The K502 & K12's original supplied electrolytic caps are rated at only 250VDC, which is exceeded when the amp is first powered on, before the tubes warm up,
and start conducting. Just call it bad basic designing on their part...
Replace them with same uF value caps, but with 350 or 400 VDC rating, then they should last a long time.
There was a question posted, a while ago, how to determine if an amplifier is Class A, just by looking at the schematic.
All single ended amplifiers must be Class A, they can be Class A1(no grid current) or Class A2, (draws grid current, needs to be driven through an interstage transformer). Class A means that the power tube draws current through the entire audio waveform.
Class AB1, AB2 are Push-pull. A PP amp can be Class A, and if the power tube cathodes are shared, then a cathode bypass capacitor isn't necessary, as the cathode current does not change. AB1 and AB2 require the bypass cap, else severe distortion, and reduced power output will result.
A seriously overdriven and underbiased single-ended amp will be driven into Class C, which is absolutely horrible for audio purposes, but is perfectly acceptable for
efficient RF power amplification, but then this is far off from the original topic
For a more technically accurate method, measure the power tubes' cathode-to-grid 1 voltage, and cathode / plate current, and then apply these to the the published tube curves and operating data to determine if it is truly Class A or not.
A Class A PP amp will deliver approximately twice the output power of a single ended amp, using the same kind of tubes. Class AB1 and AB2 can deliver up to 4X the output power of single-ended.
/ed B