This past Friday, I finally had completed the pending repairs on various combo amps, and finally got to the other Ampeg SVT that a client's driver had brought over for service. I had only briefly glanced at the front, with my eyes seeing it as an SVT-VR, when in fact it was an SVT-AV. Thrown off by the light grey front panel. The gaffer's tape sticker only stated 'Noisy--Tube Issue?'
After having to remove the rear bottom corner guards which would then allow the power amp chassis to be slid out without those obstructing the chassis, and removed the vented grille panel, noting the absence of the 120mm fan normally mounted to it as in the case of an SVT-VR. I turned the cabinet around, pried the fan/grille panel up and out, disconnecting it from it's chassis connector. Then, removed the four preamp chassis mtg screws, and went to slide the preamp chassis forward and out to address the wiring harnesses.
The clear vinyl 'garden hose' sleeving that Ampeg uses on these was, in this case, quite hardened from the years of use/disuse. So, that fought me while getting the harness wiring unplugged and extracted. I normally replace that garden hose sleeving with Tech Flex, using a Clauss 'Flipper', which is a palm-fitted VERY SHARP scissors that work great with lacing tape and spot ties...one of those tools I've owned for 50 years. Not this time, though. Cutting that vinyl sleeving can issue really nasty cuts, so I had to resort this time to using Blue Bird Tin snips, just to keep my fingers from harm. Finally got of that material cut off and removed.
I had already removed the six Groove Tube 6550C power tubes. Now, getting the small truss head screws threaded into the power tube socket's standoffs, which takes a 2mm/5/64" Hex Drive tool to remove, on a good day. This wasn't a good day, I was only able to release 9 of the twelve screws holding the spring hold-down clamps' lugs under the screw head, while the remaining three screws merely stripped in the hex drive pattern of the screw head! I hate that about Ampeg's insistance on using them. After soaking the remaining three with WD 40, pounding on the screw heads with hammer/pin punch, having no luck with trying to get any of my pliers or cutters to grip the tapered edge of the screw heads, I left for the day, after further soaking the screws with WD-40. The following morning, I tried again, this time trying my 100 yr old EIFEL-FLASH PLIERENCH that I had inherited from my great-uncle decades ago. I was able to just barely grip on the edges of the screw head, with the solder lug under it's had to which the springs of the hold-down clamps were attached, the screw heads loosened. I tried the 2mm hex drive tool at that point, and was able to un-thread the last three screws.
I was certain I was going to have to cut slots into the screw heads using my Foredom hand grinder & cut-off discs.
After removing that power tube PCB from the chassis, I was able to first install the Tech Flex, slipping the wiring thru and securing them, then removed the rear panel mounting hardware and the three front screws holding the power amp's man PCB in place. Once I lifted it up, I had access to the two loose output xfmr mounting screws to tighten, then scrutinized that PCB's solder joints. All six solder tabs of the two bias pots had severe solder joint fractures. I thought back to the label on the cabinet: "Noisy-Tubes". Yeah...right! The bias supply cap (100uF/250V) had a large bulge where the vent had erupted, as did the first power supply filter stage (100uF/450V x 2). So, I had to remove/replace those while I was at it. Solder joint fractures on the two driver tubes and preamp input tube, as well as on the XLR connector, though this time, I didn't find fractures on the two 5-pin right-angle headers that normally are also fractured.
I did find an issue with V4 & V6 cathode resistors R38 & R39. These were all 10 ohm/3W 1% power resistors, and measured 10.0 ohms, except for those two. I unsoldered and removed both, finding each now measured correctly, but no resistance across their solder pads, now vacant. I put them back in, and again got 9.29 ohms and 9.43 ohms. I looked at all of the circuits attached to those resistors, not finding anything to account for that, and replaced them with new identical resistors. Same readings again. I finally took my Xacto knife and carefully ran it between the traces to the main area of the network associated with those resistors, cleaned up the residue and checked the readings again. Now measuring 10.0 ohms. Strange one.
