I have a couple of thoughts. Wouldn't hurt to go ahead and replace the coupling caps with some new film caps just as cheap insurance. Also there are two caps to ground in the bias supply which may be leaking and drawing the bias supply down (or up depending on your point of view).
One thing to check would be with the power tubes still removed measure the voltage across the 10K resistor. It should be virtually zero as there should not be any DC current at all. If the cap across the 56k resistor is leaking it would show a voltage here. A leak in the cap to the right would not show up here however. Another possibility that would show up as a voltage drop on the 10K would be if there is a partial short in the grid wiring or damage to the tube socket that is causing current to flow into the grid pin on the socket.
Just a few things to look at.
One thing to check would be with the power tubes still removed measure the voltage across the 10K resistor. It should be virtually zero as there should not be any DC current at all. If the cap across the 56k resistor is leaking it would show a voltage here. A leak in the cap to the right would not show up here however. Another possibility that would show up as a voltage drop on the 10K would be if there is a partial short in the grid wiring or damage to the tube socket that is causing current to flow into the grid pin on the socket.
Just a few things to look at.
I would keep using these!
If the bias supply did not change with the B+ power on then the coupling caps are probably ok. However you could change them just to be sure!
6L6 pin out...
http://www.r-type.org/pdfs/6l6g.pdf
Pin 4 & 6 ?
If all the components are OK
What voltage do you have between the 10K & the 56K and what voltage do you have before the 10K...to Gnd?
The 10K to Gnd is to see if you have at least -55V if not you can forget biasing at -55.
Regards
M. Gregg
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alright, so I have yet to make any measurements with this thing powered up aside from through the tube sockets. A bit nervous about doing it, so if I were to do it, what are any and all safety precautions. Even ones that seem obvious. Was hoping (and still imagine I probably could) I could fix this thing without ever having to make measurements with it plugged in....
fell about a foot or two... I'm not positive this was the culprit, but almost positive...
Thier probably using the spare pin to mount the resistor. Have a look at the link for pin out of 6L6.
The caps are probably OK.
I assume these tubes are replacement since the fault?
You need to know if you have more than -55v before the 10K IE do you have enough bias available?
Regards
M. Gregg
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yep checked out the pinout and saw that pin 6 wasn't anything. still, with the amp off, and just checking through the sockets, I'm getting about .5k for the two resistors. on the schematic it says they should be 470k, although on the layout it does say just 470. either way, something to look at?
Safety question answer: With amp off and caps discharged clip the common probe to ground and the pos. lead to the point to be measured. Check that the clips do not touch anything but the desired test point. Check it two more times. Keep your face away from the chassis and your hands out. Apply power and not reading. Shutdown. Lather, Rinse, Repeat. 
Pin 4 is indeed the screen grid. Pin 6 is unused (at least in 6L6 but keep that in mind if you want to switch to other tubes in the family and be sure that Pin 6 is not used in them). They are probably just using pin 6 as a tie point before going on to the screen supply.
You have to lift one leg of the resistor (disconnect) to measure otherwise you are measuring everything else in the circuit too.
Pin 4 is indeed the screen grid. Pin 6 is unused (at least in 6L6 but keep that in mind if you want to switch to other tubes in the family and be sure that Pin 6 is not used in them). They are probably just using pin 6 as a tie point before going on to the screen supply.
You have to lift one leg of the resistor (disconnect) to measure otherwise you are measuring everything else in the circuit too.
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anyone on the safety precautions of making measurements while the B+ is on? One hand in pocket, so clip the - to the chassis? Can I just have the chassis sitting on my workstation/how do you guys usually set up the chassis for this kind of stuff? I'll start making these measurements, but it frankly scares the **** out of me. I know the whole "take it to a tech" argument, but I'm just a college grad and am too broke, yet I need my amp up and going. SOOO I will go in and make measurements if I need to, but I'm wondering all and any safety precautions that are necessary
OK the concert amp ab763 uses 470 ohm.
the concert 6g12 uses 470K
If you are not sure then its probably a case for a tech!
Remember to connect a load or speaker so you do not damage the OP Tx. If you are powering up!
Please test the power supply caps before you touch anything after power down!
Test the bias before the 10K to gnd. You can do this with it in stand by! Set meter to DC highest voltage. A meter set to AC volts sometimes will show 0 Volts if connected to DC!
Remember the caps on the B+ will be live after power OFF so make sure you discharge!
If you are going to test.. tell us and then tell us when you have powered OFF and the results!
You should not do this on your own! If you get a shock their will be no one to help you!
So now its up to how you want to proceed! Don't use a cheap meter. Make sure your leads are good! no splits etc.
You can do this on a work station .. its at your own risk!
Just for interest
M. Gregg
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I don't have a chassis cradle so I just have to improvise a way to secure it but I make ^%&$ sure that it ain't going to move. I am unwilling to probe a hot chassis as I am never in that big a hurry and am too shaky to even consider it. I always clip both leads and then turn on the power. I have seen what 120AC can do to a meter probe I never what to find out what 300VDC can do.
Its quite common for amps to have different components if the value they show is 470 Ohm then that should not be a problem. All caps will hold charge if they don't they are stuffed!
What bias voltage do you have before the 10K?
Don't get over confident the amp is just a dangerous as the first time you measured!
Never forget to discharge!
The values are from a book of amp schematics I have! So should be correct. Remember the two amps are very similar.
Regards
M. Gregg
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m gregg, here is the schematic I am referencing. http://ampwares.com/schematics/concert_6g12.pdf
the schematic above says 470k, the layout says 470 ohms, the resistors are measuring around 550 ohms
the schematic above says 470k, the layout says 470 ohms, the resistors are measuring around 550 ohms
Its to low!
What voltage have you got across the bias caps the one we need to know is the one after the diode. The diode connects to the cap and the 10K. We need to know if you have more than -55V<<<this is important.
You should have the schematic printed off by you!
Regards
M.Gregg
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