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

Question for the more tube savvy among us – thanks in advance for your input.


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 good, 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!

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128x128mhwalker
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mhwalker OP
18 posts
 

Thanks gang. Yes, Almarg has helped me with other tube-related mysteries before. Knock knock, Almarg?


I powered the amp down and removed the bad tube. It's getter flashing is "evaporated", and it looks like pictures I've seen of tubes that lose their vacuum or just go bad. Luckily I have a few backup 5U4 types around


My main question remains, though: In a Cary SLI-80 integrated, how can the amp seemingly work just fine with one of its two rectifier tubes blown? And, for that matter, can I run the amp with only one rectifier indefinitely (not that I would)?

I plan to contact Cary this week and see what they say.


BTW - The bad tube is (was) a Philips 5R4GYS from Uncle Kevin's shop. I purchased a matched pair from a fellow A'Goner a about a year ago. Sounded great, and the huge coke bottle style looks awesome. But nothing lasts forever. If an angel gets its wings every time a bell chimes, then what happens every time a vintage NOS tube goes bad? ;-)

Thanks again to those that chimed in.


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almarg
9,670 posts
 
Thanks, guys. The only explanation I can think of is that each tube is not in fact dedicated to just one channel, but instead the outputs of the two are paralleled. The purpose of doing that may be to reduce the sag in the B+ voltage applied to the output tubes that will occur to some degree at times when high output power is called for.

If so, I don’t think it would be safe to run the amp with just one good tube. For one thing, if the filament of the bad tube is not drawing much or any current the reduced load on the winding of the power transformer which supplies the filament voltage could cause that voltage to rise to levels that may prove harmful to the good tube, eventually if not sooner. And of course the B+ voltage will be affected as well, although I wouldn’t want to speculate as to how.

Good luck. Regards,
-- Al

gs5556
1,237 posts
 
There are diodes in series to protect the power supply circuit. Just saw a picture of the innards of the amp. The rectifiers are tied in parallel to the AC mains and the diodes are upstream of pins 4 and 6.

What happens is that if either rectifier tube shorts, the diodes continue to rectify the forward voltage instead of frying the transformer or caps with an ac surge.

Replace the tube -- continuing this operation changes the B+ voltage (may be a few volts or tens of volts higher) and places stress on the amplifier.
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almarg
9,670 posts
 
Gs5556, I’m looking at this photo of the interior of the amp. I assume that the thick yellow wires from the power transformer carry the filament voltage to the rectifiers, and they are pretty clearly connected in parallel to both tubes. And one of the two filament terminals on the left-most rectifier socket is connected to a 47 uf filter capacitor immediately above the socket, with the DC voltage supplied to the downstream circuitry for both channels apparently being taken from that point (note that there are no other connections to the filament pins of the other tube). Also, it looks like pins 4 and 6 of each tube (the two anode pins, as you realize) are probably connected to the corresponding pin of the other tube. So it appears likely that the two tubes are simply paralleled, and both channels of the amp would therefore continue to function (albeit probably not in a safe manner) even if one tube is open or even removed.

Or if the anodes are not paralleled, but the circuit is configured as shown here, if one tube is open or even removed the circuit would be converted into a half-wave rectifier, and both channels of the amp would also continue to function (again probably not in a safe manner).

Are the diodes you referred to the two things that have a black wrapping and are just below the sockets in the photo I linked to? If so, I wonder if those are fuses rather than diodes.

In any event, the bottom line is that there are plausible explanations for both channels of the amp still being able to function, with both rectifier tubes apparently serving both channels, but we are all agreed that the amp should not continue to be operated with one tube not functioning.

Best regards,
-- Al

gs5556
1,237 posts
 
Al:

Same picture I saw. The diodes are under the rectifier sockets and have the black heat shrink. The heavy red ac wires enter the anodes and the cathodes are connected to pins 4 and 6 and then they’re jumped over to the second tube socket. The rectifiers are just passing along positive voltage and providing the voltage drop.

Apparently Cary was guarding against the improbable cathode to plate short -- which seems to be what happened to OP’s rectifier tube. The 5u4G drops 44 volts across DC so the amp as running right now is that much higher to the other tube plates and should not be played.
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