My comment was more of a general statement because there are some bias circuits that sum the currents and some that average them and some that sample only 1 or 2.
With some of the servo-bias circuits, you wouldn't have any idea that something was wrong until the remaining tubes "automatically" baked themselves to death.
I have studied output tube biasing schemes to death, including all kinds of advanced schemes that include feedback to determine if and when there is a fault so that the amp can be shut down to prevent additional damage. For example, monitoring screen and plate (or cathode) currents as well as voltages to determine when faults might be actively occurring. I even developed a circuit that allowed for the shutdown of the B+ in the event of a serious overcurrent fault without blowing fuses or opening resistors.
These days, I am so focused on improving the performance potential of class D platforms that I don't do much with tube output stages anymore.
I get your comment was generalized, which is why I referred to the schematic.
FYI, Per the Operator's Manual, the amp is designed to blow a few fuses in the event of tube failures, and continue to operate temporarily with the remaining good tubes.
Comments on the bias schematic suggest that the LEDs light if the bias is out of range. I believe constant red would indicate a tube is running too cold or totally off, and constant blue would indicate a tube is running too hot. If any of the blue LEDs are constantly on I suggest turning the amp off and taking it too a tech.
@Former Rogues my best advice is take the amp to a tech. Barring that, I suggest you carefully read pages 17-19 of the Owner's Manual (OM). It provides troubleshooting advice and also recommendations for replacing the tubes. Basically if you use the same type and grade of tubes that are in the amp, you should be able to simply install new tubes and keep rocking unless the amp has a problem that needs to be repaired.
I do suggest checking to see if the amp already has blown fuses associated with any illuminated red LEDs. The OM suggests replacing the fuse with the same type and rating of fuse. If the fuse blows again it suggests a tube problem, and the manual says to replace the tubes with the same type and grade of tubes.
It is possible that a bad tube can damage surrounding circuitry before the fuse opens. A tech would open the amp and do an inspection before installing new tubes. That way there is less risk of damaging brand new tubes or doing further damage to the amp.
One way to mitigate the risk a bit would be to use the old tubes in your amp to confirm all of the tube sockets are working properly. Say the amp has blown fuses in sockets 1 and 2, you could assume the tube are bads rather than testing by installing new fuses and seeing if they blow again. To test the tube sockets install a fresh set of fuses and move the working tubes from sockets 3 and 4 to sockets 1 and 2. If all tube sockets test good, buy two more tubes with the same characteristics as your existing tubes. Keep in mind this posses more risk to the amp and your old tubes than if a technician opens the amp and does proper troubleshooting.
The OM says you can buy tubes from Laney dealers. Also I believe you will be able to order tubes with the correct characteristics directly from TAD, since that is the brand of tubes shown in the OM.
Here is an image of a TAD KT88
KT88-STR is the model. TAD has sold various models of KT88s, so see what is in your amp, and try to get the exact same type if possible. Currently it looks like they only sell KT88-STR and KT88-STR Black Plate. It's hard to read the labels in the manual, but it looks KT88-STR to me.
A98 11A I believe is just manufacturing and batch number data. I don't think you need to be concerned with this, but you never know.
PC is plate current
TC is transconductance
PC and TC are the most important characteristics to match the tubes. If TAD no longer sells the same exact model that is in your amp, they should be able to still provide tubes with PC and TC characteristics that are compatible with your tubes (providing your amp has TAD tubes).
Most tube vendors grade and match tubes using PC and TC, but there is no agreed upon testing standard. So if you want to try and replace the tubes without going to a tech, the tubes need to be matched by the same company that matched the tubes in your amp. So if your amp has TADs, your probably in luck. If the tubes were matched by a different vendor, use the other vendor for best matching.
Keep in mind the tubes in your amp are old, so the PC & TC characteristics may have drifted from when they were tested. This is why it may be better to just buy a whole new set of tubes (or at least a full matched quad) and keep your old tubes as spares.
Regarding the popping noise your amp is making. Turn the volume control all the way down. The volume control is at the very end of the preamp, so if the noise stops when the volume is turned to zero the problem is not in the output section. If the problem continues, you could try turning off the output tubes in pairs by powering down the amp and pulling the fuses associated with the tubes you want to power off. Power the tubes off one pair at a time, and specifically pull the fuse for one odd numbered tube and one even numbered tube, in order to keep the current balanced in the output transformer.
When you turn the amp back on, monitor the status LEDs and listen for problems. If anything weird happens turn the amp off immediately. If you are only running a total of 4 tubes in the amp, the impedance the amp expects is doubled. I.E. a 2 ohm output becomes a 4 ohm output, a 4 ohm output become an 8 ohm output, and an 8 ohm output becomes a 16 ohm output.
