• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Maximum Ic before auto shutdown

I'm building a kt88 PP amp for my son and I have a little SCR circuit that will shut down the power transformer if cathode current of any of the kt88's exceeds a certain limit. This is done by opening a normally-closed power relay.
I can set the circuit to trigger the SCR at any current. What is a reasonable maximum Ic to choose? The idle current will be 50-55ma per tube.
Thanks for your help.
I don't know if you should bother with the SCR, etc. Remember, you have considerable energy stored in the B+ PSU's reservoir capacitor. Disconnecting from the AC mains will not immediately stop the excessive KT88 current.

If the amp employs "fixed" bias, you can size the wattage rating of the "idle" current test/set resistor so that the part also functions as a fuse. Better a few cents and some time spent on replacing a metal film resistor than having to replace costly O/P "iron".
A 10ohm, 1/8W resistor will fry at around 111ma. Does that sound like good enough protection?

That part may "pop" during legitimate musical transients. If it does, try 1/4 W. parts and so forth. Find, by experimentation, the wattage that just survives under peak musical demands. There is definitely a balance between adequate protection and unnecessary failures.

BTW, the normal heat "under the hood" of properly working circuitry is a factor in the total "computation". Testing should be done, when the unit has been operating for a while.
There's nothing wrong in monitoring the output tubes cathode current to trip a protection circuit and cut off B+ if some safe limit is exceeded. This is the most elegant and non destructive way to protect your amp and tubes and is used in some commercial products (Ayon, and probably others). The original (Genalex) Ik max. rating is given for 175 mA but at that current you will be way over maximum plate dissipation and the tube will heavily red plating if not already destroyed. I would select a tripping point corresponding to about 120% max. plate dissipation, which should be around 90 to 100 mA depending on the plate voltage used in your amp.

Using (cathode) resistors as protection fuses looks easier but is actually more tricky: firstly, resistors are not fuses and not intended for that purpose, their specific overload characteristics before opening (time vs dissipation vs voltage vs t° ,etc...) are usually unknow and not guaranteed . Most resistors are too slow to open and can whitstand a dissipation overload of many hundreds of % for seconds before bursting into flames, with resulting damages to surrounding parts or the PCB. You MUST use special "flame-proof" safety (fusible) resistors and find the right value/wattage by a trial and error process: wrongly dimensioned they will either open prematurely during a heavy musical transient (false detection) or won't give any protection if anything really goes wrong. Once you've found the right type, wattage and value they're fine but it's still a non user-resettable protection and the amp must be opened to replace them after a failure.