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  • Ampeg SVTCL

    Quick Question.....I had a blown tube which took out a 10 ohm resistor as well......Picking through a bunch of pulled 6550's from previous repairs and the amp started to make all kinds of hissing and static sounds......turns out it was one of the 12AU7's that crapped out....now I have a thump sound in the speaker every 4 or 5 seconds.....and the red bias light for BIAS 2 will flash every time it happens...I even removed all the tubes except for two and switched them to different pairs of sockets...still the same thing....also removed the pre-amp tubes and no difference...anybody ever run into this kind of issue before?? It is getting late here so I am going to sleep on it tonight and try it again tomorrow...I have a schematic but it is for the older version.....this amp has the bias lights on the back panel.....hopefully somebody might be able to shed some light on this one.....
    Cheers

  • #2
    That's one I haven't come upon thus far, amidst all the SVT-CL's, -AV's, -VR's and Heritage, as well as teh SVT2-PRO or earlier SVT II's and original SVT's. An expensive question is...do you have a fresh sextet of 6550's or KT-88's to try in place of the one's you're salvaging? I too go thru the pulls looking for one or two replacements of the same mfgr to restore the four or five good/matched tubes. I can't yet think of what in the system would be giving your a very low frequency cyclic thump, though it does sound like an electrolytic cap charging up and letting loose. Hmmmm........what's in the protection sensing circuit that could do that with a near-matching time constant?

    C14, in the feedback loop of IC1A, in the fault circuit diagram of the preamp schematic...there's two resistors that feed the inverting input of IC1A...R24 (470k) and R18 (220k). Neither yield a RC time constant that high, but....maybe C14 is involved in this 'hickup'.

    Following all he cathode sense circuits that both talk to the Bias Light circuits as well as sum their DC potentials to the Fault circuit summing IC IC2B, the output of that feeds both the AC Control Relay and the Fault Shutdown circuit. C12 in the portion that feeds the AC Control Relay is a 220uF cap, with it's companion resistor R43 have a long time constant...but, they're all well-isolated from the output of the amplifier.

    Does this happen with the Power Amp input jack 'dead-patched' (shorting plug inserted, or open circuit jack,breaking the feed from the preamp?

    Kinda grasping at straws here, as I don't see what has a time constant like that which would couple into the power amp circuit to cause a low frequency cyclic thump. Substantial enough to trigger the RED bias light off of the upper half of the power amp stage only. Odd one for sure.



    I gather the Fault light hasn't been tripping since you removed the failed tube(s).
    Logic is an organized way of going wrong with confidence

    Comment


    • #3
      Originally posted by nevetslab View Post
      That's one I haven't come upon thus far, amidst all the SVT-CL's, -AV's, -VR's and Heritage, as well as teh SVT2-PRO or earlier SVT II's and original SVT's. An expensive question is...do you have a fresh sextet of 6550's or KT-88's to try in place of the one's you're salvaging? I too go thru the pulls looking for one or two replacements of the same mfgr to restore the four or five good/matched tubes. I can't yet think of what in the system would be giving your a very low frequency cyclic thump, though it does sound like an electrolytic cap charging up and letting loose. Hmmmm........what's in the protection sensing circuit that could do that with a near-matching time constant?

      C14, in the feedback loop of IC1A, in the fault circuit diagram of the preamp schematic...there's two resistors that feed the inverting input of IC1A...R24 (470k) and R18 (220k). Neither yield a RC time constant that high, but....maybe C14 is involved in this 'hickup'.

      Following all he cathode sense circuits that both talk to the Bias Light circuits as well as sum their DC potentials to the Fault circuit summing IC IC2B, the output of that feeds both the AC Control Relay and the Fault Shutdown circuit. C12 in the portion that feeds the AC Control Relay is a 220uF cap, with it's companion resistor R43 have a long time constant...but, they're all well-isolated from the output of the amplifier.

      Does this happen with the Power Amp input jack 'dead-patched' (shorting plug inserted, or open circuit jack,breaking the feed from the preamp?

      Kinda grasping at straws here, as I don't see what has a time constant like that which would couple into the power amp circuit to cause a low frequency cyclic thump. Substantial enough to trigger the RED bias light off of the upper half of the power amp stage only. Odd one for sure.



