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This past week's efforts was filled with setting up a newly acquired Ampeg SVT-VR, though it could have been inventory from our New York operation, as I found during disassembly, the preamp PCB wasn't the latest generation. The S/N and our company Asset # weren't in my inventory database, so I started from scratch.
Upon lifting the amp (with help) to the check-out bench at my 'still temporary' shop, I felt both large transformers shift and move within the cabinet. After removing the front grille, unplugging the fan connector from the chassis, turning it around to remove the rear grille panel, when I tipped the chassis up, sure enough, all five chassis mtg screws were loose. Ampeg only uses flat washers under the heads of the M6 x 30mm Chassis mtg screws, which ALWAYS loosen in transit, to the point of falling off, never to be found again, unless you're lucky. So, removed those, adding 1/4" split lock washers under each screw head as I added them to my hardware collection lid during disassembly.
Another indication of vintage, the corner guards found on front and rear cabinet corners don't have the relief in the fold up onto the rear cabinet edges. Older ones have to be removed in order to slide the chassis out of the cabinet, while the newer models have the relief. So, removed those as well. Unplugged the two preamp harness connectors. This one didn't have the separate ground wire that mounts to the chassis screw adjacent to the right-most connector. Older ones, that screw tends to be loose, and just spins when you try and unmount the keps nut holding the solder lug in place. If you're lucky, the factory placed thread-lock compound on the nut/screw shank, while both just spin in the process! They finally moved that wire into unoccupied space in the adjacent connector, and make that ground connection from the preamp chassis to the main chassis INSIDE the power amp chassis.
I slipped off each hold-down clamp from the Sovtek 6550 WE power tubes, marking each tube per position on the base for reference when I get to the tube evaluation stage. Removed the hold-down clamps....make that attempted to remove them. M3.5 x 10mm Truss Head screws, 2mm hex drive, which all too often just strip on your 2mm Hex driver. I just recently ordered Stainless Steel M3.5 x 10mm Truss Head Philips drive screws, as well as #6-32 x 3/8" Truss Head SS Philips drive screws for the earlier SVT-CL or SVT-VR hold-down clamps, being fed up with the battle of getting those hold-down screws removed. The reason the Truss Head screw is used is it has the lowest head height. The base of the 6550 or KT-88 power tubes sit over the head of those screws, and if using standard pan head screws, the base is now rocking on both of those screws instead of sitting firmly into the tube socket.
After I got the hold-down clamps removed, I removed the remaining three M3.5 mounting screws holding the cover plate in place, and lifted out the power tube PCB assembly. I immediately saw one of the two right-angle 1/4" fast-on female terminals scorched, with the first inch of white 14AWG (I think) wire brittle and also discolored. I fished out the bundled excess heater wire from the heater xfmr, cut the tie wrap to get the extra length of wire, and cut the two heater wires back a couple inches, now having fresh wire to work with. Stripped & tinned each, long enough to fold the wire over and crimp onto the PCB's male terminals when I reassemble the amp.
Then, sat down to cut, prep and install the two sizes of hi temp silicon rubber sleeving to fit over the hold-down clamps spring/hat joint (which digs holes into the tube glass, often poking a hole in the tube for failures). Added shorter pieces to fit onto the wings to cradle the top of the tube. Then, using the smaller ID tubing, cut to the circumference of the ID of the hold-down hat, slit open so I could slip that tubing into the ID of the 'hat', cushioning the hold--down clamp onto each tube. Our tube breakage problems have vastly reduced over the years from this treatment. Still get tube failure, but not from having the clamp spring joints drill holes into the glass. Set those clamps aside, and moved onto the chassis.
The core bolts on the Power and Output xfmrs are 8mm hex nut, with slotted screws at the opposite end. Often the slots are filled with varnish from the impregnation process, though on these two xfmrs, that wasn't the case. Sure enough, all four core bolts of the two large xfmrs were loose. Tightened them up, taking care not to over-tighten. I've yet to snap one in the process, but have done so over the years on other xfmrs that were loose. Usually happens on those which are saturated in the impregnation varnish.
I turned the chassis upside down to address the Main Power Amp PCB, as well as the AC Mains PCB and Output Tube PCB. I removed the AC Mains PCB. Newer vintage amps have replaced the #10 Philips sheet metal screws (which threaded into the IEC Mains Connector) with smaller M3 machine screws, with small hex nuts and lock washers, with thread lock compound. Of course getting those out is easier than putting them back into place.
