Adding external safety ground live to a two prong amp with 123VAC on the chassis

Discussion in 'Amp Tech Center' started by peteb, Feb 11, 2019.

1. petebFriend of Leo's

Apr 25, 2003
Just interested in verifying the function of the old fender death cap.

The chassis is made live by inserting the two prong plug back wards.

What this shows is that one end of the death cap gets 123 VAC and then the other end, the chassis gets grounded. The full 123 VAC must develop across the death cap and it must be a small amount of current as the amp appears to be unaffected.

Dec 29, 2012
United States
3. petebFriend of Leo's

Apr 25, 2003
Thanks Rob.

I get excited when the theory tests out like that.

I came up with an even simpler functional test for the death cap if an ammeter is available.

1. Plug the amp in or set the ground switch Wrong and verify that the wall voltage is on the chassis.

2. Connect an ammeter between the chassis and ground or neutral.

I just did this. The current measured 2.11 mA of AC current.

Working the equation backwards, the 0.047 mF death cap performs like a 0.046 mF cap should.

When the numbers work out that exactly I am always amazed. That’s one nice thing about electricity that the numbers often work out very cleanly.

What does the test show about the function of the death cap?

1 the cap has not failed open

2 the cap has not failed closed

3 when subjected to 60 HZ AC voltage across the cap, the current allowed thru the cap is the right amount of current for the size of the death cap and the current level, the current level the user may experience, is within safe levels.

We know the cap is not open because it was the path for the wall voltage to get on the chassis.

It’s not closed or the fuse would blow when the hot chassis gets grounded.

The current level being a safe low level of current can be checked two ways. With an ammeter, measure the current directly. Without an ammeter, monitor the plate voltage when the hot chassis gets grounded out. If the plate voltage is unaffected like it should be, then it is reasoned that the current is an acceptable low level.

Last edited: Feb 13, 2019 at 10:49 AM
4. aerhedTele-Afflicted

Age:
59
Dec 24, 2016
Boulder, WY
Grrrr, hate the edit function.

Last edited: Feb 13, 2019 at 10:57 AM
5. petebFriend of Leo's

Apr 25, 2003
The dang things are very consistent.

I measured the death cap current in this manner in four old amps.

The measured current was 2.11, 2.01, 2.12, and 2.23 mA.

6. petebFriend of Leo's

Apr 25, 2003
This video shows it better and is a truer test.

The chassis is hot, when the chassis gets grounded the plate voltage is unaffected.

And look how little and light the contact is, it doesn’t take much.

7. LynxtrapTele-Holic

Feb 29, 2016
EU
If I may, I will add few points.

4. Flip ground switch to hot, play you guitar and kiss anCap SM58 with your rendition of "Sparks Will Fly".

5. The cap will eventually fail.

6. Is it better to have the cap there or to remove it?

8. petebFriend of Leo's

Apr 25, 2003

I can’t explain the large shocks people get.

I’ve experienced the small shocks, on the fingers, on the lips.

Now I’m finally seeing how the design works. The way Fender did the two prong amps is the best way. It’s pretty fail safe as long as the cap is working and the caps seem to work for a long time.

Lynxtrap,

Can you explain the last question.

The purpose of the death cap is RF suppression, although there ar those that believe it also plays a safety role, but to be RF suppression, should the cap be on the hot line? The neutral line? Or either one?

9. LynxtrapTele-Holic

Feb 29, 2016
EU
It was late and I was feeling a bit grumpy
I was really after your opinion about it. I might be wrong, but have been getting the feeling that you're trying to prove that the cap is safe or even to be recommended?

Can't say for sure, but I wonder if they are even legal in many countries today. Say if you tried to get a new amp cleared for release on the market in the US or Europe.

10. petebFriend of Leo's

Apr 25, 2003
Thanks Lynxtrap,

I think the subject of the death cap has been covered.

I wouldn’t recommend it, it is not safe.