      I gather the Fault light hasn't been tripping since you removed the failed tube(s).
      Thank you for your reply.....The Fault Light has not tripped since I pulled the defective tube and the 10 ohm 2W resistor associated with that tube was open....
      I don't have a new matched set of tubes for this....very expensive.....close to $400......This thump shows up in both bias lights...With bias2 only lighting the red led and bias one lighting up the green led first and lighting up the red led if you increase the bias pot....depending on what tubes are installed on the bias 2 side, even the red led won't light....however you can adjust the amp for practically zero hum from the speakers...I'll check out those components that you mentioned tomorrow and see what I can find.....I have about 20 pulled tubes from other repairs over time so I should be able to get three to match pretty close....I will have to install them into a socket one at a time and set the bias voltage to a certain level...then swap them in and check the individual bias voltage for each tube...aka current draw.....I'll let you know how I make out...if anything else comes to mind let me know...
      cheers

      Comment


      • #4
        Originally posted by nevetslab View Post
        That's one I haven't come upon thus far, amidst all the SVT-CL's, -AV's, -VR's and Heritage, as well as teh SVT2-PRO or earlier SVT II's and original SVT's. An expensive question is...do you have a fresh sextet of 6550's or KT-88's to try in place of the one's you're salvaging? I too go thru the pulls looking for one or two replacements of the same mfgr to restore the four or five good/matched tubes. I can't yet think of what in the system would be giving your a very low frequency cyclic thump, though it does sound like an electrolytic cap charging up and letting loose. Hmmmm........what's in the protection sensing circuit that could do that with a near-matching time constant?

        C14, in the feedback loop of IC1A, in the fault circuit diagram of the preamp schematic...there's two resistors that feed the inverting input of IC1A...R24 (470k) and R18 (220k). Neither yield a RC time constant that high, but....maybe C14 is involved in this 'hickup'.

        Following all he cathode sense circuits that both talk to the Bias Light circuits as well as sum their DC potentials to the Fault circuit summing IC IC2B, the output of that feeds both the AC Control Relay and the Fault Shutdown circuit. C12 in the portion that feeds the AC Control Relay is a 220uF cap, with it's companion resistor R43 have a long time constant...but, they're all well-isolated from the output of the amplifier.

        Does this happen with the Power Amp input jack 'dead-patched' (shorting plug inserted, or open circuit jack,breaking the feed from the preamp?

        Kinda grasping at straws here, as I don't see what has a time constant like that which would couple into the power amp circuit to cause a low frequency cyclic thump. Substantial enough to trigger the RED bias light off of the upper half of the power amp stage only. Odd one for sure.



        I gather the Fault light hasn't been tripping since you removed the failed tube(s).
        Just getting back to this amp....as the owner is not in a hurry and I had a boat load of stuff to get out....anyway, I found the proper schematics for the amp......The main ckt board is version 04....I replaced that blown 10 ohm resistor and I also checked out all the components on the output tube board as well....everything checked out ok.....I re-seated the heater and output transformer connectors on the output board as well.....(there were a couple of times that the heaters didn't light) so....

        I have the amp powered up and looking at the amp from the rear.......the three tubes which reside on the far right hand side was where a tube blew out and took out the 10 ohm resistor....The other side of the amp had all the resistors in tact......When I try to adjust the bias with the two bias controls, the three tubes on the far left hand side have no problem in biasing up.....the tubes that are on the right won't bias up...only the red led lights.... but the hum associated with an out of spec bias can be eliminated by adjusting that associated bias pot.....

        So.....I took the tubes on the left side,(which bias up great), and installed them on the right side.....and installed the tubes from the right side and placed them on the left.....( I switched them around)....the problem remained associated to the right side.....the tubes that are now installed in the left side will light up the red light first and then immediately light up the green light.....(so they are off but not by very much so they should work ok especially for testing purposes)....

        I switched the two 12AU7's around as well but didn't change anything....I replaced the 12AX7 tube with another one and no change....I left these three tubes exactly as is and then I swapped the two sets of output tubes back to their original positions....Once again the left side biases up great but the right side doesn't....only the red led lights up...and I can eliminate the hum by adjusting the right bias control....

        I pulled the main board and looked for overheated components, bad solder connections loose connections, etc....I did solder up some connections that I thought looked questionable and I checked all hi wattage resistors...everything checked fine....so I re-installed the board and ........same thing......It is staying powered up and the fault light is not tripping......and the tubes are fine.....so I have a problem with a component somewhere but I can't seem to track it down.......and...once again, I am open to suggestions.....
        Cheers

        Comment


        • #5
          It seems to me that the ground track for the right side tubes is missing (burned). Especially that the 10 Ohms resistor was burned. Have you check ground track around the resistor?

          Mark

          Comment


          • #6
            Originally posted by MarkusBass View Post
            It seems to me that the ground track for the right side tubes is missing (burned). Especially that the 10 Ohms resistor was burned. Have you check ground track around the resistor?

