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Thread: Ampeg SVT

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    Ampeg SVT

    Good afternoon.....anybody here on this forum with experience with these amps??? I have one here that had some hardware rolling around and blew the main fuse.....outside of that I can't find anything blown.....but it won't come out of protect mode.....(the front panel light flashes between green and red at a fast rate)....and with such high voltages and current in this unit, you just can't have the boards hanging off the chassis..... so how do you guys service these monsters when you have serious issues besides the regular tube issues......I can't get any voltage readings on the output tubes but that must be due to the protect.....and you can't measure any voltages with the tubes removed as well......comments (and insults...lol) kindly welcome....
    Cheers

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    Old Timer Tom Phillips's Avatar
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    There are many versions of the "Ampeg SVT."
    Please post the exact model number/name of the SVT you have. A copy of the schematic would be good to include to speed the help process. Photos too if you can.

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    Quote Originally Posted by Tom Phillips View Post
    There are many versions of the "Ampeg SVT."
    Please post the exact model number/name of the SVT you have. A copy of the schematic would be good to include to speed the help process. Photos too if you can.
    Thanks Tom for the quick reply.....It is an Ampeg SVT CL........it uses six 6550 output tubes and two 12AU7's and one 12AX7......I am getting ready to email Loud for the schematics.....(should have them by tomorrow) and I'll post it for you.....
    Cheers

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    Supporting Member The Dude's Avatar
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    Schematics should be the ones in post #1 of this thread:

    http://music-electronics-forum.com/s...&highlight=svt

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    Senior Member nevetslab's Avatar
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    Usually when the SVT-CL has gone into protect mode, it's one of the power tubes that has failed, though with hardware loose inside....very possibly one of the power or output xfmr mtg screws, they can land in the wrong place and do serious damage.

    Before powering up again, I'd remove the preamp assembly first (from the front). Unplug the Power Switch wires (usually BLK), and the Standby Wires (usually Yellow). If other color codes, take notes so you can restore them. Unplug the 5-pin power connector to the preamp PCB, the Ribbon cable that carries the I/O to/from the preamp, and the two 2-cond cables at the left end of the PCB. White wires are the Heater wires that plug into J5, and the Blue wires are DC rectified wires of the heater, connecting to J6.

    Then, remove the 5 chassis screws that secure the power amp chassis to the cabinet, and slowly slide the chassis out.

    I'd start by removing the power tubes, one by one, and marking their positions with a Sharpie on the tube base. Positions are as follows, looking at the rear, left to right. Row closest to the rear panel....V4, V2, V3, then back row (closest to front of amp)....V5, V6 and V1. With the power tubes out and set aside, you still need to find what was rolling around loose inside. You can remove the power tube PCB assy, 8 screws hold it into place. Two large white wires, with rt angle fast-on connectors carry the 6.3VAC heater voltage to this PCB, and distributes it to the rest of the system..by way of the short ribbon cable at one end, and the other end of the white 2-cond cable that goes to the preamp. That leaves the BRN wire (O/T Primary C/T), the Blue and Blue/White wire that are the Primary upper/lower taps. With those unplugged, you can remove the power tube PCB assy, exposing the top side of the main power amp PCB. You can now start looking for damage on the main PCB.

    You won't be able to power up the chassis until you have the power tube PCB assy plugged back in, since it carries the heater voltage that ultimately runs the AC mains relay board that turns on the HT Xfmr.

    With the power tubes removed, and the preamp re-connected to the power amp chassis wiring, I'd try powering up the amp again.....best be done with a Variac & a power analyzer/ammeter so you can see what the AC mains current draw is. In standby, no power tubes installed, it will draw 200mA in Standby, and around 640mA in Operate (HT Xfmr now on).

    If the amp (without power tubes) comes up and is no longer in Protect mode, you can try installing two power tubes at a time. If you lost one of the power tubes, it will fire Protect as soon as you switch out of S/B. Might see a bright flash of light in the bad tube, so be ready to go back to S/B immediately.

    Let us know what you find so we can direct you further. I deal with SVT's on a regular basis, maintaining a sizeable inventory of them at CenterStaging in Burbank, CA.

