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Got the amp home a week later, in it's head-shell of course, and it was doing it at home as well. So I tried the same thing, and it went away again.
Replaced the rectifier tube, and it went away full stop.
Weird huh???
The reason I suspected the recto tube in the first place was because back in the day if you remember my first SigX amp, and the famous 12AY7 tube experiment incident .... it actually turned out to be a dodgy recto tube back then too!!! The old one sounded like a bloody rainstick!! Except I'd never had an amp with a rectifier tube until that point, so I didn't know it was broken!!
Anyway. If it were me, I'd be buying a preamp tube from some good stock, and test it out in every position in the amp first. If that doesn't make the issue disappear, I'd do similar with a set of poweramp tubes (always good to have a backup replacement of those anyway!) and if that doesn't cure it, then I'd be opening the amp up to check for dodgy joints.
The thing that makes me think it's not a bad joint is the fact that Nerine said it was a smooth volume drop - I don't think a bad joint would result in that. I think it would result in way more erratic sound output, with the volume jumping all over the place in a very random and inconsistent manner.
I'm no expert though
Your input is appreciated though, it’s entirely possible there is something I haven’t thought of. Quite a few times I’ve even put the amp back together thinking I’ve fixed the ‘obvious’ cause of the problem, only for it to come back! I’m sure most techs have .
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Standby switches are useful .
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The amp should get plenty warm enough though. It’ll be on like 2-3 on the master for the duration which is pretty loud on an 800 (when not attenuated).
I bloody knew it would be. Typical isn’t it.
It’s a head scratcher, this one...
Whether the PT was the cause or it whether it failed due to some other component dying, I don't know...because I can't get hold of replacement transformer to test it.
Yeah. I’ve kinda resided myself to the fact that it’s likely something a bit more serious that could possibly take down the entire amp.
The slow power-down over 10-20 seconds or so can only be one of two things:
1. valve(s) cooling down due to a filament problem. Either a faulty valve, a bad contact between the valve and the socket, or a filament supply circuit failure which could include the power transformer.
2. high-voltage supply dropping as caps(s) discharge due to the supply being interrupted. If it's this it must be downstream of the power valves or it would fade out a lot quicker than over 10-20 seconds.
The question is which, and where.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I ended up speaking to Marshall.