grounding a tube amp?

[jspartz]

Hello,

I have a Sano bass amp that when I am using it, it shocks me when I touch the rest of the PA. The Sano has a two prong electrical plug. I would like to ground it so my bass playing is not such a shocking experience.

Not sure where to start. A little direction would be great!

Jason

[RodC]

Do you have a schematic? You need to be very careful, high voltages in there. Also you have to make sure the frame is not part of the circuit for your incoming 120. There should be no connection to the frame and part of the input power, look for a transformer.

ALSO does your amp have a polarity switch? If so look for the "Death Cap" that could be bad and should be eliminated. No current code allows anthing connected to the mains in such a mannor.

NOTE WARNING
If you dont know what you are doing take it to someone who does. If you ever get shocked via hands on strings and lips on mic you could get killed and many have! Just do some googleing and you will see lots of cases.

[Scodiddly]

Grounding an amp is definitely the way to go. Actually it's not that difficult to do, mostly you're removing the RF coupling cap (aka "death cap") and other stuff that was meant to simulate a ground by coupling to one of the AC lines.

The usual disclaimer - serious safety issues, get a professional if you're at all unsure! Also, make sure you're working on an amp that has an actual power transformer, not a "hot chassis" design. Some cheapo amps did away with the power transformer, definitely a risky approach - if you see tube numbers starting with unusual numbers such as 35 or 50 then you've likely got one of those evil things.

Anyway, the typical setup had a capacitor between the chassis and one of the wires in the 2-prong plug, often through a polarity switch. The thinking was that the capacitor wouldn't pass any dangerous AC current, just RF and other noise, and that if you set the polarity switch right you'd be connecting to neutral anyway (50% chance, right?). That's in the days when grounded outlets were rare and PA systems didn't really exist in the modern sense. So what you'd do is remove the capacitor, remove any connections between the chassis and the 2-prong plug wiring (probably completely disconnect the polarity switch). You'd then take away the 2-prong plug and put in a 3-prong plug, connecting the line and neutral wires to where the 2-prong wires went and then the ground wire directly to the chassis.

While you're at it, make sure the fuse is the absolute first thing that the line wire hits when it comes into the amp. That way if the power switch shorts out the fuse can still do its job.

[jspartz]

Ok,

The two wires coming in split. One goes to the polarity switch and the other goes to the fuse. It does have a good sized transformer in the power supply. I do not have a schematic so maybe I need to spend a little time tracing out the circuit.

Jason

[RodC]

Ok,

The two wires coming in split. One goes to the polarity switch and the other goes to the fuse. It does have a good sized transformer in the power supply. I do not have a schematic so maybe I need to spend a little time tracing out the circuit.

Jason

If you are getting shocked chances are the cap for the polarity is leaking. This is why they call it the "Death cap", you should eliminate that circuit and ground it. If it has a polarity switch I think it must have a transformer, most of the polarity switches were on the secondary of the power transformers. When the cap goes bad there is no isolation from the transformer.

[brianroth]

The "death cap" is on the primary of the transformer (ie, AC mains side). **IF** you must have it in the circuit for some reason, there are UL approved caps intended for connections from mains to chassis ground.

Bri

[RodC]

The "death cap" is on the primary of the transformer (ie, AC mains side). **IF** you must have it in the circuit for some reason, there are UL approved caps intended for connections from mains to chassis ground.

Bri

They are not always on the primary side, check out the old ampegs. They actualy are on the secondary side. You have to do some tracing because the polariy switch is on the preamp section but it is connected to the secondary of the power transformer.

http://www.drtube.com/schematics/ampeg/svt6550-jp.gif

http://www.drtube.com/schematics/ampeg/svtpre-jp.gif

If you take a look the polarity cap is connected to Pin 2 of the connector that runs from the preamp to the poweramp section. Note where pin 2 is connected. The ground for the powersection which is connected to the center tap of the secondary.

(In this case it has 2 in series for higher voltage handling, but the ones I have disabled only has 1)

[nclayton]

They are not always on the primary side, check out the old ampegs. They actualy are on the secondary side. You have to do some tracing because the polariy switch is on the preamp section but it is connected to the secondary of the power transformer.

Hi. No big deal, but just to clear up, if there's a cap on the secondary side of the transformer then it definitely can't be serving the same purpose as the exterior "death cap", since the transformer itself isolates the AC line from the amp and balances it, making a ground reference for the AC line impossible to make on that side.

On the SVT the polarity switch and cap actually are on the primary. Looking at the schems. I think the confusion might be that there are two different transformers (HT and filament) with two different primaries. You might be interpretting the primary of the HT transformer as a secondary since it has the "standby" switch breaking it. This works with the SVT because when "ON" the filament primary is attached to AC so heaters warm, then when you turn on the standby switch the HT primary is attached to AC so the amp gets high voltage and runs.

