Ampeg SVT-CL problems

[smith5414]

Hello, just joined this forum to see if anyone could help with a problem I am not sure if this is the right section. I recently bought a used ampeg svt-cl classic bass head, and am having an issue with the amp shutting off after several hours of use. After practicing for 2-3 hours it will completely shut off but I can still hear the fan running. The front fault led goes from green to black (off) and the signal is lost but the tubes still glow and the fan still blows. After this happens, I put it back into standby, and turn it off and then back on, and the red led comes up for only a second and then fades to black again. To my knowledge it is not a biasing issue either, the lights on the back are green with no signal and red when signal is introduced. I've tried it running into the wall by itself, and through an extension cord into my house. The sound quality seems to be fine when it is on and working and there doesn't seem to be any signal/volume lost. Sometimes it will play for as long as 6 hours until it fails, it also seems to be running hot but i know that is a characteristic of tube amplifiers. I took it to a local ampeg certified technician, who could not get it to fail (it worked fine for him), but i think this is because he was not running a consistent signal for a long duration, which is understandable considering time constraints. He resoldered some different wiring harnesses but said that he could not identify the problem without having the amp fail while in his possession. I've read several blogs about the protection circuit sometimes being very sensitive and thought the problem might lie there. I would be grateful for any help in diagnosing the problem, I play in a touring band and need to fix it as soon as possible.

Regards,
-Zach

[Billy R aka DynaFreak]

Ampeg issued a couple of tech notes that relate to your symptoms. what is the serial number of your amp?

[smith5414]

Ampeg issued a couple of tech notes that relate to your symptoms. what is the serial number of your amp?

The serial number is BJNDR30048, this thing has been a pain, only owned it for a month. The weird thing is that the led goes out completely and not into fault mode. Any help would be awesome though, thank you.

[Alex R]

Some of these have a cutout circuit designed to kick in if it starts to draw too much current through the output tubes. I've seen a couple of these where this circuit turns the amp off after a couple of hours as you describe. As I recall the Ampeg service announcement mod didn't cure it for my customer. I then tried to disable the circuit but failed somehow. I think i was asking questions about it on here before I gave up, so you might find the thread if you search.

Sorry not to be more positive but I have had a couple more complaints abot later model SVTs switching themselves off since then and I must confess that I just diverted them to an appointed Ampeg service centre using the service announcement as an excuse, no idea whether they solved it or not but it would be nice to hear someone found a way around this.

Of course there are other reasons why your amp might be doing this!

[smith5414]

Some of these have a cutout circuit designed to kick in if it starts to draw too much current through the output tubes. I've seen a couple of these where this circuit turns the amp off after a couple of hours as you describe. As I recall the Ampeg service announcement mod didn't cure it for my customer. I then tried to disable the circuit but failed somehow. I think i was asking questions about it on here before I gave up, so you might find the thread if you search.

Sorry not to be more positive but I have had a couple more complaints abot later model SVTs switching themselves off since then and I must confess that I just diverted them to an appointed Ampeg service centre using the service announcement as an excuse, no idea whether they solved it or not but it would be nice to hear someone found a way around this.

Of course there are other reasons why your amp might be doing this!

Hm, thank you for the info, I played it last night for several hours without it failing, I put a small fan behind it and it was considerably less hot than it has been the other times I've played it. It is hard to get it to someone because I play at night mostly and the failing is sporadic. I have several shows coming up and I'm paranoid about it just cutting out. I emailed Ampeg a couple times but they haven't been much help. What was the service mod if you don't mind me asking? I think the tubes are originals but I'm not sure, they are Sovteks, I didn't think that was the issue though because of the LED going out completely and the bias always working perfectly. Did the ones you work with go out completely like this? I appreciate anymore help you can give me, hoping I can get enough specifics to pinpoint the issue, there are just so many factors. Thanks again.

[Enzo]

At least you contacted them, but Ampeg won;t be able to help you. Imagine calling General Motors and asking why your car sometimes won;t start.

This amp has dual power transformers. One is the low voltage transformer - the filament transformer. When the amp is on, it is powered up. It heats all the tubes and has the low voltage supplies. The high voltage transformer is turned on after a brief delay and can be turned off by the protection circuits. You panel indicator lights run off the low voltage supplies. There should always be some light going. MY suspicion is you are losing your low voltage supplies. J19 is the connector from the transformer to the circuit board, try wiggling that to see if your lights blink out. I'd be resoldering its pins for GPs.

[tubeswell]

...
This amp has dual power transformers. One is the low voltage transformer - the filament transformer. When the amp is on, it is powered up. It heats all the tubes and has the low voltage supplies. The high voltage transformer is turned on after a brief delay and can be turned off by the protection circuits. ... MY suspicion is you are losing your low voltage supplies. J19 is the connector from the transformer to the circuit board, try wiggling that to see if your lights blink out. I'd be resoldering its pins for GPs.

I just fixed a SVT-CL where the 6.3V spade connectors from the filament tranny onto the output tube board had oxidised between the contacts causing resistance and voltage spikes/arcing at the spade connectors (the spade connector was blackened from the arcing). Those spikes had gone back through the filament transformer (which also supplies the +/-15VDC rail for the bias supply opamp comparators) and grown in size (because of the ratio between the 6.3V and the bias sensor circuit supply windings) and fried the diodes in the rectifier for the +/-15V supply (and taken out the filter caps for that supply). Once I got that figured out and repaired, along with soldering the 6.3 winding ends directly to the output tube board spade connectors (to prevent more arcing), and replacing opamps where necessary, it all works again. So, it pays to check out what Enzo said about the filament transformer.