Quinny
Active Member
- Joined
- Nov 4, 2022
- Messages
- 54
- Reaction score
- 102
My 73 JMP is at the tech this week, having gone silent recently. Was due a service and check-up, so dropped off at local chap to give it the once over.... HT fuse had blown, one bad power tube in there which we assumed caused it. New tubes, bias set, general check and test and I picked it up today. 60 seconds in just as I put volume up to around 6, HT fuse blew again. I use a PS100 with my old Marshalls, so took that along with the amp back to tech - this time he let me hang around while he rigged up and did various tests, which was GREAT to see, very interesting. Initially all looked well, amp checking out OK and PS100 ruled out as a suspect. When the volume gets up around 5+ we noted one of the measurements (apologies, I'm not tech enough to know which one) was hovering around 600mA, which of course began pondering as to whether this was blowing the 0.5A HT, but it wasn't doing so during testing. (The initial blown HT was 1A instead of the specified 0.5A, in there from when I bought the amp a year or two ago, so maybe some history there and someone decided to just up the fuse rather than deal with the actual issue.) Various signals input and measurements taken at various stages, all looked rock solid on the various scopes and screens used.
Eventually we plug a guitar in and he says set the amp up like you'd normally play it - for me on a NMV JMP that's around 6 just as the gain is good and not starting to mush. Played a while, all good.... but, played another few chords and as I was playing there was some manner of surge internally, which blew the HT. Was great to see the event captured on the test kit - a big wave from somewhere and fuse gone again. The amp was previously tested with signal generator at full volume in all inputs but for whatever reason that wasn't causing the event to occur (as I type, I wonder if my playing is the problem!!). I've left it with him, he's going to get it happening reliably/consistently then work out what's going on.
Probably a whole heap of reasons this might be, but just for fun was wondering if anyone had seen this before and might have a guess what the issue will turn out to be?
On the plus side, for the brief period it worked earlier today when I had it home it sounded notably better for a service, so I'm excited as hell to get it back. This is the amp - May 73 turret board 50W - my favourite!
Cheers!
Eventually we plug a guitar in and he says set the amp up like you'd normally play it - for me on a NMV JMP that's around 6 just as the gain is good and not starting to mush. Played a while, all good.... but, played another few chords and as I was playing there was some manner of surge internally, which blew the HT. Was great to see the event captured on the test kit - a big wave from somewhere and fuse gone again. The amp was previously tested with signal generator at full volume in all inputs but for whatever reason that wasn't causing the event to occur (as I type, I wonder if my playing is the problem!!). I've left it with him, he's going to get it happening reliably/consistently then work out what's going on.
Probably a whole heap of reasons this might be, but just for fun was wondering if anyone had seen this before and might have a guess what the issue will turn out to be?
On the plus side, for the brief period it worked earlier today when I had it home it sounded notably better for a service, so I'm excited as hell to get it back. This is the amp - May 73 turret board 50W - my favourite!
Cheers!
