Ampeg SVT

[bsco]

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

[Tom Phillips]

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.

[bsco]

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

[The Dude]

Schematics should be the ones in post #1 of this thread:

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

[nevetslab]

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.

[Ted]

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

[nevetslab]

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.

[TimmyP1955]

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.

[nevetslab]

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.

[bsco]

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.....

[bsco]

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.....

[nevetslab]

SVT Still in Protect Mode (flashing RED Status LED)

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.

[nevetslab]

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.

[bsco]

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

[nevetslab]

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