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    Last edited by nevetslab; 08-03-2018 at 05:12 AM.
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    You might want to check the Ampeg Technical Information Bulletin referenced in this thread:

    http://music-electronics-forum.com/s...25530#poststop

    Older SVT-CLs need to be modified to use modern 6550s, it's work checking that the mods in the TIB have been performed.

    More SVT CL Service Bulletins here:
    http://music-electronics-forum.com/s...ad.php?t=34501

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    Senior Member nevetslab's Avatar
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    I should have said something about safety up front. Digging into any product like this, or even simple ones that get their power from the AC mains, there is shock hazards inside. And tube amps with their high voltage supplies, there can still be stored charge in the supply caps. I've been nailed at least once by stored charge in an SVT-CL in pulling the Power Tube PCB assembly out. Under normal operation, they will discharge to near zero and become safe to handle, but, in abnormal status, where failures have occured, those discharge paths are sometimes not there.

    All of us working inside this sort of gear have a discharge tool to bleed off stored charge in the power supply caps. Mine is a Dale 100 ohm 50W power resistor with short test probe leads attached to it that I've been using for decades. I don't go digging inside without discharging all of the supplies, making it safe to handle. And, of course, disconnecting the amp from the wall is paramount!

    There are numerous posts on MEF for repairs & troubleshooting methods on the SVT-CL, SVT-AV, SVT-VR amps. Go to Advanced Search and type in Ampeg SVT-CL, and you'll find many pages of posts....I've posted many there, and currently have one running, posted a few days ago, regarding an amp that wouldn't come out of Standby, but wasn't in Protect mode. It maps out the control logic in how these amps power up.

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    Senior Member TimmyP1955's Avatar
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    Remove the mains inlet board and make sure there's no loose hardware under there - I've had that happen. While it is out, check that the transformer bolts are tight, and inspect the mains inlet's solder joints.

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    Senior Member nevetslab's Avatar
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    Same with under the Output PCB. One of the O/T mounting bolts is under it. You do have access to the power transformer mtg bolts, the hidden one under the AC mains PCB assy. The other pair of O/T mounting bolts are hidden under the main PCB, which would have to be lifted out and up to gain access to them.

    While you have the AC Mains/Relay board out, inspect for solder fractures.....I regularly find them on the IEC mains connector, the fuse holder as well as the Grounding Screw bracket. On the output PCB, not uncommon to find them on the phone jacks, the NL4 connector as well as the impedance switch and the Transformer Wires connector, and the feedback connector.

    Since you have the chassis out for service, look closely at the solder connections of the two bias pot, the phone jacks, the XLR connectors, as those are being used as mechanical support to the main PCB! Look also at the 0.156" spaced 5 pin header connections, one being input from the low voltage Xfmr, the other the I/O voltages to the preamp on the front edge of that PCB. And, on the tube connectors.......in general, give it a good inspection. The sheer weight and mass of that SVT amp over time inflicts damage to connections. Same holds true in the preamp....the front panel pot solder connections are a common problem, as are the harness connector headers.

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    Quote Originally Posted by nevetslab View Post
    Same with under the Output PCB. One of the O/T mounting bolts is under it. You do have access to the power transformer mtg bolts, the hidden one under the AC mains PCB assy. The other pair of O/T mounting bolts are hidden under the main PCB, which would have to be lifted out and up to gain access to them.

    While you have the AC Mains/Relay board out, inspect for solder fractures.....I regularly find them on the IEC mains connector, the fuse holder as well as the Grounding Screw bracket. On the output PCB, not uncommon to find them on the phone jacks, the NL4 connector as well as the impedance switch and the Transformer Wires connector, and the feedback connector.

    Since you have the chassis out for service, look closely at the solder connections of the two bias pot, the phone jacks, the XLR connectors, as those are being used as mechanical support to the main PCB! Look also at the 0.156" spaced 5 pin header connections, one being input from the low voltage Xfmr, the other the I/O voltages to the preamp on the front edge of that PCB. And, on the tube connectors.......in general, give it a good inspection. The sheer weight and mass of that SVT amp over time inflicts damage to connections. Same holds true in the preamp....the front panel pot solder connections are a common problem, as are the harness connector headers.
    Thank you for your info...I had to go do some other things for the last few days and I am ready to get back at this one again....I do have one question......when you first turn on the power,and leave the amp in standby. the front led flashes between red and green at a fast rate.....is this normal?? and when you turn the standby switch to the on or run position, the front led still flashes......which will indicate a problem and the amp goes into protect mode....but when it is actually in standby it appears that it is still in protect mode...unless that is normal......If I can get that clarified, that would be a great help.......
    Cheers,
    P.S.Yamaha is now the agent for Ampeg.....

