Fisher 500C C82/C84 ceramic capacitor function

[ej25awd]

Looking at the 500C schematic, I'm wondering what's the real purpose of these low value 18pF ceramic capacitors (C82 & C84) between the phase splitter AC coupling capacitor and the 1k resistor on the 7591A grid since the upper half of the amp doesn't have these 😕 RF decoupling perhaps but why just the lower half? TIA

 

[Artsaudio]

Looking at the 500C schematic, I'm wondering what's the real purpose of these low value 18pF ceramic capacitors (C82 & C84) between the phase splitter AC coupling capacitor and the 1k resistor on the 7591A grid since the upper half of the amp doesn't have these 😕 RF decoupling perhaps but why just the lower half? TIA

This is just a guess since I can only see the schematic and not the actual wiring layout... The small capacitor (it's 10pf in my schematic) either squelches an ultrasonic oscillation or squelches some RF detection. RF detection problems are usually more prevalent in the earlier amplification stages, so my best guess is squelching an oscillation. Why only only one half? Again only a guess, but I'll be wiring differences (length of wires, etc.) make the other half more immune to the problem. This sure looks like a "band-aid" so solve a problem, and if the capacitor values changed over time, it's even more likely a band-aid...