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SVT-2 Pro: Hum when Input UNplugged; SNARL coming out of STBY
I had an SVT-2 Pro arrive a couple of days ago. UPS did a number on it, bashing up the box to the point that it wasn't square any more, poking holes through it, and dropping it hard enough to smash the rack handles on both sides into the faceplate. Ouch! 
At first glance the amp seems OK electrically and the amp sounds fine. It's loud and potent, just like you'd expect from and SVT. The idiot lights for bias seem to be working fine -- it's easy to bias the amp using the LEDs to reach the desired endpoint discussed in the operating manual: turn the bias knob counter-clockwise so that no LED are on, then clockwise until the GREEN LED comes on by itself, stopping before the RED LED comes on; doing it this way I can get the GREEN LED to come on, and I have a reasonable amount of farther travel with the bias adjustment before the RED LED comes on, at which point you're supposed to back off until the RED light goes out but the GREEN light stays on. According to the manual, if the GREEN LED stays on for a long time before the RED LED comes on, that indicates that your tubes are well matched. The set of 6 Sovtek 6550WE seem to be reasonably well matched by this criterion.
The amp does do a couple of oddball things though, and since I'm not familiar with the amp I'd like to ask about them:
1. It hums when there's nothing plugged into the input. Sounds like 120 Hz though I haven't put a scope on it yet. Hum goes away when a guitar is plugged into the input, which makes the amp become dead quiet with no hum. Twiddling with the controls shows that when the input is unplugged the GAIN control will bring the amount of hum up and down. In other words, with nothing plugged into the input the amp is silent when the GAIN control is turned to zero, but hum appears when the GAIN control is turned up. Plugging a guitar into the input silences the hum, regardless of the GAIN setting.
Looking at the schem, the GAIN control is just a volume pot that's located after the first gain stage:
Just as you'd expect, pressing the MUTE button silences the hum, and engaging the -15dB pad (marked as "ATTEN" on the schematic) decreases the hum significantly. That the problem is limited to the first gain stage has me thinking that the problem could be a bad shorting jack on the amp's input, where the input isn't being properly shorted to ground when the guitar cord is removed. I'm thinking that this has to be a problem with dirty/corroded contacts or a bad jack that needs replacement. Is there anything else that I should be looking for?
2. When I power on the amp, everything seems OK as I turn on the power switch. When coming out of standby, there's a gigantic magnetic bump that occurs when the time delay relay that controls the standby circuit engages.
Specifically,
A) if I leave the STANDBY in the OFF position and power-on the amp, the amp's status LED will illuminate RED. When I switch the STANDBY to ON, a few seconds will pass, and then the time-delay relay will activate, switching the status LED to GREEN. So far so good...
B) If I leave the standby in the ON position and power-on the amp, the amp's status LED will illuminate RED, until the time-delay relay engages to power-on the standby circuit. When the relay activates, the status LED changes from RED to GREEN automatically. So far so good...
I took off the 12 screws to remove the top cover plate to peek inside of the amp. After I had finished looking, I laid the top cover plate back in place on the top of the amp, but I didn't screw it back down. I powered the amp off. No problem so far... but when I powered the amp back on, I heard a BANG! when the standby relay engaged; evidently there was enough of a current surge in the amp to make the cover plate jump and BANG! when the amp came out of standby. This was a physical noise in the amp chassis, with no noise coming from the speakers.
Is this normal? It sure scares the hell out of me. I'm sure that I could make the BANG! go away by bolting the topcover down, but I'd like to be sure that there's nothing wrong. FWIW the amp sounds fine, except for the hum problem when nothing's plugged into the input jack and the loud bang when coming out of STBY.
3. Transformer Hum when Biased Hot. The amp gives out a faint audible angry hum if the amp is biased hot enough to make the RED LED come on, or if after I've gotten the GREEN LED to come on, I keep turning the LED clockwise toward the setting that makes the RED LED come on. Backing off to the point that the GREEN LED just comes on makes the amp much quieter. The angry hum can also become audible if the push and pull sides of the amp aren't balanced / biased to similar levels (using the LED indicators).
I haven't been able to get inside of the amp and take actual cathode current measurements. Actually, I don't think there's a way to access the resistors while the amp is assembled in a functional state, as the chassis requires disassembly and removal of the output tube board to reach the resistors that are located on the amp board below it. I can, however, pop-off the bottom cover and reach the solder side of the amp board's PCB. It looks like I'd have to find the solder points on the back side of the PCB that correspond to the cathode resistors on the PCB to use as test points. Either that or get a Bias King and take pass-through current measurements at the tube sockets. It looks like some measurements that we take for granted in Fender/Marshall circuits -- like using an ohmmeter across a current sensing cathode resistor -- may be difficult/impossible to make while the amp is assembled and running, as the amp's configuration uses stacked PCBs and requires some disassembly and unplugging of cards to get to some of the boards. Ugh.