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    Ok. Did some more poking around and managed to power it up with the power amp board hanging off the chassis and access to the pre-amp board as well.....(I did remove the 12AX and the two 12AU's in order to get the board out...not sure if this makes a difference)......the 15V supplies (both of them) are resting at around 4VDC...... the high side of the 47 ohm resistors were at 15V + and -.........I checked the IC on the pre-amp board and that checked ok....I had another new one and popped it in...same issue....so.....there is something going on with the pre-amp board but everything checks out fine.....unless I am missing something here....there is supposed to be a +135 line coming into the pre-amp board and going through a 1M dropping resistor to the IC....that voltage is not there either....so am I correct in assuming that the unit needs that 135V in order to power up?????????..........I am going to have a few beer now and lite up the BBQ.....and relax....any suggestions as usual are greatly appreciated.....

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    Senior Member nevetslab's Avatar
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    SVT Still in Protect Mode (flashing RED Status LED)

    Quote Originally Posted by bsco View Post
    Thank you for your info...I had to go do some other things for the last few days and I am ready to get back at this one again....I do have one question......when you first turn on the power,and leave the amp in standby. the front led flashes between red and green at a fast rate.....is this normal?? and when you turn the standby switch to the on or run position, the front led still flashes......which will indicate a problem and the amp goes into protect mode....but when it is actually in standby it appears that it is still in protect mode...unless that is normal......If I can get that clarified, that would be a great help.......
    Cheers,
    P.S.Yamaha is now the agent for Ampeg.....
    Yes. When the amp has gone into Protect mode, it remains in that state, with the HT Xfmr turned off, until the amp is physically powered off. When it is powered back on, it won't be in protect. If the condition (whatever it is) hasn't been corrected, it will trigger IC2B on the main power amp board to go into Fault Mode. There is a difference between Fault Mode and Protect Mode. Fault mode, the output from IC2B will be around -14VDC, whereas in Normal mode, it will be around +14V. The output in Fault mode at the 5-pin connector J20 (on power amp PCB) is dropped to -0.6VDC at Pin 1. That signal passes to the Preamp PCB where the difference between Fault and Standby Mode is the presence of the +345VDC applied thru a 1M resistor R34 to the comparator circuit IC1B. In Protect Mode, there is NO +345VDC, since the HT Xfrm has been turned off by the control line going out J12 on the power amp PCB to Q1 on the AC Mains Relay board via J36.

    The slow oscillation for the LED to flash is by way of IC1A.

    Now, what is causing your SVT-CL to be in Protect Mode, or NOT coming out of Standby Mode, which was a condition I just had to deal with in the past couple weeks, and posted a thread on that subject.

    Usually it takes a power tube having failed or pulling abnormally high current to trigger IC2B to change states. It's set up as a comparator, and has two states: Normal (output at Pin 7 is High +14V) or Fault (output at Pin 7 is Low -14V). If there are no power tubes installed, and it's in Protect Mode, then it's possible IC2 has failed, or parts in the circuitry have gone bad, or, parts in the summing network feeding the inverting input (Pin 6) have gone high and holding at that level. Associated with this Fault Comparator circuit is the Bias Window Comparator, that sometimes suffers a failed IC or part feeding the two Window levels.

    Now, in your amp, your initial post reported of loose hardware inside. Dangerous condition, and you may still be suffering from the results of the some damage caused. I've had HV rectifiers fail on the main Power Amp PCB, but that will blow the mains fuse on every attempt to power up. I've also had LV rectifier failure, or cap failure that doesn't pull high current, but causes misbehavior. That was the case on what I had recently written up, where the AC Mains Relay board kept trying to turn on the HT Xfmr, but the LV power supply couldn't properly charge up and remain up.

