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

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

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.