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The paper/oil caps will pop without warning or provocation, electrolytics dry out and are known to go leaky both physically and electrically. Disk ceramics are quite stable and I've had numerous film caps go bad from being exposed to the high temps and voltages in vintage tube gear. Another to consider are the carbon comp resistors that get baked from radiated heat, nothing like a 6L6 or pair of KT88's to cook them. 50+ years of absorbing moisture cold only to be baked dry again when the heat is on, I'm also an audio guy and have over 40-plus years of experience with tubes so I know what you're dealing with. Case in point is my Magnavox T-940 AstroSonic. The IF gain would drift down as the set warmed up due to the resistors drifting up thus dropping the plate voltage. As an experiment I dynamically tested each resistor in circuit, Simpson 260 on the leads while heating the suspected resistor with a soldering iron at 45 volts (220F tip) and watched it drift up beyond the 10% tolerance rating. The set spent its first 14 years entertaining us kids, been in the family since new and it's going to get all new resistors in the near future.
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