    When I troubleshoot the control circuitry on an SVT-CL, I do it with the power amp PCB mounted and wired up, the 3 tubes for the power amp stage in place (both 12AU7's and the 12AX7), but NO power tubes installed. I have the power amp chassis upside down on the bench with the Preamp connected via AC / Standby wires and the I/O harness wiring connected. You have to have all that connected to complete the control circuitry. Then, with the parts placement drawing of the power amp PCB in hand, and time to find all the pertinent circuits on the foil side of the main power amp board, I go probing carefully in the areas of concern. If the Power Amp will turn on and remain in Operate, there is DANGEROUS HIGH VOLTAGE PRESENT! Only in rare cases am I running the main PCB and tube PCB live and dangling in some controlled state while probing, and actually can't think of a time that I've done that.

    You might want to read thru my recent SVT-CL post "Ampeg SVT-CL ht xfmr cycling on and off" on July 31, I think it was, as it does describe how the control circuitry behaves, or misbehaves as was that case until I found the culprits.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by bsco View Post
    Ok. Did some more poking around and managed to power it up with the power amp board hanging off the chassis and access to the pre-amp board as well.....(I did remove the 12AX and the two 12AU's in order to get the board out...not sure if this makes a difference)......the 15V supplies (both of them) are resting at around 4VDC...... the high side of the 47 ohm resistors were at 15V + and -.........I checked the IC on the pre-amp board and that checked ok....I had another new one and popped it in...same issue....so.....there is something going on with the pre-amp board but everything checks out fine.....unless I am missing something here....there is supposed to be a +135 line coming into the pre-amp board and going through a 1M dropping resistor to the IC....that voltage is not there either....so am I correct in assuming that the unit needs that 135V in order to power up?????????..........I am going to have a few beer now and lite up the BBQ.....and relax....any suggestions as usual are greatly appreciated.....
    If your +/- 15V supplies are sitting only around +/- 4V, you have issues in the power supply for starters. +135VDC? no....+345V or in that range, yes. To get the amp to power up, the control voltage going out J12 on the power amp PCB to J36 on the AC Mains Relay board to feed Q1 MPS A13 Darlington by way of a 220k resistor, that voltage needs to be around 10V, and not drifting. At one point, the SVT-CL had a time delay circuit that ramped up that control voltage to around +10V, but later eliminated the added 220uF cap and discharge diode. I had low supply voltages on the +/- 15V supply, which is fed from a +/- 20V supply, whose discrete diode bridge is fed thru a pair of 0.4A thermistors. I had two questionable thermistors, and replaced them with a pair of 10 ohm 2W resistors, had to lay them on their sides to be out of the way of the power tube PCB. Then, I had aged filter caps, all of which were causing the absence of correct bipolar supply voltages. You may be dealing with that issue on this amp.

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    Ok. Thank you for the info nevetslab.....much appreciated....I never pulled the output tubes...I will do that and re-install the 12A tubes....the schematic I have is not very clear and that is why I had said +135V....anyway, I will do some more testing later today to see what I can determine...All the tube heaters light up fine...I pulled the output tubes and gave them a quick test and there were no shorts.......I checked the boards for bad solder connections but I couldn't see anything...I did touch up some connections on the tube sockets anyway...I'll let you know how I make out.....
    Cheers

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    Senior Member nevetslab's Avatar
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    More Ampeg SVT-CL documents

    Besides the schematics g1 had pointed you to, which were on my earlier SVT-CL ht xfmr cycling thread, here's the power amp PCB parts placement guide to assist in troubleshooting.....remembering that you'll be looking at the foil side of the PCB, but it is the road map, if you will, in finding where all the parts are. I thought I had an Ampeg drawing showing the foil pattern of the power amp. I do have that for the preamp board, and that's here too.....zip files.

    PWA Power Amplifier PCB Schematics and Parts List (419xxp2).zip
    PWA Preamp PCB Layout (51912PWA_0).zip

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    Quote Originally Posted by nevetslab View Post
    Besides the schematics g1 had pointed you to, which were on my earlier SVT-CL ht xfmr cycling thread, here's the power amp PCB parts placement guide to assist in troubleshooting.....remembering that you'll be looking at the foil side of the PCB, but it is the road map, if you will, in finding where all the parts are. I thought I had an Ampeg drawing showing the foil pattern of the power amp. I do have that for the preamp board, and that's here too.....zip files.

