Ok so this issue started a few months ago around last september and is getting worse. My head is a HA3500 and is about 10-12 years old. Its been a great amp and is well used. When the issue started it presented itself as an intermittent low volume crackle that wold ride on the back of certain notes down the lower end of the fret board. At first It only seemed to happen with my passive bass plugged into the passive input jack and I put the problem down to the bass. I spent an evening replacing the Pots, jack and Capacitor (I thought this was the issue)and re wiring in my bass. and at next practice all seemed solved.... for an hour or so.....then the issue returned. I switch to another bass, an active one and plug it into the active socket and all seemed OK. Bamboozled by my issue, I continued playing that active bass for a few weeks while trying to solve the problem with the passive jack. I took the lid of the head and checked the connections on the jack, they all seem good. Just for good measure I cleaned out the inside of the chassis with an airline and changed the tube for a brand new 12AX7 Groovetube. Still the problem persisted with the passive channel. We had a few gigs over the Christmas period and I continued with the active channel and active bass with no issues then we had a long break with January and February off. First rehearsal this year first week of march, I plug in the active bass and noticed that crackle is present, much like when i first noticed it on the passive channel. I had another cab available, so just to confirm this is the head I plugged in the other cab. The problem persists. I also retried the passive bass, its that bad the amp sounds very sick indeed. I am 100% sure the issue is with the head. I have also noticed that when a loud guitar strum is played on a different guitar this can cause a low level crackle on my amp from across the room. The crackle seems unaffected by changes to ether the pre amp settings, compressor or EQ but is louder with louder volume settings. So i'm asking for help to identify where my problem lies. The issue is present on the above video at 4:05 -4:20 ish and again 5:03 Ish I'm guessing a bad solder joint somewhere, are there any known week areas? are theses prone to failing with age? A bad cap maybe? Dodgy transistor? Anyone Able to take a stab? I have gigs approaching and really need this Damn think back up and running. Thanks in advance.