My personal feeing is that the term ‘death cap’ over states the real danger involved and I really question the original purpose of the cap. Is it there solely for RF suppression? I doubt it since modern amps don’t have RF suppression caps. I personally feel that one of the purposes of the cap was to make a two prong system that was relatively harmless to the un-informed user.

11. BendyhaTele-AfflictedSilver Supporter

Mar 26, 2014
Northern Germany
Reading in electronics magazines from the 1950's, it seems that it was quite common to get buzzing and popping noises on the radio, television and amplifiers due to dirt comming in with the mains power (superimposed wave forms riding on the (ideally) sinus AC supply) .
There were many articles written on building power line filters to place before your radio...also how to lessen the generation of interference from the main culprates - electric motors; lots of the then older appliances from the 1920 - 40 not having a built in filter cap, and many were likely to have sparking brushes.
The earlier light-bulbs were also a very prevalent cause of hum in the house power supply of yesteryear.
All these, and other LF interferences where audible in an amplifiers output, and could be lessened with the use of your so called death cap of 0.05µ - 1µ .....(not the best line filter)
As to RF suppression......a smaller cap (ca.150pF) at the guitar input jack would be more effective.

12. petebFriend of Leo's

Apr 25, 2003
thanks Bendyha

so its not radio frequency suppression, its just general noise suppression.

maybe now days the power from the source is filtered or cleaner for other reasons.

Can you answer the LAST question?

if the purpose of the cap is a line filter, should it be placed on the hot, neutral, or either line?

Dec 29, 2012
United States
Old amp builders could have left the metal chassis floating, with no death cap, but then the chassis would not act as an RF shield. A grounded chassis will sink electromagnetic induced noise to ground. I'm a ham radio operator and you can have the nicest, highest antenna around but if you ground it then all RF goes to ground and it's quiet. The death cap is there to provide the chassis with a ground without electrocuting people 50% of the time.

If a death cap fails as a short it can kill you.

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14. petebFriend of Leo's

Apr 25, 2003
Thanks Rob,

I think you are right about the cap letting the EMI/RFI off of the chassis as opposed to being a line filter preventing EMI/RFI from reaching the amp.

I think that helps answer the last question. Is the hot or the neutral line supposed to connect to the cap?

If it’s the neutral line, which makes sense, then the cap may not work as a line filter. Meaning that the cap is not a line filter. It is a ground path like Rob says.

And the size of the cap is chosen to get the desired impedance and resulting current flow at 60 cycle 120 VAC.

Thank you Rob, you have all the answers.

Last edited: Feb 14, 2019 at 9:15 PM

Dec 29, 2012
United States
The death cap can act as a line filter if the amp chassis has a 3 prong power cord. Fender did make some amps like this. The death cap would do nothing in it's normal neutral to chassis position because the chassis is already grounded, but flip the ground switch and connect the death cap hot to chassis and you have a filter, a cap between hot and ground. I don't know if Fender intentionally did that but that's what they ended up with.

16. BendyhaTele-AfflictedSilver Supporter

Mar 26, 2014
Northern Germany
Wrong!
The cap should always be on the same side as the fuse. So....if the cap fails; the fuse blows.
The fuse should always be on the LIVE side. So....if there is a problem/short, the live is isolated from the circuit - a blown fuse on the neutral side is the most dangerous senario.
ERGO...the cap should always be on the LIVE line.

Dec 29, 2012
United States
Bendyha, blackface and silverface amps put the death cap before the fuse and power switch. In my layouts I recommend "powering" the ground switch/death cap with the output of the fuse.

BobbyZ likes this.
18. BendyhaTele-AfflictedSilver Supporter

Mar 26, 2014
Northern Germany
True, I was not thinking of the amps with ground-switch, just those with the non-switchable cap have the cap on the transformer side of the fuse....like the OP champ.

Dec 29, 2012
United States
Oh, sorry. Yea, the Champ is wired so the fuse feeds the death cap if the amp is plugged in backwards.

20. LynxtrapTele-Holic

Feb 29, 2016
EU
Have you considered not recommending the cap and ground switch? I mean, maybe it would be better to not have them there at all if people build amps using the layouts?

Bendyha likes this.
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