            Mark
            Actually I didn't....the track does look to be ok but I will double check just to make sure....if the ground track was open at the resistor, wouldn't that be the same as the resistor burned open???and tripping the Fault light?? The fault light is not tripping out now...but it was before......
            Cheers,
            Bernie

            Comment


            • #7
              On the main PCB, Cathode resistors for V1, V2, V3 & V5 are sharing the ground buss that runs along the edge of the PCB, while V4 & V6 are on a different ground trace, that does join up with the others. Reading between the lines of 'bsco's failure notes, the failure is on the upper half (V1, V2, V3), though I don't know which one. If the ground trace opened for that set, you'd also have V5 without a ground return. Easy enough to check with an ohmmeter
              Logic is an organized way of going wrong with confidence

              Comment


              • #8
                When you had the power tube PCB out, I assume you checked the screen resistors on that board. The correct value is 220 ohm 1/2W. They used to have 22 ohm with a diode across each, but those got changed out to 220 ohm. I've been using small body 220 ohm 1W Metal Oxide in my service on these. I've often found solder fractures on the power tube socket pins on that board, though it's not as common as the fractures on the rear panel parts of that main board below it. The bias pot terminals are prone to develop solder fractures, and if not fixed, you'll have bias problems on them. I go hunting with surgical loupes and fiber optic headware (as seen in my avatar) Anything looking dodgy, I de-solder and re-solder. I've also found bad conduction with lead-free solder joints. Hate that stuff!!

                You may also be dealing with failure in IC3 & IC4, which are the two comparator IC's that run the bias tally lights. Threshold voltages are 147mV and 294mV, if memory serves.

                When I'm putting an output stage back together, I have the power amp chassis standing on end (block under the power transformer), so I can work with just one pair of tubes at a time (such as V4, V3....V3 being upper half, V4 lower half). The bias lights won't have anything to do with plate current flow. I monitor that thru the individual cathode resistors, fond along the edge of the main board (V3, V1, V2, V5 reading towards the center edge, and V6, V4, set in from the edge a bit, above a cluster of electrolytic caps C31 thru C36). Check & print out the component placement of the board, so you have a 'road map' of the component placement to check as you go.

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                These images were from a set-up where I had tack-soldered small test hoops on each cathode resistor, allowing the scope to hang off any of the cathodes for observing the effective current flow under drive of each tube. Not needed in just getting basic readings across the cathode resistors in batching and grouping.

                I always mark the power tubes on their bases as to which tube is which as I remove them. Doesn't mean they will end up in that arrangement on the re-build.

                I will set initial bias with one pair, and usually with the bias pots set around 11 o/clock, looking for 22mA on each side as a target. Then, I'll go thru one pair at a time, and mark down what each measures as, before I start loading up the sockets. In your case, I'd take one known good pair, and move them to each tube socket in the triad, to verify each tube position is good, with regards to screen, input grid circuits go, as well as cathode.

                Once I have known good tube socket circuits, then I'll look at the nominal current levels, and may juggle the tubes to group them in best match. I often have to re-balance the upper vs lower to get a better balance during this phase of re-build. The Bias Lights won't tell you anything until all three tubes on each triad are installed. (assuming the comparator circuits are working correctly). I'll try to park the two groups centered around 22-24mA, with no more than 3mA in either direction, if I"m so lucky. It's a juggling act. If you've got a couple tubes well outside the range of the rest, say one reading 15mA, and another 33mA, with the rest in the 20-24mA range, you'll have to see what can be best balanced with the high one and low one....as you do have separate bias pots to work with.
                Last edited by nevetslab; 11-09-2015, 01:26 AM. Reason: wording
                Logic is an organized way of going wrong with confidence

                Comment


                • #9
                  Originally posted by nevetslab View Post
                  When you had the power tube PCB out, I assume you checked the screen resistors on that board. The correct value is 220 ohm 1/2W. They used to have 22 ohm with a diode across each, but those got changed out to 220 ohm. I've been using small body 220 ohm 1W Metal Oxide in my service on these. I've often found solder fractures on the power tube socket pins on that board, though it's not as common as the fractures on the rear panel parts of that main board below it. The bias pot terminals are prone to develop solder fractures, and if not fixed, you'll have bias problems on them. I go hunting with surgical loupes and fiber optic headware (as seen in my avatar) Anything looking dodgy, I de-solder and re-solder. I've also found bad conduction with lead-free solder joints. Hate that stuff!!

                  You may also be dealing with failure in IC3 & IC4, which are the two comparator IC's that run the bias tally lights. Threshold voltages are 147mV and 294mV, if memory serves.