    PWA Power Amplifier PCB Schematics and Parts List (419xxp2).zip
    PWA Preamp PCB Layout (51912PWA_0).zip
    Ok. Thank you very much....I will see if I can get at this over the weekend but if not I will be early next week......Have to work on the convertable top and mix up some concrete this weekend......busy trying to do some stuff here around the house.....thanks to everybody for all the help so far......I do have a question nevetslab......this thing goes into fault mode as soon as the power is applied....and in standby...I re-installed those 12a tubes and removed the output tubes....and same thing......so there must be a problem with the low voltage supply which I will check out.....that front panel light is supposed to light up red when it is powered up and placed in standby mode.....right??

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by bsco View Post
    Ok. Thank you very much....I will see if I can get at this over the weekend but if not I will be early next week......Have to work on the convertable top and mix up some concrete this weekend......busy trying to do some stuff here around the house.....thanks to everybody for all the help so far......I do have a question nevetslab......this thing goes into fault mode as soon as the power is applied....and in standby...I re-installed those 12a tubes and removed the output tubes....and same thing......so there must be a problem with the low voltage supply which I will check out.....that front panel light is supposed to light up red when it is powered up and placed in standby mode.....right??
    YES. In Standby mode, the panel light is RED. It won't go into Protect mode until attempt to come out of S/B.....assuming there is a fault condition. With the power tubes removed, and you're still getting Fault status, it remains in that mode, even after going back to S/B, until you power down, which resets the crude anlog logic circuits. So, as I'd outlined in the earlier threads and previous post, you'll have to trouble-shoot the control voltage circuits to find the fault. I gather you're not blowing mains fuses, which you did at the beginning of this pursuit. Take your time, be safe.

    I've never intentionally unplugged the High Voltage Secondary connections to the power amp PCB so the high potentials aren't on the board while troubleshooting the low voltage control circuits. They are discrete connections...J13 thru J18...right-angle insulated 1/4" Fast-on terminals. I've always managed to steer clear of the high potentials. If you're nervous about it, unplug them, but make sure to document it, so you know EXACTLY where each wire/connector has to go in restoring order. Happy hunting!

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    Ok. Thank you very much nevetslab.....I will let you know how I make out.....and yes these amps do make me kind of nervous when working on them as I am not used to them.....
    Cheers

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    I am back.......I did what you suggested nevetslab.....I disconnected the HV connections from the main board and taped them up.....and I removed the output tubes as well so they won't get damaged or cracked......when powered up and left in standby, the front panel light flashes......the 15V rails are down to approx. 4V each.....so I have a problem with the LV supplies.......now I can work without getting fried.......I have some schematics here from various sources so hopefully I have the right ones....the board number on this one is 2033854 REV E00.....I'll let you know how I make out......

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    Senior Member nevetslab's Avatar
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    OK....good luck on the hunt.

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    Quote Originally Posted by nevetslab View Post
    OK....good luck on the hunt.
    OKKKKK.......I ended up taking that main board out completely as voltage checks were just sending me around in circles......so I started doing resistance checks and esr checks on caps....all the diodes checked ok as did the caps....I discovered that the IC for the DI was shorted pins 1,2,3,and4...pins 4 and 8 also showed a lower than normal reading....I removed it and sure enough...so I removed the other ic's and they also didn't read right on some pins.....so I am going to replace all of them.....I was thinking about mounting them in sockets and just running a very small dab of silicone on the ends of each IC where it meets with the edge of the socket....at least that way, if it ever comes back for repair it will be very easy to replace them....and the dab of silicone will keep them secured in the sockets while the amp is in use........I will have to get some TL072's Monday....I have a bunch of TL074's on hand.....I'll replace these early next week and let you know how I make out......Thank you very much for your help and all your advice on how to tackle these amps...I have to admit...I felt much safer with the HV removed from the board....
    Cheers

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    Senior Member nevetslab's Avatar
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    Leave it to the previous mfgr owner of Ampeg to have selected 16V rated 220uF caps for C2/C27 with outputs at +/- 15V. Any time I have to dig into there, they get replaced with 35V parts, as the size is still suitable, lower loss/ESR. Sounds like you found what was dragging your supplies down. You might also check the two Thermistors CB1 & CB2 (look like 220pF ceramic disc caps...that small). I had just pulled two out on an amp that were very high in resistance for the function....40 ohms and 220 ohms. 2-10 ohm NTC 1/2A thermistors are probably what they should be. 10 ohm/1/2W would suffice in lieu of using thermistors there.

