My Tubelab SSE has been running fine (since sorting out the defective binding post) but this past weekend it has acted up. Shortly after turning it on (and enjoying it immensely, BTW), the sound suddenly faded and as I watched, the filaments dimmed I thought "blown fuse?" Checked it and nope. It's fine. Waited few minutes for things to cool, and switched on again. Things started up fine and I was beginning to think that the CL90 in the mains hot leg was acting up, but as I was watching, both output tubes suddenly (like a switch was flipped) lit up with a pretty blue glow. No drastic change in sound, but obviously not right. Shut it down, cycled one more time, same thing, but now the blue glow is only on the left side output tube.
So it appears that the output tubes are getting too much current. Yes? If so, why? The IC's going south is my first guess, for a couple of reasons. First, the "binary" nature of the appearance of the tube glow. Fine one second, glowing the next. Seems like failed resistor or cap would have failed in a different manner. I just don't know. Second, heat could have been an issue. The chassis plate under which everything is mounted gets pretty warm, at times. If the heat sinking was already marginal on the IC's (as I'm given to understand it is) they certainly could have failed in the short time they've been in service.
I've ordered new IC's, and another set of heat sinks (will screw them back to back), and will add a fan to the chassis.
Am I on the right track? What other components could have cause this behavior? Why did the amp shut itself off in the first place? Is the CL90 suspect too?
So it appears that the output tubes are getting too much current. Yes? If so, why? The IC's going south is my first guess, for a couple of reasons. First, the "binary" nature of the appearance of the tube glow. Fine one second, glowing the next. Seems like failed resistor or cap would have failed in a different manner. I just don't know. Second, heat could have been an issue. The chassis plate under which everything is mounted gets pretty warm, at times. If the heat sinking was already marginal on the IC's (as I'm given to understand it is) they certainly could have failed in the short time they've been in service.
I've ordered new IC's, and another set of heat sinks (will screw them back to back), and will add a fan to the chassis.
Am I on the right track? What other components could have cause this behavior? Why did the amp shut itself off in the first place? Is the CL90 suspect too?
Good film capacitors can last a lifetime, so as long as the voltage rating is above 500v I wouldn't figure on that yet especially if both tubes are acting the same.
I don't see a problem with the blueness, unless there are sparks involved. The power dropping out does seem odd. Seems like something may be loose.
I could be wrong, but IC's seem to either work, or not, nothing in between so I'd count those out.
I don't see a problem with the blueness, unless there are sparks involved. The power dropping out does seem odd. Seems like something may be loose.
I could be wrong, but IC's seem to either work, or not, nothing in between so I'd count those out.
These were new EH 6L6GC's. I'd have to say burple. It extended out the spacers all the way to the glass.
560R
If these failed, they'd fail open, no? Essentially shutting off the current?
Tube rectifier, though the behavior is essentially the same with SS.
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Quite.
The coupling caps were upgrades. I'll swap the original Mallory parts back in and see what happens.
Blue glow is normal with many tubes. I don't have EH 6L6s myself, so I can't say for sure. If it's a soft blue glow against the glass then it is nothing to worry about. It comes from stray electrons hitting the glass (there is lots of info in the Internet about this). Glowing red/orange plates and bright blue/purple flashes are what you need to worry about. It takes until the tube is fully warmed-up for the blue glow to show up. It's odd that you only get it on one side of the tube now.
I'm a little unclear as to when you noticed this happen. Was it early on or after it had been playing for some time? Since the amp is cathode biased, there just isn't much that can go wrong. The CCSs have no impact on the output tube bias. The only thing that can affect it are the coupling caps and maybe the cathode bypass caps (if they shorted somehow).
I'm a little unclear as to when you noticed this happen. Was it early on or after it had been playing for some time? Since the amp is cathode biased, there just isn't much that can go wrong. The CCSs have no impact on the output tube bias. The only thing that can affect it are the coupling caps and maybe the cathode bypass caps (if they shorted somehow).
I think your onboard components are good. If your heaters are going out there has to be an issue closely related to the power transformer, but probably not the transformer itself. Either something is loose (could even be in the wall), or perhaps an unhappy switch, cable or socket? You could try bypassing that CL90 altogether.
Nope. Not the chip. I have accidentally blown (shorted) the chip (by poking around with a scope probe on a live amp) and the amp keeps right on playing! The channel with the blown chip looses about 3/4 of its volume, but the chip can not affect the operation of the output stage. If the chip failed open, the affected channel would have no sound.
If the purpleish glow is inside the tube, and not on the glass itself and you say "It extended out the spacers all the way to the glass." That is usually an indication of a gassy tube. A gassy tube contains air or another impurity. It will ionize at a certain combination of voltage and temperature causing excessive tube current. I woild try a sdfferent set of output tubes.
Thanks, everyone. I'll cross the IC's off the list, I guess. I will bypass the CL90 and replace the mains switch. As you recall, this started with things just shutting down on their own, so getting that squared might be a good first step.
Still, the glow was not there before, so I'm a little spooked about it showing up all of a sudden.
Emboldened by the assurances that nothing immediately terminal was afoot, I fired it up again. Same drill - As I watched things warm, the B+ came up and the blue glow began. Looking carefully though, I realize that what I was seeing on the spacers was actually reflection/refraction from the glow around the cathode leaking out though the small gap between the plate and spacer.
Anyway, seconds later, lights out. Like I had hit the switch. Maybe five seconds later, back on. B+ came up almost immediately, even before the filaments had lit up all the way. Maybe 40 seconds after that, out again. Switched off, scratched my head for a few minutes, and switched on again. Normal startup and operation, save for that glow.
Now, it is <i>just</i> possible that these tubes have always behaved thus and I never noticed. I rarely use them, running instead a pair of KT88's which don't glow, and a pair of EL34's which always have, albeit just slightly visible through the slits in the plate.
So the most immediately troubling thing is the intermittent power failure. The CL90's definitely coming out of the loop.
If the filaments are dimming as you say, it sounds like you have one or more flaky or high resistance connection between the wall and your power transformer.
Worst case scenario would be a defective power transformer; unlikely, but possible.
Hard parts like switches, etc., do not fail very often.
Let us know what you find.
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