I'm getting a light shock

[pbman1953]

I finished my Fender Super Twin Bass amp project. I installed the chassis in a Mojotone Fender-like cab. There are 2 metal strips on the top of the cab. The bolts go through the cab top and chassis to tighten with a nut.


If I happen to touch the metal strips while changing the metal speaker cab jack, I get a light shock.


Does anyone know why? Is this normal ?

[72Blazer]

I'm still new but we'll definitely need detailed pics of the guts of the guts inside the chassis as well as the schematic/layout used.  Any special mods you installed?  Death cap? etc..

Someone will be able to assist though.

Vr
J.

[acheld]

If your chassis has a permanent and dedicated connection to earth, that should not be happening.

Is your chassis grounded?

Is your wall outlet properly wired?   You'd be surprised how often it is not . . . I always carry an outlet tester (one like this   https://www.amazon.com/Receptacle-Tester-Klein-Tools-RT110/dp/B01AKX3AYE/ref=sr_1_7?crid=JOFASABWQ4Z7&keywords=outlet+tester+plug&qid=1648911707&sprefix=outlet+tester%2Caps%2C137&sr=8-7

Check these two items first.

[shooter]

Fender Super Twin

new build or brought back to life?
if original, was it a 2 prone'r?

[sluckey]

It's magic. But simple solution... Don't do that!

Read this short thread about a very similar situation...

     https://el34world.com/Forum/index.php?topic=28634.msg315352#msg315352

[pbman1953]

3 prong plug with the green wire screwed and soldered to the chassis


The PT  ground and the OT grounds are grounded together.


 The death cap was removed before I got it

[tubeswell]

3 prong plug with the green wire screwed and soldered to the chassis

Picture please.

[pbman1953]

3 prong green ground

[pbman1953]

PT black wire ground

[pbman1953]

OT Ground.


From the PT wire ground picture, there's a black wire coming to the EXT speaker jack. Then from there, a black wire goes over to the Spk jack , where the OT connects

[tubeswell]

I wouldn’t trust that mains ground connection. Looks old and it may be failing. Install a new mains cord with a new safety ground. Clamp the bare wire strands securely and firmly to a ring terminal before soldering. Then bolt the ring terminal securely to the chassis with it’s own dedicated bolt and locking nut. (Make sure the chassis is clean before attaching the bolt.)


Also make sure your speaker Jack sleeve is wired to the OT secondary common.

[PRR]

> If I happen to touch the metal strips while changing the metal speaker cab jack, I get a light shock.

How??? The strips bond to the chassis their screws. The speaker plug is in the jack which is firmly screwed to the chassis. The chassis is a heavy hunk of metal. The strips and plug "can't" be at different voltages.

Something you are not telling us. Crappy connections. Standing on dirt or concrete. Silk undershorts in dry-dry weather.

[pbman1953]

I wouldn’t trust that mains ground connection. Looks old and it may be failing. Install a new mains cord with a new safety ground. Clamp the bare wire strands securely and firmly to a ring terminal before soldering. Then bolt the ring terminal securely to the chassis with it’s own dedicated bolt and locking nut. (Make sure the chassis is clean before attaching the bolt.)


Also make sure your speaker Jack sleeve is wired to the OT secondary common.

Ok,


I'll buy a new wire. I have ring terminals, thanks

[pbman1953]

> If I happen to touch the metal strips while changing the metal speaker cab jack, I get a light shock.

How??? The strips bond to the chassis their screws. The speaker plug is in the jack which is firmly screwed to the chassis. The chassis is a heavy hunk of metal. The strips and plug "can't" be at different voltages.

Something you are not telling us. Crappy connections. Standing on dirt or concrete. Silk undershorts in dry-dry weather.

Here are the strips. Think of a Bassman and other early 60's Fenders.

[pbman1953]

I wouldn’t trust that mains ground connection. Looks old and it may be failing. Install a new mains cord with a new safety ground. Clamp the bare wire strands securely and firmly to a ring terminal before soldering. Then bolt the ring terminal securely to the chassis with it’s own dedicated bolt and locking nut. (Make sure the chassis is clean before attaching the bolt.)


Also make sure your speaker Jack sleeve is wired to the OT secondary common.

When you say secondary common, please explain. From there's the power wires Red, Brown & Blue, white/brown & white/ blue, then the speaker wires- black and green. There's only one set of speaker wires because the OT only has a 4 ohm out put

[pbman1953]

The black speaker ground wire goes to the speaker jack and it's using the chassis as it's ground from the compression to the chassis.

[sluckey]

changing the metal speaker cab jack

Please explain exactly what this means.

[pbman1953]

Meant to say while touching

[tubeswell]

There's only one set of speaker wires because the OT only has a 4 ohm out put

In that case, the end of the secondary which is not the end with the NFB loop attached.

[sluckey]

Meant to say while touching

Still unclear.

Is the "metal speaker cab jack" on the separate speaker cab box? If so, maybe you have the wires reversed in your speaker cable. IE, the OT ungrounded "hot" lead is actually connected to the sleeve of the speaker cab jack. Make sure your speaker cable plugs are wired tip to tip and sleeve to sleeve.

[pbman1953]

I have 2 heads , one on top of the other. I switch the cab back and forth between the 2 heads. Individually.


The cab's speaker wire has  a metal, Switchcraft, cover. The amp's stock jack is some kind of shorting plug. After a review you all sent me I decided, since I ordered a new OT with 4 and 8 ohm outputs, that I'd remove the the shorting jack and install a standard 1/4" non- shorting jack.

I sent a picture of the stock jack and next to it the jack I replaced it with. Why do they use a grounding jack?

After installing the standard jack he issue seems to have been cured. I can touch both the chassis and the speaker jack while it's plugged into the speaker cab


Thanks!

[sluckey]

Why do they use a grounding jack?

The grounding jack puts a ground on the OT hot lead only when nothing is plugged into the speaker jack. This will protect the amp. It's much better to have a short on the OT secondary than an open circuit.

[pbman1953]

That seems confusing. Wouldn't that be a direct short?


Next question-


I'll be receiving soon an OT with 4 & 8 ohm out puts.  There are 2 holes and I can use one  for each ohm value. Should I install new jacks with shorting plugs?

or

one shorting jack with an ohm selector  toggle ?

[mresistor]

That seems confusing. Wouldn't that be a direct short?
Yes  as Sluckey said it protects the OT from harm caused by flyback voltage that would occur if it was open, not shorted with nothing plugged in.

Next question-


I'll be receiving soon an OT with 4 & 8 ohm out puts.  There are 2 holes and I can use one  for each ohm value. Should I install new jacks with shorting plugs?

or

one shorting jack with an ohm selector  toggle ?


The answer to that should be obvious.   If you want to select between two different impedance taps of an OT you should use an impedance switch.   

[sluckey]

one shorting jack with an ohm selector  toggle ?

Do this.

[pbman1953]

Cool, that's how I have it on my Traynor


Thanks!

[PRR]

Wouldn't that be a direct short?

On smaller tube amps, particularly self-biased, shorting the output is safe. The power intake does not change (much) with load, and the idle condition is the hottest possible condition. On The Other Hand, an open un-loaded output and any signal risks voltage kick-up in the OT, which will eventually puncture the insulation.

Transistor amps are mostly different because they will suck more power "as needed", or when shorted.

However, is this the SIX 6L6 amplifier? It is likely to aim at transistor design thinking for most power. The best thing is to NEVER mis-load it.