Ned

[RodC]

They are not always on the primary side, check out the old ampegs. They actualy are on the secondary side. You have to do some tracing because the polariy switch is on the preamp section but it is connected to the secondary of the power transformer.

Hi. No big deal, but just to clear up, if there's a cap on the secondary side of the transformer then it definitely can't be serving the same purpose as the exterior "death cap", since the transformer itself isolates the AC line from the amp and balances it, making a ground reference for the AC line impossible to make on that side.

On the SVT the polarity switch and cap actually are on the primary. Looking at the schems. I think the confusion might be that there are two different transformers (HT and filament) with two different primaries. You might be interpretting the primary of the HT transformer as a secondary since it has the "standby" switch breaking it. This works with the SVT because when "ON" the filament primary is attached to AC so heaters warm, then when you turn on the standby switch the HT primary is attached to AC so the amp gets high voltage and runs.

Ned

The ground that the polarity cap is connected to is on the secondary side, thus if the cap is shorted out there is no isolation through the transformer.

The other legs of the switch are connected to one of the 2 incoming AC lines, so, if the cap is shorted out and the switch is connected to the hot leg of the AC the amp then has 120V on the ground on the secondary side of the amp.

Look close, the cap for the polarity is connected to the orange wire on the secondary of the transformer. (Which is the ground for the amp) Note all the rectifiers on the same connection? They are not on the primary side of the transformer.

[nclayton]

Okay, I just misunderstood what you were saying. I thought you were talking about which AC was being referenced to ground, not which ground the AC was being referenced to. Obviously it doesn't make sense to use a "death cap" on one of the stepped up AC voltages on the transformer's secondary since it's already isolated from the AC line. But I can see now you're saying it's the AC line that's being shunted, not the transformers' secondary, it's just being shunted to a "different" ground.

I'm still not convinced this is any different from normal, though. I think that the ground shown inside of the power transformer (the orange) is strapped to the chassis at some point, even if that connection isn't explicitly shown on the schematic. I don't see any "earth" connection outside the transformer since this schematic only shows a two prong AC cord. I think it's all the same ground.


Ned

[RodC]

I'm still not convinced this is any different from normal, though. I think that the ground shown inside of the power transformer (the orange) is strapped to the chassis at some point, even if that connection isn't explicitly shown on the schematic. I don't see any "earth" connection outside the transformer since this schematic only shows a two prong AC cord. I think it's all the same ground.
Ned

Agreed, what is weird is that these models have a 3 prong plug, so it just leaks to ground. Still shocks the crap out of you if you have a weak ground.

I think we were saying the same thing, I was just trying to point out that you are loosing the isolation.

[brianroth]

OK...I'm sure there are exceptions to any rule! Typically (Fender, Marshall and many others), the Death Cap was on the primary/mains side, connected from the wiper of the polarity switch to chassis.

Now I need to go study that SVT schemo <g>.

Bri

[RodC]

OK...I'm sure there are exceptions to any rule! Typically (Fender, Marshall and many others), the Ceath Cap was on the primary/mains side, connected from the wiper of the polarity switch to chassis.

Now I need to go study that SVT schemo <g>.

Bri

I belive we are all correct, I did not understand what you guys were saying at first. I thought you were maintaining that the isolation was intact if the cap was shorted. (Nothing on the secondary side) In the SVT case I bet it is like nclayton stated, the chasis is probably the same circuit. I was just trying to point out why the death cap could ruin the isolation.

[workshed]

Not to hijack this topic, but is it as dangerous to add a three-prong plug to a solid state amp? I have this amp that I'd like to add a three-prong plug to:

http://www.workshed.com/transfers/personal/rosac/

If it's something I can manage myself, I'm willing to give it a try, but I also don't want to end up dead for the sake of DIY.

-Bret

[nclayton]

it's always less dangerous to work on a solid state amp than it is to work on a tube amp, but you can still get zapped. The main thing is to just be really really careful if you ever have to work on an amp when it's plugged in. If you're changing the power cord I would assume you'll probably unplug it first.

There's a lot of "don't do it if....." warnings on this thread, but I'd just say go ahead and do it. It's easy to do, and 99.9% of amps in the world have power transformers. If the .1% guy tries to install a grounded cord on an amp that runs directly from the AC line, the worst that will happen is his amp's fuse blows the first time he turns it on... well...I guess the WORST is actually your breaker might blow, but big deal.

If your amp is the type that employs a so-called "death cap" you can still add a three prong cord and you don't necessarily have to remove the cap (although if you can find it, it's a good idea to snip it).