bsco 8/2/2018 1:48 PM
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 kindly welcome....
Tom Phillips 8/2/2018 1:58 PM
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 8/2/2018 2:31 PM
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 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.....
The Dude 8/2/2018 3:29 PM
Schematics should be the ones in post #1 of this thread:
nevetslab 8/2/2018 9:57 PM
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 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 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 8/3/2018 8:04 AM
You might want to check the Ampeg Technical Information Bulletin referenced in this thread:

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:
nevetslab 8/3/2018 11:59 AM
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 8/3/2018 8:47 PM
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.