Parallel Amps

[C Moore]

The problem with this is that one amp will only be connected to ground via a series of instrument cables and jack sockets.
These aren't rated for / capable of conducting fault current, eg if the power transformer shorts and sends line voltage to the chassis / input jack shield.
If the only link to safety ground is at the far end of > 30ft of instrument cables, jack contacts, PCB traces etc, then it's likely that the weakest link in that chain will blow when passing tens of amps of line current.
In such a circumstance, the whole pedalboard / guitar metalwork / musician could get pulled up towards line voltage.

In the light of the above, I hope that it is now apparent that such a tenuous connection to ground is unsuitable for maintaining electrical safety.

Yes...I have always been told to Never Lift The Mains Ground.....Seems like a mistake.
But how do you lift the signal ground, (of 1 of 2 parallel amps) and still have a complete circuit in that second amp.

Thanks Pete

[Reeltarded]

You lose gain across the para. A Deceiver mirrors your pickup impedance to many outputs, expensively. Everything else sounds different, like when you turn the knob on your guitar to 8 and the highend is yesterday, adios, goodbyebyes.

My amps have 600+ volt wires inside. I don't know what capacity the shield on a heavy WW signal cable has but I know for sure it can handle ground for long enough to blow a fuse on the main amp if something melts and the same jacks can handle enough current to arc weld with for long enough to blow the mains.

Maybe I'm a lucky idiot.

Blackmore playing three Majors with coily signal cables strapping them and look at the polarity switches. They had no common. Get the screwdriver and flip poles til it stops humming. I remember what that was like. 3x2 combinations. Don't be wrong when you eat the mic. Yikes. Lightning.

[Zippy]

Don't be wrong when you eat the mic. Yikes. Lightning.

You lose more roadies doing sound check that way...

Yikes indeed, that makes me recall the good ol' days of setting everything up, then, with guitar in hand, tapping the mic to see if the grounds were right.

Yeah, lip sparks... Hated that!

[xtian]

Another device to solve this problem is a transformer-islolated ABY switch, like Lehle:

http://lehle.com/IT/ABY-Switcher

[pdf64]

how do you lift the signal ground, (of 1 of 2 parallel amps) and still have a complete circuit in that second amp

With guitar rigs, signal ground is generally connected to line ground, so the signal input will get its ground reference via the line cord.
Unfortunately that may still result in hum interfering with the signal, so some sort of transformer coupled interface is needed, eg Palmer, Lehle.

In the old days folk used to screw around with electrical safety and generally get away with it.

Thankfully we're getting a little more risk averse when it comes to fatal hazards.

[C Moore]

Another device to solve this problem is a transformer-islolated ABY switch, like Lehle:

http://lehle.com/IT/ABY-Switcher

This (or similar) look like smart way to go.
I am no Saint. I have used a 3 to 2 adapter to lift the ground on the "B" amp.....but I never liked doing it.
Thank for that link btw.......

[pdf64]

I don't know what capacity the shield on a heavy WW signal cable has but I know for sure it can handle ground for long enough to blow a fuse on the main amp if something melts and the same jacks can handle enough current to arc weld with for long enough to blow the mains

In the interest of safety, to flog a dead horse still further, it may be beneficial to highlight the point that jack connectors / instrument cables are absolutely not rated to carry line fault currents.
My guess is that its a flip of coin whether the line fuse or the signal ground path blows open first.
Also consider that whilst passing that current, the signal ground would be pulled up pretty much to line voltage, ie the guitar strings / metalwork will be live, either for the few seconds it may take for the line fuse to blow, or for a more prolonged time, if the signal ground blows open.

[Reeltarded]

All great points. Truly. I don't want anyone to not be as methodic and turn up missing.

I am pretty paranoid.