The Neutral terminal of the IEC mains PCB connector had solder joint fracture, as did one of the horizontal Mains Fuse holder terminals. Repaired those joints. Lead-free solder in use, which I hate. All other solder joints looked fine. Tightened the power xfmr mtg screws while I had the PCB lifted out. Those were tight, surprisingly. Put the AC Mains PCB back into place. Then, removed the Output PCB assy. Getting that board out of the chassis is MUCH harder than that of the SVT-CL. I work on SVT-CL's more often, so getting this board out takes having to re-learn how to do it, as it will NOT just lift out...board is too long for that. Gotta remove the xfmr secondary connector & feedback connector, spin the board CW, while lifting the output connector end up until you find it has cleared. Only found one bad solder joint, though all too often, I find the impedance switch with solder joint fractures, as well as on the phone jack terminals. Just one back connection to repair, though I also found one of the fiber washers from the phone jacks was missing. Wasn't still sitting on the inside surface of the chassis. Getting that PCB back into the chassis is always harder, making you wonder how the hell you even got it out of the chassis!
I next stopped to fabricate a brass shield to fit around the Standby Switch in the preamp. A standard feature of Ampeg SVT-VR amps is the HF buzz on Channel 2. That's from the AC mains wires adjacent to the input connector to V5 preamp tube, right next to the switch. I only got varying success with lead dress to that S/B switch. I've since been cutting, punching and folding 5mm thick brass shim stock, soldering the housing together, then applying mylar insulating tape around the bottom and sides to prevent shorting to the preamp components adjacent and below the shield. I still have to route the AC mains wires connected to the S/B switch away from the input connector. The later generation SVT-VR PCB layout has that input connector moved between V4 and V5 tubes, but, you still have to add the shield to minimize the AC mains field being picked up by the preamp tube's grid circuit. Removed the main Preamp PCB for inspection of solder joints, all looked ok.
Then, moved on to the power tubes. I had closely inspected the tube PCB, as well as measured the plate resistors & screen resistors. Found the plate resistors all within a couple percent of each other. I folded the heater wires over the PCB's male fast-on terminals and soldered those into place. Plugged in the O/T primary wires and the I/O connector at the one end of the board. I then installed the clear Lexan Power Tube Service Plate, and mounted it to the six tube sockets, then to the chassis. I have a slot cut into the plate to access the input grids, as well as ground and the cathode circuits of the two sets of tubes. I connected the preamp to the chassis, connected 4 ohm load to the speaker connector, and turned on the amp without power tubes installed. I first wanted to both measure as well as trim the bias controls to set the driver tube's bias voltage the same. This would allow me to see the real difference in transconductance of the power tubes, as I measure the plate or cathode current thru each pair of tubes I plug in. On this amp, the bias voltage was -42.0VDC. I trimmed both the same, then, after powering back to S/B, let the charge drop, and plugged in V3 & V4, using those tube sockets to test each pair of upper/lower halves during this current measurement process. With just two tubes at a time, I measured their cathode current and recorded the findings. I found the range between 22mA and 26mA. I ended up swapping two tubes to get a better balance overall.
.Then, I loaded all six tubes into place, having remarked the two tubes I swapped. The service plate allows connecting the DMM between the plate resistor's source and individual plate, so you can accurately measure each tube's plate current. I recorded those readings, trimmed up the bias to get an average of 24mA per half. In the SVT-VR's Bias LED circuit, it has a window range between 64.4mA and 78mA. So, 72mA plate current is about in the middle of the window.
I cooled down the power tubes, then removed them, removed the Lexan Service Plate, installed the metal cover plate, and mounted each of the hold-down clamps, now fitted with the shock mount tubing, and using the Philips Drive M3.5 Truss Head screws.
I then moved the chassis back into the cabinet, locking into place with the chassis mtg screws. Turned the cabinet around and installed the preamp. Then, moved the amp back to the test bench, spun it around to have the rear panel facing me. I powered up the amp, 4 ohm dummy load connected to the output, and set my oscillator for 40Hz. Switched out of Standby, and plugged in the signal, connected the output to my audio analyzer so I could adjust the Balance control for minimum distortion at 25V RMS output @ 40Hz. The Amber LED should light at minimum distortion, though there's no guarantee on that. That LED's circuit is a differential amp, fed from both upper/lower sets of power tube cathodes, with output feeding thru signal diodes to the next differential amp, one diode reversed so positive half-sine wave signal is fed to the second diff amp. That feeds a two stage transistor circuit that light the Amber LED. The first Diff Amp has 2.2uF caps across the 470k feedback & shunt resistors to give a 1 sec time constant. So, I got the balance set. I then plugged the 4 ohm test speaker into the amp output, and powered up. Turned the preamp controls up, one channel at a time. I was hearing more hum than normal, plus hearing a noisy tube.
Turned the amp around, removed the preamp again, and opened the cover plate. I removed V1 & V2 tubes, and powered back on again, to see what changed. Still hearing hum, so switch back to S/B, and swapped the two 12AU7 tubes V2 & V4, swapped V5 with V1 and listened again. Lower hum, but still hearing noisy tube, so I swapped V1 with V3 (both 12AX7's), and listened again. That sounded better. Then, fed in burst pink noise to check the tone circuits. All sounds normal again for an SVT-VR.
Upon lifting the amp (with help) to the check-out bench at my 'still temporary' shop, I felt both large transformers shift and move within the cabinet. After removing the front grille, unplugging the fan connector from the chassis, turning it around to remove the rear grille panel, when I tipped the chassis up, sure enough, all five chassis mtg screws were loose. Ampeg only uses flat washers under the heads of the M6 x 30mm Chassis mtg screws, which ALWAYS loosen in transit, to the point of falling off, never to be found again, unless you're lucky. So, removed those, adding 1/4" split lock washers under each screw head as I added them to my hardware collection lid during disassembly.
Another indication of vintage, the corner guards found on front and rear cabinet corners don't have the relief in the fold up onto the rear cabinet edges. Older ones have to be removed in order to slide the chassis out of the cabinet, while the newer models have the relief. So, removed those as well. Unplugged the two preamp harness connectors. This one didn't have the separate ground wire that mounts to the chassis screw adjacent to the right-most connector. Older ones, that screw tends to be loose, and just spins when you try and unmount the keps nut holding the solder lug in place. If you're lucky, the factory placed thread-lock compound on the nut/screw shank, while both just spin in the process! They finally moved that wire into unoccupied space in the adjacent connector, and make that ground connection from the preamp chassis to the main chassis INSIDE the power amp chassis.
I slipped off each hold-down clamp from the Sovtek 6550 WE power tubes, marking each tube per position on the base for reference when I get to the tube evaluation stage. Removed the hold-down clamps....make that attempted to remove them. M3.5 x 10mm Truss Head screws, 2mm hex drive, which all too often just strip on your 2mm Hex driver. I just recently ordered Stainless Steel M3.5 x 10mm Truss Head Philips drive screws, as well as #6-32 x 3/8" Truss Head SS Philips drive screws for the earlier SVT-CL or SVT-VR hold-down clamps, being fed up with the battle of getting those hold-down screws removed. The reason the Truss Head screw is used is it has the lowest head height. The base of the 6550 or KT-88 power tubes sit over the head of those screws, and if using standard pan head screws, the base is now rocking on both of those screws instead of sitting firmly into the tube socket.
After I got the hold-down clamps removed, I removed the remaining three M3.5 mounting screws holding the cover plate in place, and lifted out the power tube PCB assembly. I immediately saw one of the two right-angle 1/4" fast-on female terminals scorched, with the first inch of white 14AWG (I think) wire brittle and also discolored. I fished out the bundled excess heater wire from the heater xfmr, cut the tie wrap to get the extra length of wire, and cut the two heater wires back a couple inches, now having fresh wire to work with. Stripped & tinned each, long enough to fold the wire over and crimp onto the PCB's male terminals when I reassemble the amp.
Then, sat down to cut, prep and install the two sizes of hi temp silicon rubber sleeving to fit over the hold-down clamps spring/hat joint (which digs holes into the tube glass, often poking a hole in the tube for failures). Added shorter pieces to fit onto the wings to cradle the top of the tube. Then, using the smaller ID tubing, cut to the circumference of the ID of the hold-down hat, slit open so I could slip that tubing into the ID of the 'hat', cushioning the hold--down clamp onto each tube. Our tube breakage problems have vastly reduced over the years from this treatment. Still get tube failure, but not from having the clamp spring joints drill holes into the glass. Set those clamps aside, and moved onto the chassis.
The core bolts on the Power and Output xfmrs are 8mm hex nut, with slotted screws at the opposite end. Often the slots are filled with varnish from the impregnation process, though on these two xfmrs, that wasn't the case. Sure enough, all four core bolts of the two large xfmrs were loose. Tightened them up, taking care not to over-tighten. I've yet to snap one in the process, but have done so over the years on other xfmrs that were loose. Usually happens on those which are saturated in the impregnation varnish.
I turned the chassis upside down to address the Main Power Amp PCB, as well as the AC Mains PCB and Output Tube PCB. I removed the AC Mains PCB. Newer vintage amps have replaced the #10 Philips sheet metal screws (which threaded into the IEC Mains Connector) with smaller M3 machine screws, with small hex nuts and lock washers, with thread lock compound. Of course getting those out is easier than putting them back into place.
The Neutral terminal of the IEC mains PCB connector had solder joint fracture, as did one of the horizontal Mains Fuse holder terminals. Repaired those joints. Lead-free solder in use, which I hate. All other solder joints looked fine. Tightened the power xfmr mtg screws while I had the PCB lifted out. Those were tight, surprisingly. Put the AC Mains PCB back into place. Then, removed the Output PCB assy. Getting that board out of the chassis is MUCH harder than that of the SVT-CL. I work on SVT-CL's more often, so getting this board out takes having to re-learn how to do it, as it will NOT just lift out...board is too long for that. Gotta remove the xfmr secondary connector & feedback connector, spin the board CW, while lifting the output connector end up until you find it has cleared. Only found one bad solder joint, though all too often, I find the impedance switch with solder joint fractures, as well as on the phone jack terminals. Just one back connection to repair, though I also found one of the fiber washers from the phone jacks was missing. Wasn't still sitting on the inside surface of the chassis. Getting that PCB back into the chassis is always harder, making you wonder how the hell you even got it out of the chassis!
I next stopped to fabricate a brass shield to fit around the Standby Switch in the preamp. A standard feature of Ampeg SVT-VR amps is the HF buzz on Channel 2. That's from the AC mains wires adjacent to the input connector to V5 preamp tube, right next to the switch. I only got varying success with lead dress to that S/B switch. I've since been cutting, punching and folding 5mm thick brass shim stock, soldering the housing together, then applying mylar insulating tape around the bottom and sides to prevent shorting to the preamp components adjacent and below the shield. I still have to route the AC mains wires connected to the S/B switch away from the input connector. The later generation SVT-VR PCB layout has that input connector moved between V4 and V5 tubes, but, you still have to add the shield to minimize the AC mains field being picked up by the preamp tube's grid circuit. Removed the main Preamp PCB for inspection of solder joints, all looked ok.
Then, moved on to the power tubes. I had closely inspected the tube PCB, as well as measured the plate resistors & screen resistors. Found the plate resistors all within a couple percent of each other. I folded the heater wires over the PCB's male fast-on terminals and soldered those into place. Plugged in the O/T primary wires and the I/O connector at the one end of the board. I then installed the clear Lexan Power Tube Service Plate, and mounted it to the six tube sockets, then to the chassis. I have a slot cut into the plate to access the input grids, as well as ground and the cathode circuits of the two sets of tubes. I connected the preamp to the chassis, connected 4 ohm load to the speaker connector, and turned on the amp without power tubes installed. I first wanted to both measure as well as trim the bias controls to set the driver tube's bias voltage the same. This would allow me to see the real difference in transconductance of the power tubes, as I measure the plate or cathode current thru each pair of tubes I plug in. On this amp, the bias voltage was -42.0VDC. I trimmed both the same, then, after powering back to S/B, let the charge drop, and plugged in V3 & V4, using those tube sockets to test each pair of upper/lower halves during this current measurement process. With just two tubes at a time, I measured their cathode current and recorded the findings. I found the range between 22mA and 26mA. I ended up swapping two tubes to get a better balance overall.
.Then, I loaded all six tubes into place, having remarked the two tubes I swapped. The service plate allows connecting the DMM between the plate resistor's source and individual plate, so you can accurately measure each tube's plate current. I recorded those readings, trimmed up the bias to get an average of 24mA per half. In the SVT-VR's Bias LED circuit, it has a window range between 64.4mA and 78mA. So, 72mA plate current is about in the middle of the window.
I cooled down the power tubes, then removed them, removed the Lexan Service Plate, installed the metal cover plate, and mounted each of the hold-down clamps, now fitted with the shock mount tubing, and using the Philips Drive M3.5 Truss Head screws.
I then moved the chassis back into the cabinet, locking into place with the chassis mtg screws. Turned the cabinet around and installed the preamp. Then, moved the amp back to the test bench, spun it around to have the rear panel facing me. I powered up the amp, 4 ohm dummy load connected to the output, and set my oscillator for 40Hz. Switched out of Standby, and plugged in the signal, connected the output to my audio analyzer so I could adjust the Balance control for minimum distortion at 25V RMS output @ 40Hz. The Amber LED should light at minimum distortion, though there's no guarantee on that. That LED's circuit is a differential amp, fed from both upper/lower sets of power tube cathodes, with output feeding thru signal diodes to the next differential amp, one diode reversed so positive half-sine wave signal is fed to the second diff amp. That feeds a two stage transistor circuit that light the Amber LED. The first Diff Amp has 2.2uF caps across the 470k feedback & shunt resistors to give a 1 sec time constant. So, I got the balance set. I then plugged the 4 ohm test speaker into the amp output, and powered up. Turned the preamp controls up, one channel at a time. I was hearing more hum than normal, plus hearing a noisy tube.
Turned the amp around, removed the preamp again, and opened the cover plate. I removed V1 & V2 tubes, and powered back on again, to see what changed. Still hearing hum, so switch back to S/B, and swapped the two 12AU7 tubes V2 & V4, swapped V5 with V1 and listened again. Lower hum, but still hearing noisy tube, so I swapped V1 with V3 (both 12AX7's), and listened again. That sounded better. Then, fed in burst pink noise to check the tone circuits. All sounds normal again for an SVT-VR.
Attached Files