Cary SLI-80 with one blown rectifier tube, BUT STILL WORKS just fine. How?

[Mhwalker]

Question for the more tube savvy among us – thanks in advance for your input if you have any.

I powered up my Cary SLI-80 integrated tube amp this afternoon, sat down, listened, and everything sounded great. I left the amp on all afternoon, and sat back down for more listing this evening. Still sounds great.

BUT, now that it’s dark outside and there’s less ambient lighting, I just happened to notice there’s no orange glow from the left rectifier tube (the SLI-80 has one rectifier per channel). Upon closer examination, the rectifier appears to be blown, or damaged, or something. No glow, and the getter flashing is severally discolored. I don’t have a tube tester so I can’t be 100% sure. But visually it’s pretty obvious the tube is bad.

BUT, here’s the thing I need your insight on: The amp (appears to) still work just fine – both channels sound great. Even at higher volumes. Bias is normal, and is stable.

So, what gives? Can a Cary SLI-80 amp, with one 5U4G rectifier tube per channel, still work as normal if one of the rectifier tubes blows?

I assumed, perhaps incorrectly, that if the left rectifier were to go bad, the rest of the left channel would go out also – or at least would not produce power – while the right channel would still operate as normal. I assume each rectifier is electrically isolated to only one stereo channel. Is that not correct? Are both rectifiers working as one, and if so, is the one that’s still good doing the work of 2?

How worried should I be here? If it sounds great and seems to work as normal, is it OK to continue to listen to it and wait for the next time I power it down to switch out the rectifiers? Or should I power down right away?

Thanks!

 

[dosmalo]

Even though one tube doesn't appear like the other one doesn't necessarily mean that it is blown. Most all of my tubes have a different glow even in the dark, even matched pairs and matched quads. If your rectifier tube is blown you're going to hear it and know it. I had one failing in my pre amp and could hear it.
If it were me, to be on the safe side I'd replace both rectifier tubes with new and go from there.
Especially with a nice unit like the Cary.

 

[cademan]

Depending on the design, most tube amps which utilize two rectifier tubes will still work fine with one tube burned out or missing.

Amps which use two rectifier tubes in parallel are just doubling the current. A single one in this design will still work but at half capacity.

Amps which use two rectifier tubes in a series configuration are using each tube as half the rectifier so if one went bad in this configuration, you would have hum. Yours is probably in parallel configuration. Do you have a schematic or any pics?

You really don't have one rectifier tube per channel. These are operating as one in the power supply for both channels.

The 5U4 rectifier is a very robust tube and would probably take a nuclear bomb to destroy one. You might want to try swapping the so called bad one with the good one. At least this would tell you if it were the tube or the socket.

 

[gadget73]

I've seen the inside of one of these. I'm pretty sure the rectifiers are paralleled so losing one gives half current capacity. It will work with only one, but obviously the remaining one works twice as hard. Replacing both would be a good idea. 5U4GB's are cheap enough to keep some spares around anyway. If you were nearby I'd give you a couple.

These also have big solid state rectifiers upstream of the 5U4, so the tube is really just acting like a big resistor.

 

[gadget73]

as massive as the power supply caps are in these things, its no surprise its rough on the tubes. I want to say the caps immediately off the rectifier are not large, but it feeds through a fair sized choke and into some big beer can caps after that point. The inrush current is probably quite large.

luckily the rectifier tubes are cheap, so not such a big deal.

 

[Dan ʕ•ᴥ•ʔ]

I used to own a SLI-80...it ate rectifiers for daily snacks.

 

[gadget73]

5U4GB is rated for 4.6 amps transient current, the 5AR4 is good for 3.7. I don't know that direct vs indirect has a bunch to do with it though. Probably more a function of plate size, the 5U4GB has more than the 5AR4 does.