After making solder joint repairs on the output tube PCB and the AC Mains PCB, putting those back into place, I put the main PCB back down, having also tightened up the three standoffs that support the front side of the board and secured it. Turned the chassis back over to finish with the power & output xfmrs, having also tightened the rest of their mounting screws. Moving the chassis was still yielding movement of those two heavy xfmrs, pointing to the core bolts. They were all loose. I was able to tighten them....as much as I dared, not wanting to snap those off!. Then, I turned my attention to the power tube PCB.
This vintage used #6-32 mounting screws/hex keps nuts instead of M3.5 hardware. All twelve of the hex stud-standoffs were loose enough to allow spinning if the mating screw locked up (which I've also encountered before!). After tightening all those up, I took my Starrett Tap Wrench and Greenlee #6-32 tap, and ran that into all twelve of the standoffs, which certainly needed it. Afterwards, I could easily drive the hold-down screws into the standoffs without stripping out the heads of the 2.0mm Hex driver.
Finally got thru opening up the preamp, fixing the solder joint fractures on the pots and those on J5 & J6 (Heater voltage input, Relay voltage output), and put it all back together.
Mounted the power tube PCB assembly & cover plate, minus the hold-down clamps, and re-connected the harnesses to the preamp, stood the chassis' up on end, supported so the power amp chassis wouldn't tip over, and was able to power up the chassis for the first time to preset the driver tubes bias voltage to -52V for each. Then went thru each of the 6550 power tubes in pairs, using V3/V4 as the test tube sockets and recorded the plate current readings. Swapped a pair of tubes to make a better fit, and put all of the tubes into place.
I almost got a working set of tubes. Just one tube on one side is way high relative to it's other two mates. I tried the Sovtek 6550's that came from that other SVT-CL which has the shorted power xfmr. Those are far worse in the sense of a balanced set. So, now hoping I can find yet another 6550 that will fit better into the plate current match, and then finally see if there ARE noisy tubes in this amp. Gotta get to that point first. So it goes in maintenance on Ampeg SVT's.
After having to remove the rear bottom corner guards which would then allow the power amp chassis to be slid out without those obstructing the chassis, and removed the vented grille panel, noting the absence of the 120mm fan normally mounted to it as in the case of an SVT-VR. I turned the cabinet around, pried the fan/grille panel up and out, disconnecting it from it's chassis connector. Then, removed the four preamp chassis mtg screws, and went to slide the preamp chassis forward and out to address the wiring harnesses.
The clear vinyl 'garden hose' sleeving that Ampeg uses on these was, in this case, quite hardened from the years of use/disuse. So, that fought me while getting the harness wiring unplugged and extracted. I normally replace that garden hose sleeving with Tech Flex, using a Clauss 'Flipper', which is a palm-fitted VERY SHARP scissors that work great with lacing tape and spot ties...one of those tools I've owned for 50 years. Not this time, though. Cutting that vinyl sleeving can issue really nasty cuts, so I had to resort this time to using Blue Bird Tin snips, just to keep my fingers from harm. Finally got of that material cut off and removed.
I had already removed the six Groove Tube 6550C power tubes. Now, getting the small truss head screws threaded into the power tube socket's standoffs, which takes a 2mm/5/64" Hex Drive tool to remove, on a good day. This wasn't a good day, I was only able to release 9 of the twelve screws holding the spring hold-down clamps' lugs under the screw head, while the remaining three screws merely stripped in the hex drive pattern of the screw head! I hate that about Ampeg's insistance on using them. After soaking the remaining three with WD 40, pounding on the screw heads with hammer/pin punch, having no luck with trying to get any of my pliers or cutters to grip the tapered edge of the screw heads, I left for the day, after further soaking the screws with WD-40. The following morning, I tried again, this time trying my 100 yr old EIFEL-FLASH PLIERENCH that I had inherited from my great-uncle decades ago. I was able to just barely grip on the edges of the screw head, with the solder lug under it's had to which the springs of the hold-down clamps were attached, the screw heads loosened. I tried the 2mm hex drive tool at that point, and was able to un-thread the last three screws.
