot, or a bad solder joint.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Looking at the tube layouts it's most likely V1, V5 or V6.
12AX7 Tubes are sort of like 2 tubes in one package, Half of V1 is used for Channel 1/2, and V5 and V6 are used exclusively for Channel 1 and 2.
Both channels share a power amp.
V1 is the one furthest to the right, behind the Output Transformer and V5 and V6 are the two in front that are in line with the Output Transformer.
Difference in brightness of filament glow and the blue glow when the valve is conducting are normal and not a sign of incorrect operation.
If it's a valve problem at all, it's a preamp valve.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein