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Sometimes just getting an Ampeg SVT-CL/AV/VR apart is a daunting task!

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  • Sometimes just getting an Ampeg SVT-CL/AV/VR apart is a daunting task!

    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.
    Last edited by nevetslab; 02-18-2020, 06:25 PM.
    Logic is an organized way of going wrong with confidence

  • #2
    These power board hex screws are PAIN to work with.
    I need to finish my work with the SVT-CL this month that had some serious issuses, but when I think about unscrewing power tube board it nowe makes me sick...

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    • #3
      Originally posted by boroman View Post
      These power board hex screws are PAIN to work with.
      I need to finish my work with the SVT-CL this month that had some serious issuses, but when I think about unscrewing power tube board it nowe makes me sick...
      Since I had run the tap thru all twelve standoffs, that solved that hex driver problem. I have both M3.5 taps as well as #6-32 taps, as Ampeg in the later production models changed to M3.5 hardware. These in this amp I'm still working on are #6-32.

      After I was able to power up this chassis, I've now encountered a nightmare. There's a substantial turn-on thump/pop, and, after the Status LED on the preamp panel goes GRN, I get hum, having the sound you hear when something is pulling excess current. I can dial it out by mis-adjusting the bias pots, but that's not a normal solution. I had gone thru the main power amp PCB repairing solder joint fractures, but, I don't think I've found all of them. I can set this off into it's hum state after switching out of Standby by tapping on any number of locations. I've dead-patched the power amp input, so it's NOT coming from the preamp. The first time I encountered this....the preamp chassis touched the power amp chassis (both standing on their sides, giving access to the bottom side of the power amp PCB). I though I had found it, finding I had failed to fully tighten the grounding wire inside the preamp chassis, but that wasn't it.

      So, the hunt is on. I've swapped out the two driver tubes with new ones. I think I'll return to the previous drivers, as these new ones, I have to crank the bias pots to 3 o'clock instead of 12 o'clock positions just to have the same plate current levels. I've replace the 1st power supply filter caps C15/C16 (100uF/450V), along with the bias supply cap C23, having found the release vents had bulged out. I checked ESR on the rest of the caps this morning in-circuit, and am seeing nominal readings, along with balanced voltages with C17/C18 stack, and low ripple on the supplies, highest on the 1st filter stage feeding the OT C/T, of course. I don't see the ripple change when the hum pops up after tapping on the PCB or tubes to excite the condition.

      So, I guess I'll start by sequentially re-soldering parts on the input tube of the power amp. I had already found I had failed to complete re-soldering the cathode of V1-pin 8, but repairing that wasn't it. Just hoping I don't have to replace parts on the fly, as getting into/out of the main PCB is such a PITA!
      Logic is an organized way of going wrong with confidence

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      • #4
        After de-soldering and re-soldering the input tube V1 of the power amp chassis, then doing same with the plate, cathode resistors and coupling caps to the driver stage tubes, I no longer had the vibration-sensitivity that was causing the hum waveform to come and go. I gave a listen, but it sounded distorted. With the scope connected, back to sine wave and dummy load, I saw asymmetrical clipping. Swapped the driver tubes, having first gone back to just one pair of power tubes to make matters simple. The problem moved with the driver tubes. Measured DCV on the two drivers, finding only one of them was conducting, and nothing on the other. So, I swapped both driver tubes out, not having another similar Electro-Harmonix ECC81. Put in a pair of new J/J ECC81's, and reset the bias. The bias pot settings are now much further clockwise then before, but, such is life. I at least now have good signal appearing on both, so it wasn't a component issue, which would have required more disassembly. Put the rest of the power tubes back in, rebiased and gave a look. I can now get full power out, so connected the speaker, patched signal into the preamp, and it sounds like an SVT-AV again.

        Now, I just have to cool the power tubes down, remove them, add the hi temp sleeving onto the hold-down clamps (protection on the power tubes during transit), and put it all back together. This amp took a lot more time to restore to working order than I had planned on spending. Tube noise indeed!
        Logic is an organized way of going wrong with confidence

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        • #5
          I really enjoy these 'tales from the trenches,' thanks for sharing. I particularly like the ones that make my day seem like cake.

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          • #6
            Right?!?

            And @nevetslab reminds me of some golfers I know... They can play 18 holes, then sit and recount every detail of every stroke. Somehow, though, watching golf is soporific, while these accounts are much more interesting. Go figger.
            --
            I build and repair guitar amps
            http://amps.monkeymatic.com

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            • #7
              Always satisfying to find the amp went back together without a hitch. Not that I had planned on it taking me 17 bloody hours to get thru with all the details. When I first preset the bias levels for the upper/lower output stage to -52VDC, I recorded the plate current of each tube, installed in single pairs. That yielded two ranges of plate current among five of the six tubes, with the sixth tube much higher, after grouping the tubes into two triads. I pulled those Groove Tube 6550's out, and put the Sovtek Wing C 6550 tubes in from the SVT-CL (with the shorted power xfmr), but that set was all over the map....I wasn't able to get anything reasonable in two sets, though they all did work. The Groove Tube 6550's weren't Sovtek....might be Svetlana (will have to look at some I have on hand to verify). I don't normally mix mfgrs tubes within a set, but in this case, all I had that was close was one of the Sovtek tubes, and it balanced ok. I had thought of using a triad of Sovteks with a triad of the GT's but couldn't even get a decent triad from that Sovtek set to do that with.

              It IS satisfying to get to the conclusion of one of these restorations and be able to pound on the cabinet, jar it, shake it, not feeling a 30 lb weight within shift on you, and nothing objectionable happens from your physical abuse. Solid amp again as it should have been. Sometimes these things just take you for a ride if they know they can coerce you into it.

              And, just to make my day more complete, the client's employer (AuntieM) dropped off a pair of Ampeg SVT4-Pro's that need work. The Fender Hot Rod DeVille 410 that accompanied them will be simple. Just when I thought I was going to be able to slip away to spend a week doing loudspeaker maintenance at the warehouse. At least I enjoy the chase!
              Logic is an organized way of going wrong with confidence

              Comment


              • #8
                I get your point.
                My SVT-CL has the same symptoms. Excessive hum (like twice the normal) but it wass running OK. After I took it apart to trace hum (tubes were not an issue), strange things happened. The R14 blew, then a screen resistor blew (all without power tubes on board). In the end, no voltage to the preamp tubes is fed, they're not glowing and the amp stays in red (does not go out of stanby anymore) I have recapped whole power/bias section. No bad solder joint. I have spent so many hours tracing the problem. But you know, those problems were striking in a cascade, so it's hard for me to judge what it wrong now. I suspect one of the ribbon cables, I just bought one to test, we will see.

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                • #9
                  Originally posted by boroman View Post
                  I get your point.
                  My SVT-CL has the same symptoms. Excessive hum (like twice the normal) but it wass running OK. After I took it apart to trace hum (tubes were not an issue), strange things happened. The R14 blew, then a screen resistor blew (all without power tubes on board). In the end, no voltage to the preamp tubes is fed, they're not glowing and the amp stays in red (does not go out of stanby anymore) I have recapped whole power/bias section. No bad solder joint. I have spent so many hours tracing the problem. But you know, those problems were striking in a cascade, so it's hard for me to judge what it wrong now. I suspect one of the ribbon cables, I just bought one to test, we will see.
                  So, you're now in Protect mode (hence, no HT voltages to run the preamp/power amp tubes. Now, very strange that one of your six screen resistors would blow under this condition. Or, did you just discover it was open. Can't blow with no current flow thru it.

                  The protection circuit starts with sensing voltage off of six power tube cathode resistors R35-R40 (10 ohm 3W 1%). The cathode voltage is sampled and summed thru the 2.2k resistors and diodes, and feed the comparator circuit at U2B. It's comparing that voltage with what you get at the junction of C13/R48/R50. So, the bipolar supplies (+/-15V) MUST be up and running. The output from this comparator goes both to the LED circuit in the preamp, as well as to the AC Mains Relay board. The nominal level when all is well is +14V, while Fault condition it's -14V. In the preamp, that Protect signal will be either +14V (normal), or -0.6V (Fault). There's +345V that comes into play thru R34 and summed with -15V thru two resistors R34 & R5, which feed IC1B. It's there to also tell you the HT supply is on....otherwise, you'll just get RED. IC1A is the flashing oscillator. You only get that +345V if the AC Mains board's relay is pulled in. So, you'll have to do some digging. The Bipolar supplies are, of course, on the main power amp PCB, underneath the power tube PCB. There's a pair of thermistors CB1 & CB2 that have to be good in order to have the +/- 20V unregulated supply, which then is zener regulated down to the +/-15V. I've had the zeners fail, the caps at either side of the 30 ohm/5W dropping resistor fail. So, that protection circuit sends you all over the place in the amp to spot the loony.
                  Logic is an organized way of going wrong with confidence

                  Comment


                  • #10
                    Originally posted by nevetslab View Post
                    So, you're now in Protect mode (hence, no HT voltages to run the preamp/power amp tubes. Now, very strange that one of your six screen resistors would blow under this condition. Or, did you just discover it was open. Can't blow with no current flow thru it.
                    It blew before I got a proetect mode fault. Before it was OK.

                    The protection circuit starts with sensing voltage off of six power tube cathode resistors R35-R40 (10 ohm 3W 1%). The cathode voltage is sampled and summed thru the 2.2k resistors and diodes, and feed the comparator circuit at U2B. It's comparing that voltage with what you get at the junction of C13/R48/R50. So, the bipolar supplies (+/-15V) MUST be up and running. The output from this comparator goes both to the LED circuit in the preamp, as well as to the AC Mains Relay board. The nominal level when all is well is +14V, while Fault condition it's -14V. In the preamp, that Protect signal will be either +14V (normal), or -0.6V (Fault). There's +345V that comes into play thru R34 and summed with -15V thru two resistors R34 & R5, which feed IC1B. It's there to also tell you the HT supply is on....otherwise, you'll just get RED. IC1A is the flashing oscillator. You only get that +345V if the AC Mains board's relay is pulled in. So, you'll have to do some digging. The Bipolar supplies are, of course, on the main power amp PCB, underneath the power tube PCB. There's a pair of thermistors CB1 & CB2 that have to be good in order to have the +/- 20V unregulated supply, which then is zener regulated down to the +/-15V. I've had the zeners fail, the caps at either side of the 30 ohm/5W dropping resistor fail. So, that protection circuit sends you all over the place in the amp to spot the loony.
                    My God, this is brilliant explanation! I'll save it for the day I'll get back to this amp... I do think, still, it might be the ribbon cable.

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                    • #11
                      While digging thru my notes to find the labor hours I had spent on the SVT-CL I was servicing back on 2/05/20, which had the shorted power transformer, I read thru the notes on the client' SVT-AV, the amp in this thread. When I was writing up the invoice for the client, I had discovered I installed ECC81 tubes for the drivers. 12AT7's! Opps! I had then opened the amp back up, removed the ECC81's and put in the correct ECC82 (12AU7) drivers, and took a short cut in biasing up the amp....using the bias LED's to land the operating point in the middle range between No LED and RED/GRN for each channel. Then, fed 100Hz into the Power Amp input, dummy load, and used the distortion analzyer to land the bias for best balance. Removed the signal, verified the AC mains current/power was still the nominal 2.2A/230W @ 120V/60Hz. All sounded nominal.

                      I was a little surprised nobody caught that mistake. As time goes on, I mix ECC82's and ECC81's up in my head, as I did that day when I thought I was thru with the amp. Old age catching up with me, I guess.

                      The client needs a fresh set of tubes on his SVT-AV, and was wanting to pick up the broken SVT-CL with the shorted PT. I had left off not yet finding a replacement PT for it, and pay me for the labor I had put into it (6hrs, and was willing to take 3 hrs labor's fee back then). I'm hoping he might be willing to let me buy the amp as-is for, say $75, since it still needs a new main Power Transformer, as well as a new matched set of KT-88 power tubes, and someone's labor to put it back together. Not a mere trifle of $$$. My thoughts for that amp is to turn it into a power tube test fixture for SVT's. No desire to own it as a working amp these days, it being way to heavy to lug around. But, it works really nice for sorting power tubes! We'll see what happens.
                      Logic is an organized way of going wrong with confidence

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                      • #12
                        I fixed one SVT-CL and that was the last one. I can turn work away if I want and that mess of an amp cured me of ever wanting to work on one again.
                        --Jim


                        He's like a new set of strings... he just needs to be stretched a bit.

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