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Thanks, g1....
bsco, there's no fast way that I've found in dealing with the power tubes on the SVT CL, AV or VR (which is a different animal). The Protect circuit actually will help you in finding the culprit. You can operate the amp with only one pair of power tubes installed at a time. The Bias LED's won't be of service, as their 'window' range is dialed in for a sextet of tubes. I normally have the amp and preamp out of the cabinet, with the power amp chassis standing on one end (using a small block under the power xfmr to keep it upright. I also have replaced the 'garden hose' sleeving that Ampeg installs with Tech Flex, so I can position the preamp instead of the garden hose positioning it how IT wants. In the post g1 gave you above, scroll up to the beginning of that post, and you'll see my test set-up for screening power tubes, in pairs.
I've also made a couple diagrams to assist me when I'm just looking at the back side of the power amp chassis....it shows the location of the six cathode resistors, and the two bias pots, and which tubes they control...I tend to forget in spite doing this all the time.
The first image is viewed from the bottom of the chassis. I normally have my ground lead clipped onto one of the cathode resistors that appear along that bottom edge (amp is sitting on the power xfmr end). The second image is top view of the power tube mtg plate, showing the grouping of the upper and lower halves of the output stage.
Now, if you don't have any spare power tubes, you're kind of out of luck with regards to being able to cobble a working set. But, you may be able to cobble a set out of the best four tubes you have. I"m normally measuring the plate/screen current thru the cathode resistors, and will pre-set the bias voltage so the upper and lower halves are equal. Then, I can see just what each tube is pulling. May have to tweak the bias until you're in the window around 23mA (0.230V across 10 ohms). Even if I don't get close matching in two pairs, I can group what I've measured for the best pair for upper and lower, then with the two bias pots, adjust them for near equal bias total. You can see after setting up the amp with what you do have working, if it will run without tripping the protect circuit. Usually it's just one tube, sometimes two. As I only toss out the known bad tubes, over the years, I've accumulated 6550's and KT-88's from Sovtek, Svetlana, Electro-Harmonix, J/J, Tung Sol and others, so I can often extend the working life of the amp without having to turn around every time I lose one power tube and spend another $250-$300 for a new set. When I do, though, I add to my pile of working 'pulls'
In the thread g1 directed you to, that particular amp had one or two tubes that misbehaved from vibration....and I was able to excite the problem by tapping on the top of the tube powered up. I'd get RED bias LED to turn on with the tapping, and then it fired the Protect circuit, saving damage within. Though, I will always have a look at the power tube PCB when I've had tube failure, just to see what the screen resistors are, and if there's any damage there.
If you haven't already looked thoroughly at all of the solder joints on the power amp PCB, I'd recommend you do so....bright light & magnification to reveal any radial solder fractures....particularly around the bias pot terminals, harness connector headers, feedback header, I/O connector header, ribbon cable header to the power tube PCB. Likewise with the output PCB & AC mains PCB. Solder fractures so often cause grief with these amps, and grow from the shear weight/mass vibrating everything in transit over time.
bsco, there's no fast way that I've found in dealing with the power tubes on the SVT CL, AV or VR (which is a different animal). The Protect circuit actually will help you in finding the culprit. You can operate the amp with only one pair of power tubes installed at a time. The Bias LED's won't be of service, as their 'window' range is dialed in for a sextet of tubes. I normally have the amp and preamp out of the cabinet, with the power amp chassis standing on one end (using a small block under the power xfmr to keep it upright. I also have replaced the 'garden hose' sleeving that Ampeg installs with Tech Flex, so I can position the preamp instead of the garden hose positioning it how IT wants. In the post g1 gave you above, scroll up to the beginning of that post, and you'll see my test set-up for screening power tubes, in pairs.
I've also made a couple diagrams to assist me when I'm just looking at the back side of the power amp chassis....it shows the location of the six cathode resistors, and the two bias pots, and which tubes they control...I tend to forget in spite doing this all the time.
The first image is viewed from the bottom of the chassis. I normally have my ground lead clipped onto one of the cathode resistors that appear along that bottom edge (amp is sitting on the power xfmr end). The second image is top view of the power tube mtg plate, showing the grouping of the upper and lower halves of the output stage.
Now, if you don't have any spare power tubes, you're kind of out of luck with regards to being able to cobble a working set. But, you may be able to cobble a set out of the best four tubes you have. I"m normally measuring the plate/screen current thru the cathode resistors, and will pre-set the bias voltage so the upper and lower halves are equal. Then, I can see just what each tube is pulling. May have to tweak the bias until you're in the window around 23mA (0.230V across 10 ohms). Even if I don't get close matching in two pairs, I can group what I've measured for the best pair for upper and lower, then with the two bias pots, adjust them for near equal bias total. You can see after setting up the amp with what you do have working, if it will run without tripping the protect circuit. Usually it's just one tube, sometimes two. As I only toss out the known bad tubes, over the years, I've accumulated 6550's and KT-88's from Sovtek, Svetlana, Electro-Harmonix, J/J, Tung Sol and others, so I can often extend the working life of the amp without having to turn around every time I lose one power tube and spend another $250-$300 for a new set. When I do, though, I add to my pile of working 'pulls'
In the thread g1 directed you to, that particular amp had one or two tubes that misbehaved from vibration....and I was able to excite the problem by tapping on the top of the tube powered up. I'd get RED bias LED to turn on with the tapping, and then it fired the Protect circuit, saving damage within. Though, I will always have a look at the power tube PCB when I've had tube failure, just to see what the screen resistors are, and if there's any damage there.
If you haven't already looked thoroughly at all of the solder joints on the power amp PCB, I'd recommend you do so....bright light & magnification to reveal any radial solder fractures....particularly around the bias pot terminals, harness connector headers, feedback header, I/O connector header, ribbon cable header to the power tube PCB. Likewise with the output PCB & AC mains PCB. Solder fractures so often cause grief with these amps, and grow from the shear weight/mass vibrating everything in transit over time.
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