Early 70's Ampeg SVT Grounding Earth lead mod.

[Seans]

A little bit in the dark here, but is a mod for a grounding lead modification the normal thing these days for the twin power lead Ampegs or what, does it affect the sound, was no grounding normal in the 70's as it was sometimes there and sometimes not, etc etc. I haven't had any problems with the amp yet and am quite happy with the hum etc but just thought I'd ask, cheers guys.

[billfitzmaurice]

I thought with UK power a 3 prong mains lead was a necessity.

[Seans]

Originally Posted by billfitzmaurice

I thought with UK power a 3 prong mains lead was a necessity.

Well yes, as its 240v, but as the 1970 SVT has no spur on the transformer for 240v I am running a step down Tran to 110v, but really I would like to know if this mod was the norm for you guys on 110v, or do you just run the twin leads still.

[johnk_10]

i run a 3-prong grounded AC cord on my '69.

[JimmyM]

mine was changed over to 3 prong a long time ago. if you haven't done it, do it, just for the peace of mind of knowing you won't die if you touch someone else's guitar. ok, i'm exaggerating a bit, but shocks can be quite nasty and take a lot out of you.

[Seans]

Ok sounds good, I just need to decide whether to run a fixed cable or cut a hole for a socket, not sure I want to do the latter, but, it is the safest option if someone trips on your power lead it doesn't pull the amp off the cab.

[Seans]

Looking at the schematic would the polarity switch need to be disconnected?.

[Jerrold Tiers]

Originally Posted by Seans

Looking at the schematic would the polarity switch need to be disconnected?.

You may as well disconnect it.....

The point of the polarity switch was to counteract the leakage current from transformer to chassis.... because originally the 2 prong US plug could be plugged in either way, so the switch reversed the compensation.

Now, if you use the 3 prong, the chassis is grounded anyway, so the switch has no function.

[Seans]

Many thanks, will it matter if its left for a while or should it be disconnected asap.

[BassmanPaul]

Originally Posted by Seans

Ok sounds good, I just need to decide whether to run a fixed cable or cut a hole for a socket, not sure I want to do the latter, but, it is the safest option if someone trips on your power lead it doesn't pull the amp off the cab.

A hard wired connection is better than a plug and socket. Cutting a hole to mount and IEC socket will devalue your vintage amp. Running your cables properly so they are not tripping hazards is something that should be practiced by all. However we do have drummers.............

Paul

[BassmanPaul]

Originally Posted by Seans

Looking at the schematic would the polarity switch need to be disconnected?.

Remove the capacitor, leave the switch.

Paul

[Seans]

Originally Posted by BassmanPaul

A hard wired connection is better than a plug and socket. Cutting a hole to mount and IEC socket will devalue your vintage amp. Running your cables properly so they are not tripping hazards is something that should be practiced by all. However we do have drummers.............

Paul

Thanks Paul, problem is our singer is quite an athlete and a good flat surface like an SVT........

[BassmanPaul]

Barbed wire is a good deterrent I have always found. Tell him not to abuse the tools of your trade. I had a guitar player once in the 'Pool who thought it was OK to walk on my guitar case with my bass in it. He didn't do it again after I stood on his Strat!

Paul

[Kindness]

Originally Posted by Jerrold Tiers

Now, if you use the 3 prong, the chassis is grounded anyway, so the switch has no function.

I believe you. However, I know someone that is generally quite knowledgeable about tube amp design and function and he believes that, particularly in these old Ampegs, adding a three prong cord while keeping the cap in place allows you to use the polarity switch to achieve a lower noise setting. Is that possible? I have no practical experience as I have always removed the cap from the circuit when installing a three prong cord.

[Seans]

Paul, perhaps I should leave the earth wire close to the amp and let someone who goes near make the connection for me, got be a good 56volts

[Seans]

Originally Posted by BassmanPaul

Remove the capacitor, leave the switch.

Paul

And leave the cap wires unconnected or join them up?.

[BassmanPaul]

Originally Posted by Seans

And leave the cap wires unconnected or join them up?.

Cut both ends of the cap out of the circuit and leave it that way. Don't do ANYTHING else.

Paul

[BassmanPaul]

Originally Posted by Kindness

I believe you. However, I know someone that is generally quite knowledgeable about tube amp design and function and he believes that, particularly in these old Ampegs, adding a three prong cord while keeping the cap in place allows you to use the polarity switch to achieve a lower noise setting. Is that possible? I have no practical experience as I have always removed the cap from the circuit when installing a three prong cord.

A line filter partially consists of high voltage AC capacitors between phase to ground, neutral to ground and sometimes phase to neutral. Leaving the cap in simulates this to a certain extent. My view is that with the weakness of a typical line cords ground pin, if the cap (possibly many years old) leaks you can easily get a live chassis situation. I prefer not to take that risk.

Paul

[Seans]

So not knowing where to connect my live would it be the fuse?.

[BassmanPaul]

That should be obvious from the original power wiring. You should check what is acceptable for your local electrical code. Back in the day UK amps would feature a double pole mains switch switching both phase and neutral. In the Americas that would be strictly against code. I usually wire the phase to the fuse to the switch. Thus if anything shorts the phase will be disconnected. The phase should go to the back (furthest from the mounting) connection. This will prevent a shock if someone tries replace the fuse with the AC connected. IE the hot end will contact last.

Paul