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?



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See if the tube is cold (carefully!). If it's cold and no glow, tube is dead and will not work. The only explanation is there has to be a diode in series with the rectifier. 
I have no idea, but have you tried swapping it with the other 5u4g to see if anything changes?

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.

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

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.
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


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.
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.
Thanks, Gs. A minor point, but I think you meant to say pins 2 and 8 for the cathode/filament connections. Those being the cathode/filament pins of a 5U4 (4 and 6 are the anodes), with pins 2 and 8 being paralleled between the two sockets in this case and one of them (pin 8) being connected to the 47 uf filter capacitor and subsequent circuitry as I mentioned.

So, again, per my earlier comment it seems explainable that both channels would continue to function in this case even if one of the rectifiers was dead/open, which consistent with your use of the word "improbable" would seem to be a more likely occurrence than a cathode/filament-to-anode short.

Best regards,
-- Al

Gs5556 10-29-2017
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.
Re the comment I provided above in response to this statement, I just realized that the anodes and cathodes you are referring to are most likely the anodes and cathodes of the protection diodes, not the anodes and cathodes of the rectifier tubes.

If so, I am in agreement with your comment. But in any event the fact that the cathodes/outputs of the two rectifier tubes are connected together means that both tubes serve both channels. So both channels would continue to function (although probably in an unsafe manner) regardless of whether one rectifier tube is open or shorted or something in between, or even if one tube is removed.

Best regards,
-- Al

Al, the reason there are two rectifiers is so that they will hold up when the amp is at full power. At lower power levels one rectifier tube is sufficient.

IOW, the amp is be safe to operate (its been that way already) as long as the volume were not turned up (the loss of the filament load is already proven to not change the operating voltages significantly- other than the B+ is slightly lower right now). If high power demands are made, the remaining rectifier tube would eventually also fail.

Since I often come across old Audiogon forum posts while researching various questions on both A’Gon and Google, I thought I should close the loop on this one in case anyone else has this same rectifier question / issue with their Cary SLI-80 integrated amp and stumbles upon this post:


Here’s the explanation as to how my Cary SLI-80 was working just fine with one of its two rectifier tubes being blown:


I finally contacted Cary’s tech department, and spoke with Mark. Nice guy. I explained what had happened, that I just happened to notice one of the 2 rectifier tubes in my SLI-80 was blown, and yet the amp was working great, sounding great, and it may have been that way for several listening sessions. What gives?


Well, I – incorrectly – assumed that since the SLI-80 has 2 separate rectifier tubes, that one is for the left channel and the other for the right channel. I was wrong (that’s OK, I usually am. Just ask my wife ;-). Turns out, Cary runs the 2 rectifier tubes in parallel (and a shout-out to fellow A’Goners almarg and GS5556 who mentioned this possibility above). I’m not an electrical engineer, but I got the gist that – basically – the one remaining good tube was doing the work of 2, and that’s why the amp continued to perform just fine with one blown rectifier tube. Of course, that also meant the remaining tube was working harder, and if I had left it that way, it too likely would have blown prematurely.


I also – incorrectly – assumed that the HEXFRED upgrade that my amp has in its rectifier stage was “taking over” most of the work from the rectifier tubes themselves, and that may have explained why the amp was working just fine on one tube. Again, I was wrong. According to Cary, while the HEXFRED upgrade does “tighten up” and improve the rectification stage, the amp still relies on the rectifier tubes themselves to do the actual work, and the amp can still benefit from using higher quality rectifier tubes.


Cary also told me that part of their “ultimate upgrade” package for the SLI-80 is to completely remove the tubes from the rectification process and rely only on the HEXFREDs (and perhaps other magical circuits, I don’t know). This explains the pictures I’ve seen online of some SLI-80 amps with no rectifier tubes at all, and just small black metal covers over the rectifier tube sockets. So, if your SLI-80 amp – like mine – does have the HEXFRED upgrade, but also still uses rectifier tubes, rest assured you’re still running tube rectification, and can still benefit from rolling-in your favorite tubes (like the now ubiquitous Philips 5R4GYS, they just look so cool ;-).


I hope that helps anyone with similar SLI-80 rectification questions.


And with that, I rest my case, Your Honor.

From the sweet spot,

Mike (Mhwalker)

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We've been using HEXFREDs for the last 25 years and they are in everything we make.

If they are really added into this amp as described, the tube rectifiers are really only there for show, since the voltage drops across the HEXFREDs is a fraction of that of the tube, so it isn't doing anything except glowing.
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@atmasphere Old thread. I wanted to try FRED's in my preamp. Care to clue me in on the type you're using? B+ is 350Vdc.
I'm looking at trying these (IXYS Diode 15A 1200V HiPerFred) but heard they aren't as transparent as these (Rectifiers 15A 600V Ultrafast 19ns HEXFRED). Thanks.
@noromance You'll be alright with the 600V units and you can use the 8A version rather than the 15A version!