My amp shocked me

  • Thread starter Thread starter Toki987
  • Start date Start date
easychair said:
No, it's not. But if the hot is connected to the amp chassis by the switch being in the wrong position or the outlet being wired wrong, and you touch the ground of another piece of gear, you make a connection between hot and neutral. You get shocked.

There is nothing necessarily wrong with the amp. The above could happen in a perfectly working amp, and a correctly wired outlet, if the switch is in the wrong position.

Those switches are bad news, Toki. I'd have a tech remove it and wire the thing right. There is really not much need for the safety ground to be hooked to either leg with a three wire plug. It's easy in that amp, you should just be able to unhook it, basically.

They are handy if an outlet is wired wrong, but like I said, you can get shocked even if everything is OK, so at least be careful. Punkin has a good idea about checking. I check every time with mics and tube amps.

Sounds meritable to me. The amp has worked like a pack mule for me with no problems at all. Last time there was a shock was last yr up in Tenn at a spring festival. I didn`t see it, but I heard one of the guys on the sound crew squeal like a pig really loud, and shout something pertaining to his mother.
 
easychair said:
No, it's not. But if the hot is connected to the amp chassis by the switch being in the wrong position or the outlet being wired wrong, and you touch the ground of another piece of gear, you make a connection between hot and neutral. You get shocked.

There is nothing necessarily wrong with the amp. The above could happen in a perfectly working amp, and a correctly wired outlet, if the switch is in the wrong position.

Those switches are bad news, Toki. I'd have a tech remove it and wire the thing right. There is really not much need for the safety ground to be hooked to either leg with a three wire plug. It's easy in that amp, you should just be able to unhook it, basically.

They are handy if an outlet is wired wrong, but like I said, you can get shocked even if everything is OK, so at least be careful. Punkin has a good idea about checking. I check every time with mics and tube amps.

Good points there Easychair. I've never seen an amp equipped with a polarity reversal switch like that. I'm with you...it seems like a crazy idea. I still don't see how from the diagram that was posted how that switch actually cuases a polarity reversal though. The switch looks like it's DC/continuity isolated from ground through a capacitor. You've obviously seen this sort of set up before. I'm thinking the schematic isn't correct or the amp isn't exactly as the schematic appears.
 
punkin said:
Good points there Easychair. I've never seen an amp equipped with a polarity reversal switch like that. I'm with you...it seems like a crazy idea. I still don't see how from the diagram that was posted how that switch actually cuases a polarity reversal though. The switch looks like it's DC/continuity isolated from ground through a capacitor. You've obviously seen this sort of set up before. I'm thinking the schematic isn't correct or the amp isn't exactly as the schematic appears.

The switch doesn't cause a reversal. I may have been on a crack binge. All it does is switch the cap from one leg of the service to the other.

The cap was necessary back in the day, it grounded noise from the chassis. Tube gear components, most notably transformers, produce eddy currents in the chassis. This makes voltage, and noise from the chassis not being a ground potential. There is an imbalance. It is also a natural part of tube gear. Some trannies and designs do this more than others. The chassis needed to be tied to ground, and through the neutral was the only way. I'll admit I don't fully grok this, but the cap makes it a high-impedance connection? Apparently this is important. The three wire system gets rid of the problem by providing a direct signal connection to ground without going through the neutral. Some makers still provide the switch, though. All I can figure is that...well, I can't figure it. I'll ask THE MAN WHO KNOWS next week.

A side note- some tube amps don't like to be on GFI outlets. GFIs trip when they sense a current imbalance on the legs. If enough current is being shunted through the ground wire, the GFI will sense the imbalance, interpret it as a fault, and trip, even though the amp is working fine. It's mostly old ones, as now there are strict rules about how much current and voltage can be on a chassis.

Another- lots of old tube stuff, radios, TVs, amps, didn't have the switch. You just plugged it in the other way if there was a problem. The switch was a convenience, then a necessity when polarized plugs came along.

Back to it-
All I really know is that you can get shocked if the switch is set wrong, and the hot is connected to the chassis. And you can get shocked dead if the cap has shorted and the switch is in the wrong position. That will put full line voltage on the chassis. And the amp can seem fine. Most people who get a jolt haven't really got the whole thing, the cap is protecting them to an extent.

You are right, Toki's amp is different than that schematic. His has a three wire system, with ground connected to the chassis. The cap and switch are after the accessory outlet.

At least, that's what pretty much every other late '70s Fender has. It can most likely be removed without affecting the amp at all. And even if it has a noise issue, it can most likely be resolved another way. Many noise issues in more modern amps can be traced to design and layout and component compromises.

I find it interesting Mesa still provides these switches. And that they say "if the switch is left in the middle position, any other shocks come from other gear" without ever saying that using the switch could shock you.

:rolleyes:

Happy Thanksgiving. :)
 
Here`s the AC section of the schematic.
 

Attachments

  • bassman_135_schem_ac.webp
    bassman_135_schem_ac.webp
    36.3 KB · Views: 77
Zapped!

Hey Toki,
Long time......Once upon a time, while playing outdoors, my bass player leaned over to say something to me while we were playing and zapped me on the ear! It was definately 120 volts! Upon further investigation, the ground wire on his power cord was not making a connection, which is the same thing as ground lift. If you can't find anything else, check your cord.

bd
 
arcaxis said:
While I do agree that there is no substitute for a high quality piece of test equipment and these cheap testers could possibly give erroneous reading due to circuit capacitance and such, a cheap tester may be better than none at all. The author of the article suggests the use of a ground impedance tester($$$$ :eek: ) and he is correct that this is the best way to verify a circuit. I do agree on this. The author of the article also represents a company providing professional power quality services and maybe in my mind he might slightly tend to bias information that would prove beneficial to promoting his interests.

I still would recommend the cheap tester over nothing at all :)

:D

You are right, of course, and the problems need to be pretty severe for a regular outlet tester to fuck up. Personally, I use an ECOS 7106, which was about $100 when they were new. I think ECOS is long gone, but the 7106 shows up on ebay pretty often for $20 or so. Either that, or I use my meter.
 
I appreciate it guys. After reading what you`ve said, it appears that there could be a leak going into the chassis. I`ve heard that it could take somebody out if the conditions are just right. That`s kinda spooky.
I`ve got a Simpson 260 that I use for simple stuff, but Im no electrician. I think I`ll take the head over to a guy that mods Fenders down here and tell him to just make it so it won`t shock the hell out of me.
 
i used to have a real old tube amp when i first started playing. we played in my buddy's basement on concrete floors. i noticed that i couldn't touch the strings without feeling a bit of electricity. so i ended up having to stand on a few peices of carpet, that seemed to help me a bit.
 
Back
Top