                  When I'm putting an output stage back together, I have the power amp chassis standing on end (block under the power transformer), so I can work with just one pair of tubes at a time (such as V4, V3....V3 being upper half, V4 lower half). The bias lights won't have anything to do with plate current flow. I monitor that thru the individual cathode resistors, fond along the edge of the main board (V3, V1, V2, V5 reading towards the center edge, and V6, V4, set in from the edge a bit, above a cluster of electrolytic caps C31 thru C36). Check & print out the component placement of the board, so you have a 'road map' of the component placement to check as you go.

                  [ATTACH=CONFIG]36411[/ATTACH] [ATTACH=CONFIG]36413[/ATTACH] [ATTACH=CONFIG]36417[/ATTACH] [ATTACH=CONFIG]36418[/ATTACH]

                  These images were from a set-up where I had tack-soldered small test hoops on each cathode resistor, allowing the scope to hang off any of the cathodes for observing the effective current flow under drive of each tube. Not needed in just getting basic readings across the cathode resistors in batching and grouping.

                  I always mark the power tubes on their bases as to which tube is which as I remove them. Doesn't mean they will end up in that arrangement on the re-build.

                  I will set initial bias with one pair, and usually with the bias pots set around 11 o/clock, looking for 22mA on each side as a target. Then, I'll go thru one pair at a time, and mark down what each measures as, before I start loading up the sockets. In your case, I'd take one known good pair, and move them to each tube socket in the triad, to verify each tube position is good, with regards to screen, input grid circuits go, as well as cathode.

                  Once I have known good tube socket circuits, then I'll look at the nominal current levels, and may juggle the tubes to group them in best match. I often have to re-balance the upper vs lower to get a better balance during this phase of re-build. The Bias Lights won't tell you anything until all three tubes on each triad are installed. (assuming the comparator circuits are working correctly). I'll try to park the two groups centered around 22-24mA, with no more than 3mA in either direction, if I"m so lucky. It's a juggling act. If you've got a couple tubes well outside the range of the rest, say one reading 15mA, and another 33mA, with the rest in the 20-24mA range, you'll have to see what can be best balanced with the high one and low one....as you do have separate bias pots to work with.
                  Ok. thank you for that post.....I think I have found the problem.....actually I have two problems.......the first problem seems to be the first op-amp stage in IC3..... the tube socket that originally had the bad tube goes to pin 1 of the grey ribbon cable connector.....from there it goes to a 1K resistor.....and there are three of them there connected to the inputs of the op-amps....R72 was reading around 600 ohms both ways in circuit.....So I compared the components with the good side....IC4..... and checked those 1K resistors...checked good...So I removed IC3 and checked the resistor in question again and now it reads the correct value....I checked the chip and pins 1,2,and 3 don't read right...they are not shorted but they do measure leakage.....so I replaced it.....and once again i measured R72 and the reading was good.......But I still have a problem......the tubes wouldn't light up...the amp stays in protect.....and that happened a couple of times while I was working on it......and upon a closer inspection, the female connectors for the white heater harness are corroded from the heat generated by the high current....I will have to go to an appliance shop Monday so I can get the proper female connectors...The male spade lugs are in very good shape......these will have to be crimped....what I plan to do is tin the wires, crimp them nice and tight, then add a drop of solder to them as well.......I will post back here later next week to let you know how it worked out......Once again, Thank you very much for that post......
                  Cheers........

                  Comment


                  • #10
                    OK. The amp worked out fine after replacing the above mentioned parts....Ran it for a few days to make sure it was ok......

                    Comment


                    • #11
                      Excellent! Glad to have been of assistance. Now, if I can just get a solution to the Ashdown BTA-400 that is refusing to properly drive loudspeakers without making rude body noises and scary outbursts, I'll be a happy camper.
                      Logic is an organized way of going wrong with confidence

                      Comment


                      • #12
                        Originally posted by nevetslab View Post
                        Excellent! Glad to have been of assistance. Now, if I can just get a solution to the Ashdown BTA-400 that is refusing to properly drive loudspeakers without making rude body noises and scary outbursts, I'll be a happy camper.
                        Ahhhh...I thought you guys knew everything....lol....Thank you very much for the help....
                        Cheers

                        Comment


                        • #13
                          Don't tin before crimping. If you don't trust the crimp, you can crimp and solder.

                          Comment


                          • #14
                            Originally posted by TimmyP1955 View Post
                            Don't tin before crimping. If you don't trust the crimp, you can crimp and solder.
                            Too late......I returned the amp to it's rightful owner....what is the reasoning behind your suggestion? Cheers
                            P.S. Thank you for your input......

                            Comment


                            • #15
                              As he said, if you are doing both, solder last.
                              When you crimp a tinned wire, the crushing action of the crimp can make the solder go resistive, like a bad ("cold") solder joint.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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