I was certain I was going to have to cut slots into the screw heads using my Foredom hand grinder & cut-off discs.
After removing that power tube PCB from the chassis, I was able to first install the Tech Flex, slipping the wiring thru and securing them, then removed the rear panel mounting hardware and the three front screws holding the power amp's man PCB in place. Once I lifted it up, I had access to the two loose output xfmr mounting screws to tighten, then scrutinized that PCB's solder joints. All six solder tabs of the two bias pots had severe solder joint fractures. I thought back to the label on the cabinet: "Noisy-Tubes". Yeah...right! The bias supply cap (100uF/250V) had a large bulge where the vent had erupted, as did the first power supply filter stage (100uF/450V x 2). So, I had to remove/replace those while I was at it. Solder joint fractures on the two driver tubes and preamp input tube, as well as on the XLR connector, though this time, I didn't find fractures on the two 5-pin right-angle headers that normally are also fractured.
I did find an issue with V4 & V6 cathode resistors R38 & R39. These were all 10 ohm/3W 1% power resistors, and measured 10.0 ohms, except for those two. I unsoldered and removed both, finding each now measured correctly, but no resistance across their solder pads, now vacant. I put them back in, and again got 9.29 ohms and 9.43 ohms. I looked at all of the circuits attached to those resistors, not finding anything to account for that, and replaced them with new identical resistors. Same readings again. I finally took my Xacto knife and carefully ran it between the traces to the main area of the network associated with those resistors, cleaned up the residue and checked the readings again. Now measuring 10.0 ohms. Strange one.
After making solder joint repairs on the output tube PCB and the AC Mains PCB, putting those back into place, I put the main PCB back down, having also tightened up the three standoffs that support the front side of the board and secured it. Turned the chassis back over to finish with the power & output xfmrs, having also tightened the rest of their mounting screws. Moving the chassis was still yielding movement of those two heavy xfmrs, pointing to the core bolts. They were all loose. I was able to tighten them....as much as I dared, not wanting to snap those off!. Then, I turned my attention to the power tube PCB.
This vintage used #6-32 mounting screws/hex keps nuts instead of M3.5 hardware. All twelve of the hex stud-standoffs were loose enough to allow spinning if the mating screw locked up (which I've also encountered before!). After tightening all those up, I took my Starrett Tap Wrench and Greenlee #6-32 tap, and ran that into all twelve of the standoffs, which certainly needed it. Afterwards, I could easily drive the hold-down screws into the standoffs without stripping out the heads of the 2.0mm Hex driver.
Finally got thru opening up the preamp, fixing the solder joint fractures on the pots and those on J5 & J6 (Heater voltage input, Relay voltage output), and put it all back together.
Mounted the power tube PCB assembly & cover plate, minus the hold-down clamps, and re-connected the harnesses to the preamp, stood the chassis' up on end, supported so the power amp chassis wouldn't tip over, and was able to power up the chassis for the first time to preset the driver tubes bias voltage to -52V for each. Then went thru each of the 6550 power tubes in pairs, using V3/V4 as the test tube sockets and recorded the plate current readings. Swapped a pair of tubes to make a better fit, and put all of the tubes into place.
I almost got a working set of tubes. Just one tube on one side is way high relative to it's other two mates. I tried the Sovtek 6550's that came from that other SVT-CL which has the shorted power xfmr. Those are far worse in the sense of a balanced set. So, now hoping I can find yet another 6550 that will fit better into the plate current match, and then finally see if there ARE noisy tubes in this amp. Gotta get to that point first. So it goes in maintenance on Ampeg SVT's.
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