1995 Laney VC30, double checking extremely alarming behavior as expected

[OwenM]

Hi there

I recapped and changed a few pots on a Laney VC30 recently, I changed the 3 15uf polarised electrolytics and the big dual 50uf can capacitor. All was fairly straightforward but I noticed an alarming trait, accidentally.

There is a ground connection coming off the PCB, meeting with the negative of the big can cap and then bolted to the chassis. When the wire from the PCB to the chassis ground is removed/desoldered then I get -300v dc on the chassis compared to the guitar input signal and ground connectors, and as a result the guitar strings too...

The main earth connection (from various points on the chassis and circuit to the ground pin of the power cord) is good so I think the B+ potential is getting to somewhere on the signal path rather than a bad earth on the chassis. On the schematic only the negative of the big can cap and the HT fuse are connected to THAT ground point, but the ground of several other components meets on the PCB before coming off a pad and meeting with the big can cap and chassis ground. I suppose I could hypothesise that the B+, not grounded now, is travelling back to the ground around the input stages and that's why it's appearing on the I/P jacks, but that's just conjecture and I'm really not sure....

So what I'd like to know is, is this normal??? I'm inclined to think not...but if I remove the HT fuse then the amp dies and is safe, so is this expected behaviour, and the HT fuse is there in the event that this ground wire comes loose and the guitar strings develop a potential? That still seems a bit scary...

Or does it point to a shorted cap further up the circuit or something similar? I can't SEE a path for the B+ around the circuit on the schematic for it to somehow get into the signal path but its hard to see what other components ground at the same point so what could potentially be going on.

On the schematic the PCB of my model is 9072-4, page 6. The focus area is down the bottom in the middle. The loose wire would be severing the connection directly after pad 57. Pad 56 and 57 have the HT fuse between them, between pads 51 and 58 is the 100uf can cap.

Thanks for reading and any help that can be offered!

I'm UK btw, if that matters!

Sorry can't post attachments yet, here's a link to the schematic - https://elektrotanya.com/laney_vc30-.../download.html

[Dave H]

There is a ground connection coming off the PCB, meeting with the negative of the big can cap and then bolted to the chassis. When the wire from the PCB to the chassis ground is removed/desoldered then I get -300v dc on the chassis compared to the guitar input signal and ground connectors, and as a result the guitar strings too...

The main earth connection (from various points on the chassis and circuit to the ground pin of the power cord) is good

I prefer to think of it as +300V on the guitar input compared to the chassis (which is mains earthed). It's probably normal if you have removed the ground reference from the PCB 0V rail making its only connection to the power supply B+ meaning the whole circuit floats up to B+ voltage.

[OwenM]

Thanks very much for the reply, I'm starting to make peace with it being normal but that's quite a big and easy way to put +300v on your guitar strings. Definitely an area I will pay double attention to in future and make sure there's a really good mechanical and soldered connection at that lug!!

I would imagine having other points it grounds to along the way would create ground loops etc, but still this is the first time I've come across something in my (limited) experience recapping and fixing pots/bias etc that could easily make something very dangerous, I'd kind of relaxed a bit with working with tube amps but now I'm dutifully reminded that it's not quite as safe as I was thinking..

Note: I always take big precautions and know it's not 'safe', and I am always very careful, I'd just come some way to making peace with the common internet warnings on less electrically-minded forums saying 'don't open your amp! It's a death trap!!'

[g1]

I think it is basically a 'ghost' voltage incapable of any appreciable current or being a safety hazard. I think that is what Dave H alluded to without actually saying ('whole circuit floats up to B+ voltage'). It's like the bird on a high voltage wire thing.
Otherwise we would be hearing of electrocutions when chassis ground connections are lost or missing.

[Dave H]

I think it is basically a 'ghost' voltage incapable of any appreciable current or being a safety hazard.

There's a safe way to find out. Connect a DMM set to 'mA' where the missing ground wire should be.

[OwenM]

There's a safe way to find out. Connect a DMM set to 'mA' where the missing ground wire should be.

Thanks, yeah it shows about 1.5mA so my fears have been calmed!

Still humms a lot though... I think I'm gonna have this one apart for a bit longer yet...