    Cheers,

    Steven

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    Quote Originally Posted by nevetslab View Post
    Leave it to the previous mfgr owner of Ampeg to have selected 16V rated 220uF caps for C2/C27 with outputs at +/- 15V. Any time I have to dig into there, they get replaced with 35V parts, as the size is still suitable, lower loss/ESR. Sounds like you found what was dragging your supplies down. You might also check the two Thermistors CB1 & CB2 (look like 220pF ceramic disc caps...that small). I had just pulled two out on an amp that were very high in resistance for the function....40 ohms and 220 ohms. 2-10 ohm NTC 1/2A thermistors are probably what they should be. 10 ohm/1/2W would suffice in lieu of using thermistors there.

    Cheers,

    Steven
    I will check those thermistors and replace the caps you mentioned....I have to install the sockets and chips and then install the board and fire it up without the output tubes to see if there are any more issues but I think this might be it.............won't get back to it until early next week......I will let you know.....

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    Quick question...TL072BD (original) and TL072CN (replacement)will it work ?? only difference I can find it the thermal rating......I have the CN versions here......
    Cheers

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by bsco View Post
    ...I was thinking about mounting them in sockets and just running a very small dab of silicone on the ends of each IC where it meets with the edge of the socket....at least that way, if it ever comes back for repair it will be very easy to replace them....and the dab of silicone will keep them secured in the sockets while the amp is in use...
    If I may add my 2˘. A good quality IC socket will hold the chips sufficiently well without the silicone. I wouldn't add the silicone. It would look unprofessional and potentially cause contamination problems later.

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    Quote Originally Posted by Tom Phillips View Post
    If I may add my 2˘. A good quality IC socket will hold the chips sufficiently well without the silicone. I wouldn't add the silicone. It would look unprofessional and potentially cause contamination problems later.
    Noted Tom...Thanks......
    Cheers

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    Lifetime Member Enzo's Avatar
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    It is just a guitar amp, ANY TL072 will work.

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    Education is what you're left with after you have forgotten what you have learned.

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    nevertslab, I got it working......I ended up replacing all those chips....I checked all the components in those op-amp ckts and everything was fine.....I went around and re-soldered all the connections to all the heavy parts and connectors..they looked ok but I did them anyway.....those capacitors and thermistors were fine......the only thing I am waiting on now is the fuse holder......the owner broke it trying to replace the fuse.......anyway, I should have the holder later next week...and I will be able to get it back together......Thanks for all the help......
    Cheers

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    Quote Originally Posted by bsco View Post
    nevertslab, I got it working......I ended up replacing all those chips....I checked all the components in those op-amp ckts and everything was fine.....I went around and re-soldered all the connections to all the heavy parts and connectors..they looked ok but I did them anyway.....those capacitors and thermistors were fine......the only thing I am waiting on now is the fuse holder......the owner broke it trying to replace the fuse.......anyway, I should have the holder later next week...and I will be able to get it back together......Thanks for all the help......
    Cheers
    nevertslab....I got the amp back together as I had a new fuse holder assy ordered for it.....The amp works fine at a low volume but if you crank it up, it will go into protect mode after a couple of minutes......I am thinking it is a problem with one or more of output tubes....trouble is how do you test for this without tearing it apart again to determine which tube or tubes would be at fault.....If I have to I will have to replace all the tubes with a matched set but that is going to be expensive...I would love to hear your thoughts on this seeing as you have lots of experience dealing with these amps.....
    Cheers

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  30. #30
    don't forget the joker g1's Avatar
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    Have a read through his post about matching here: http://music-electronics-forum.com/s...l=1#post477222

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Senior Member nevetslab's Avatar
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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.

    Click image for larger version. 

Name:	SVT-CL Power Amp PCB Parts Location Bias Adj.jpg 
Views:	19 
Size:	214.7 KB 
ID:	50243 Click image for larger version. 

