Son of Ampzilla turn on pulse

I have a Son of Ampzilla that I acquired in 2014. I got great help on this forum back then for a channel drop problem. The amp has been working great until recently.

I now get a pretty severe turn on pulse, followed by a slight hum that is of equal volume in both channels. The hum is more pronounced with the inputs disconnected. The turn on pulse is the same if the inputs are connected or disconnected. The pulse has been getting worse and I am starting to be concerned for my speakers. The amp still appear to work fine as far as the sound goes.

I am thinking something is getting funky in the power supply since both channels are equally affected. Anyone run into this issue before?

The usual problem is cold solder connections. Solder is eroded by electron flow so it is best to have bigger connections than small ones. If this amp is old, then all of its connections are suspect.

The thump is from the rails not rising at the same rate and the circuit not settling in a balanced way. This amp should have resistive current sources for the complementary diff amps, which is a good thing and a bad thing. generally, they do rise without thumps, but also, until the circuit reaches a significant portion of its final voltage, the currents in it are low and the gm of the transistors is also low. This can cause little squeelly noises until the final currents are reached or gm is high enough for the frequency response of the circuit to be normal.
Sounds like the main filter capacitors, no surprise after nearly half a century.

+1 on this being the probable cause for the turn on thump.

I too am in the process of restoring a GAS Son of Ampzilla. In the power supply I replaced the aging 10,000uF/50V capacitors with new 12,000uF/50V ones.

For the hum, make sure the bolt on the diode bridge is tight.

Good luck
Two and a half years later and I finally get a chance to work on the GAS Son Of Ampzilla again. All the caps were replaced including the main filter caps. The GAS115 and 117 transistors were replaced on both channels back in 2014 to fix an oscillation issue - that has not come back.

I still get the turn on pulse but not every time. It is always on the same channel. That channel has also developed an intermittent crackling/whooshing/popping noise that is quite random in when it occurs and its exact qualities. Sometimes it will stay in that state and the audio from that channel will be distorted until it suddenly "pops" and becomes clear again. I tried hitting the components with freeze spray while it was occurring but have been unable to pinpoint anything with this method.

Anyone have any idea or recommended steps to troubleshoot? I was using a Kenwood amp in place of the GAS for the last two years and forgot how much better the GAS sounded.