I’ve seen a lot of the same complaints on various forums regarding these amps. Basically, the amp starts to get noisy and the occasional arcing can be heard through the speaker. The problem seems more exaggerated when using the tremolo circuit which is just a phase-shift LFO modulating the negative grid voltage. I’m not so sure that the blue 1/2 watt resistors inside the amp are not really metal film despite their appearance (**UPDATE** I’ve recently seen some evidence that arcing tends to happen more often in metal-film resistors. I’m not sure about how correct this is, but it would make sense in this amp). If you find yourself working on amp with these sort of issues, here is what I did to alleviate some of the nasty pops.
On the main board, change R34 from 475k to a 1M 1/2 watt resistor. Also change R8 (pictured below) from 221k to 270k. This will lower the the overall voltage swing on the grid preventing so much stress to the power tubes.
On the second board (the one that has the tube sockets), you’ll probably want to change out the metal-film resistors just to make sure they don’t start arcing. You can always tap them with a chopstick and listen for any pops in the speaker to determine if they are bad or not. I apologize for the lack of a picture, but there aren’t many components on the board. You should be able to find them all right in the middle grouped together.
R15 & R16: 100k
R17 & R18: 220k
R19 & R20L 22k
For this amp, the speaker was replaced with a 12″ Jensen C12N. The thing cleans up pretty nice and has a sound not unlike other true vintage Ampegs that I’ve heard.