Name:	Power Tube PCB Parts Location.jpg 
Views:	19 
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ID:	50244

    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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  32. #32
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by nevetslab View Post
    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).
    Boy oh boy is that ever da troof! All the rest too. A big thumbsup on an excellent post Mister Nevets! Now, I think I have a bit of TechFlex around here somewhere... can't wait for my next "modern" SVT to come thru so I can razor off that hideous garden hose tubing & heave it in the bin where it belongs. Too bad all the rest of it is such a PITA, true, how true.

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    Quote Originally Posted by nevetslab View Post
    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.

    Click image for larger version. 

Name:	SVT-CL Power Amp PCB Parts Location Bias Adj.jpg 
Views:	19 
Size:	214.7 KB 
ID:	50243 Click image for larger version. 

Name:	Power Tube PCB Parts Location.jpg 
Views:	19 
Size:	375.8 KB 
ID:	50244

    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.
    Thank you for the reply.....I removed the plastic garden hose and replaced it with something more flexable...same stuff used in automobiles to cover wiring harnesses.....If I keep the volume low the amp works fine...that is why I am leaning towards a tube issue.....I do have some used 6550's here so I will take it apart and do as you suggested......Thank you for putting up with my questions and in-experience with these beasts...After this is over I will most definately have a better understanding of how this amp works and the various techniques used to troubleshoot it without getting fried.....I'll dig those tubes out and give it a shot....I'll let you know how I make out.....I did go through all the solder connections........at least now it is powering up and is not blowing fuses....
    Cheers

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    Ok. I removed all the output tubes and just worked with two.....in locations V2 and V4.....just because they wee side by side.....I set the bias pots to the 12 o'clock position and then measured the voltage drops across the associated resistors.....I left the same tube in the V4 location and used the V2 location for checking the other tubes.....here are the results taken with a digital meter set to 2V scale.....(.135V)......(.163V)......(.137V)......(.137V) ......(.169V).......and the tube that stayed in V4 location was .123V....,.I did notice that when I was testing the different tubes in V2 socket, the bias voltage on the tube in location V4 would vary when the other tubes were inserted in V2 location...I take it that this is normal??? this doesn't seem right ??? anyway I installed all the tubes, set the bias controls until they were green and made voltage checks again.....using a 2V setting, V1 measures .135V, V2 measures .150V, V3 measures .184V.......V4 measures .185V, V5 measures .161V and V6 measures .170V.......they look like they are pretty close between the two sets.....if these are too far off let me know.....

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  35. #35
    Senior Member nevetslab's Avatar
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    If you swapped V3 with V4, does the balance get better for V1-3 & V4-6? Your target bias is 230mV (23mA). You will have a spread in both directions about that target voltage. Right now, your average for V1-v3 is 0.156, and for V4-V6 it's 0.172. If it's possible with the older tubes used to cobble this set together, if they're both the same mfgr and different from the other 4 tubes, I'd put one each in the upper and lower set, rather than having the two different tubes in with one, while the other set is all the same. Granted, I don't have hard data to show what this does over time, and if it leads to problems. It just makes good sense in the cobbling procedure.

    After getting the set balanced with your average at around 230mV, I let the amp idle for a good half hour or more, and periodically check the results. If all looks good, then cool them down so you can pull them one by one, marking each with a sharpie pen per their new position, and then put the clamps back in, the rest of the hardware securing the power tube plate, and load 'em back in.

    I hope you haven't run into any of the power amp chassis' cage nuts popping out. Just like on Marshall amps, once you install the chassis back into the cabinet, cage nuts want to refuse to accept the screws that came from them. All too often, pleading with them and trying to drive them into the threads, the screw will force them out of their square hole, forcing you to pull the chassis again. Dunno why. They ALWAYS THREAD IN FINE when they're out of the cabinet. The new Ampeg amps are horrible....they've been using cheap Chinese hardware, which is NOT tempered spring steel, and they disappear into the chassis out of their holes with the greatest of ease. I've ordered quality M6 cage nuts, and have added 1/4" split lock washers under the screw heads. Tired of SVT's coming back in from tour missing 3 of the 5 chassis mtg screws!

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