Any insights would be appreciated.
TIA
At first glance the amp seems OK electrically and the amp sounds fine. It's loud and potent, just like you'd expect from and SVT. The idiot lights for bias seem to be working fine -- it's easy to bias the amp using the LEDs to reach the desired endpoint discussed in the operating manual: turn the bias knob counter-clockwise so that no LED are on, then clockwise until the GREEN LED comes on by itself, stopping before the RED LED comes on; doing it this way I can get the GREEN LED to come on, and I have a reasonable amount of farther travel with the bias adjustment before the RED LED comes on, at which point you're supposed to back off until the RED light goes out but the GREEN light stays on. According to the manual, if the GREEN LED stays on for a long time before the RED LED comes on, that indicates that your tubes are well matched. The set of 6 Sovtek 6550WE seem to be reasonably well matched by this criterion.
The amp does do a couple of oddball things though, and since I'm not familiar with the amp I'd like to ask about them:
1. It hums when there's nothing plugged into the input. Sounds like 120 Hz though I haven't put a scope on it yet. Hum goes away when a guitar is plugged into the input, which makes the amp become dead quiet with no hum. Twiddling with the controls shows that when the input is unplugged the GAIN control will bring the amount of hum up and down. In other words, with nothing plugged into the input the amp is silent when the GAIN control is turned to zero, but hum appears when the GAIN control is turned up. Plugging a guitar into the input silences the hum, regardless of the GAIN setting.
Looking at the schem, the GAIN control is just a volume pot that's located after the first gain stage:
Just as you'd expect, pressing the MUTE button silences the hum, and engaging the -15dB pad (marked as "ATTEN" on the schematic) decreases the hum significantly. That the problem is limited to the first gain stage has me thinking that the problem could be a bad shorting jack on the amp's input, where the input isn't being properly shorted to ground when the guitar cord is removed. I'm thinking that this has to be a problem with dirty/corroded contacts or a bad jack that needs replacement. Is there anything else that I should be looking for?
2. When I power on the amp, everything seems OK as I turn on the power switch. When coming out of standby, there's a gigantic magnetic bump that occurs when the time delay relay that controls the standby circuit engages.
Specifically,
A) if I leave the STANDBY in the OFF position and power-on the amp, the amp's status LED will illuminate RED. When I switch the STANDBY to ON, a few seconds will pass, and then the time-delay relay will activate, switching the status LED to GREEN. So far so good...
B) If I leave the standby in the ON position and power-on the amp, the amp's status LED will illuminate RED, until the time-delay relay engages to power-on the standby circuit. When the relay activates, the status LED changes from RED to GREEN automatically. So far so good...
I took off the 12 screws to remove the top cover plate to peek inside of the amp. After I had finished looking, I laid the top cover plate back in place on the top of the amp, but I didn't screw it back down. I powered the amp off. No problem so far... but when I powered the amp back on, I heard a BANG! when the standby relay engaged; evidently there was enough of a current surge in the amp to make the cover plate jump and BANG! when the amp came out of standby. This was a physical noise in the amp chassis, with no noise coming from the speakers.
Is this normal? It sure scares the hell out of me. I'm sure that I could make the BANG! go away by bolting the topcover down, but I'd like to be sure that there's nothing wrong. FWIW the amp sounds fine, except for the hum problem when nothing's plugged into the input jack and the loud bang when coming out of STBY.
3. Transformer Hum when Biased Hot. The amp gives out a faint audible angry hum if the amp is biased hot enough to make the RED LED come on, or if after I've gotten the GREEN LED to come on, I keep turning the LED clockwise toward the setting that makes the RED LED come on. Backing off to the point that the GREEN LED just comes on makes the amp much quieter. The angry hum can also become audible if the push and pull sides of the amp aren't balanced / biased to similar levels (using the LED indicators).
I haven't been able to get inside of the amp and take actual cathode current measurements. Actually, I don't think there's a way to access the resistors while the amp is assembled in a functional state, as the chassis requires disassembly and removal of the output tube board to reach the resistors that are located on the amp board below it. I can, however, pop-off the bottom cover and reach the solder side of the amp board's PCB. It looks like I'd have to find the solder points on the back side of the PCB that correspond to the cathode resistors on the PCB to use as test points. Either that or get a Bias King and take pass-through current measurements at the tube sockets. It looks like some measurements that we take for granted in Fender/Marshall circuits -- like using an ohmmeter across a current sensing cathode resistor -- may be difficult/impossible to make while the amp is assembled and running, as the amp's configuration uses stacked PCBs and requires some disassembly and unplugging of cards to get to some of the boards. Ugh.
Any insights would be appreciated.
TIA
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