AMpeg SVT-CL Unholy Noise

[Kyuzo]

I posted this under "Other Amps", but it probably belongs here. Sorry about that...

Hello all, I'm an EE but new to tubes, and I'm in love with my SVT-CL - but when I threw the switch at a gig last weekend I got an unholy hissing and crackling noise. My signal level was weak at any gain/master setting (but would occasionally punch through), and the red bias LEDS flashed on and off. I was able to rule out bass, all cables and cab by substitution. The noise is there whenever I come out of standby, even if gain and master are all the way down. Schematics are on the way and I'd love to work on this myself after I get some education on how to stay alive doing so...

Anybody have any insight? Should I give in and head for the shop? Is it time to spring for a SW emulator? Man, I love this amp...

Thanks for reading!

[Enzo]

HI
Yes here is better than there, but I already responded there, so go read it there. I don;t feel like typing it again.

Welcome to the forum too.

[Kyuzo]

Thanks Enzo, much appreciated. As of last night, one red bias LED was on steady. Maybe I lost the bias voltage on one side (or in one tube)? I'm embarassed to admit it, but there was a very minor infusion of fermented grape product last spring. (Yes - I have learned my lesson.) On cleanup it didn't look like any made its way into the chassis, but you never know. Everything worked fine for several gigs after except for a brief incident with the same hiss and crackle which resolved itself fairly quickly. I know - never put a drink on your amp. These words I have writ on my soul in letters of fire. At any rate, a spare complement of tubes might be in order. I'm assuming the power section should be a matched sextet, yes?

[Pedro Vecino]

Hissing and crackling noises are associated with a problem in one (or more) preamp tubes.
If you can not adjust a side in green, one or more power tubes has been off the circuit. They are usually protected by a 220 Ohms resistor (they tend to break when a power tube fails), and when a side is good and has symmetry in its tubes (matched), measure is 26mA. through plates.
To remove the amp (if not is a rack version) is somewhat laborious, and access to the PC board where they are, too.
Regards

[Kyuzo]

Thanks Pedro. Something interesting happened last night. I plugged into my preamp out to take a look at what was (or wasn't) there. The idea was to plug a cable in and scope it to see if there was any garbage. The hissing and crackling stopped, bias lights turned green again and I had full output power. I haven't been able to make it fail again. This is going to be an interesting project...

[Amp Kat]

I would seriously consider having the tubes (6550) checked well. While most testers can't detect exactly when they will die they can detect shorts and weak tubes. It is possible that maybe the effects loop jacks were a tad corroded and when you inserted the cable it broke the corrosion and made contact as that is a place where noise and crackling can occur. You should go thru it and make sure everything is tightened very well because this amp vibrates alot because it is such a monster. I mean 6 -6550's at 150 hz's at anything past 1 is more power than most amps ever see in their life. It is a very addicting amp and I fixed one once and didn't wanna turn it back over because I was recording with it and enjoying the power and tone so much. They are one of a kind along with their earlier built brethens but they are quite dangerous and can literally hurt you with their weight. I also recommend not putting the head on the speaker cab but away on the floor because of the rattle from the cab.

[Enzo]

I find more bad 12AU7s in those than I do bad power tubes. I'm just sayin'...