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Author Topic: Death cap  (Read 27787 times)

Dennis Wiggins

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Re: Death cap
« Reply #30 on: December 16, 2014, 03:16:31 PM »

Here's an attempt to explain the purpose of the "Death Cap".  FYI only...

https://www.youtube.com/watch?v=MMTN3B-zClo

-Dennis
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Deric Craig

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Re: Death cap
« Reply #31 on: December 17, 2014, 12:53:43 AM »

Here's an attempt to explain the "Death Cap".  FYI only...

https://www.youtube.com/watch?v=MMTN3B-zClo

-Dennis

I took the time to view the entire video. There are several points made in the video presentation that I take exception to, for instance his comments about the "death cap" being connected to the "hot" side of the line to reduce hum...
Uh, Ok...

And I would prefer to have him refer to the chassis as a "common" instead of "ground" especially since "ground" does not exist in these amp examples. Any of us who do amp tech work often refer to the chassis connections as ground, but to me, it is just a possible point of confusion for a lay person.

I would wager this tech guy has not ever held a guitar connected to an amp with this cap that was not in a favorable switch position, and brushed his lips against a grounded vocal mic ...

This gentleman has some resemblance to a guy who, at one point in time, was on the fire investigation team that I was involved with. The man I had to periodically work with, was a lead county electrical inspector plus taught classes at a 'unnamed' university in electrical. More than once I butted heads with this guy on his "conclusions" on cause and origin. Sometimes individuals are not as knowledgeable as they like to perceive themselves. I could share some examples but choose to decline at this point.

I read through a decent amount of the comments below the video. I did cringe upon reading quite a few posted comments thanking this man for "debunking myths" about the death cap.
Oh boy...
I did see a few comments from some who supported the removal and mitigation of the potential issues with the cap.

There will be more comments on this video. Yes, the internet has a wide variety of information. Some of it is actually good.
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John Sabine

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Re: Death cap
« Reply #32 on: December 17, 2014, 08:56:46 AM »


Yup, hot chassis TV sets did not have exposed metal parts that users could touch. A hot chassis guitar amp that telescopes the chassis ground out to the player seems dangerous indeed. I wonder if there could be a high impedance signal isolation transformer used in series with the guitar output/amp input to segregate the power ground from the guitar. Lead guitar pickups are pretty high impedance so I am not optimistic.

Perhaps use a wireless guitar pick-up   :o .

JR

PS: I remember getting shocks while messing with an old school (hot chassis radio) while standing bare foot on the basement floor. I had one old beauty that was about the size of a small juke box, and the front hinged out to reveal a turntable for playing records (78). It also had a magic eye for tuning... I wish I still had that... probably ended up in a landfill after I dissected it. I vaguely recall it had an electromagnet in the speaker... yes it hummed. That's how you knew it was ready.  8)

 Prisons had a problem with hot chassis radios on cell blocks. Prisoners would disassemble their radios and remove the speakers create home made headphones. The problem would come when they would sit on the metal toilets with their home made headphones on. If they hadn't completely insulated themselves from the electrical circuit in the headphones they would be electrocuted. Some old speakers would use the speaker basket as the ground connection so if they were touching the basket they were contacting the circuit.
 The owner of a radio station that I worked at decided to make an on air monitor using an old desktop radio. He soldered a patch cable directly to the speaker terminals of the radio and then plugged it into the patch bay for the main console. The radio was an old hot chassis design and as a result this caused a great deal of excitement as well as a large flash and a loud bang! Amazingly the console wasn't destroyed.


Sent from my iPad using Tapatalk
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Art Welter

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Re: Death cap
« Reply #33 on: December 17, 2014, 10:54:46 AM »

My cure for this issue has been to remove the death cap and disable this portion of the AC circuit. That includes removal of the 2 conductor AC plug and replacing that plug/cable with a quality #12 or #14 grounded cable whose ground lead is soldered to the chassis of the amplifier. Soldered, not attached under a screw head or bolt.
 
Seeing the circuit, keeping in mind the guitar is also a part of that circuit, and following the guidelines below, what are your thoughts and suggestions.
 
1. The guitar can not be altered.
2. The amplifier can have the death cap removed, but can not be altered in any other way.
3. Tone can not be effected.
4. A stomp box sized unit is preferred IF the device is placed in line with the guitar output cable.
Bob,

All good, and makes me realize I probably should have done your mod on the Nordmende Isabella amp I just sold yesterday.  The guy that bought it was very aware of the potential shock hazard problems with vintage tube gear. I had labeled the "hot" side of the non-polarized two pin AC plug, and he already knew "the little guy gets you", so I don't think he will have problems.

I'm sure any solder connection you made would be robust enough to last another 50 years, but having experienced too many "cold solder" joints that look OK, but have infinite resistance, my only addition to your instructions would be the ground lead should have a mechanical screw or bolt connection to the chassis in addition to solder, what my dad (R.I.P.) would have called a "belt and suspenders" approach to safety. In addition, the ground lead should have a strain relief within a few inches of the chassis connection so there is no possibility of it flopping around in transit, which can eventually lead to the wire breaking at the connection point.

All that said, I still feel a moral ambiguity about altering classic gear, fortunately I sold my last ungrounded tube amp before seeing your post so won't have to struggle with my conscience  ;).

P.S. I would also suggest modifying point #3 to read "Tone must not be affected.

On a semi-related subject, what brand and value volume and tone potentiometers for Gibson humbuckers,  P90s, and Melody Maker lipstick style pickups are least prone to "scratchy pot" syndrome?

Art
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Bob Leonard

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Re: Death cap
« Reply #34 on: December 17, 2014, 11:08:10 PM »

CTS pots are the only way to go. Gibson historic are CTS, but they get a premium for them. Take a look at MOJO Tone for these pots.

http://www.mojotone.com/amp-parts/potentiometers-CTS
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Bob Leonard

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Re: Death cap
« Reply #35 on: December 17, 2014, 11:20:29 PM »

I took the time to view the entire video. There are several points made in the video presentation that I take exception to, for instance his comments about the "death cap" being connected to the "hot" side of the line to reduce hum...
Uh, Ok...

And I would prefer to have him refer to the chassis as a "common" instead of "ground" especially since "ground" does not exist in these amp examples. Any of us who do amp tech work often refer to the chassis connections as ground, but to me, it is just a possible point of confusion for a lay person.

I would wager this tech guy has not ever held a guitar connected to an amp with this cap that was not in a favorable switch position, and brushed his lips against a grounded vocal mic ...

This gentleman has some resemblance to a guy who, at one point in time, was on the fire investigation team that I was involved with. The man I had to periodically work with, was a lead county electrical inspector plus taught classes at a 'unnamed' university in electrical. More than once I butted heads with this guy on his "conclusions" on cause and origin. Sometimes individuals are not as knowledgeable as they like to perceive themselves. I could share some examples but choose to decline at this point.

I read through a decent amount of the comments below the video. I did cringe upon reading quite a few posted comments thanking this man for "debunking myths" about the death cap.
Oh boy...
I did see a few comments from some who supported the removal and mitigation of the potential issues with the cap.

There will be more comments on this video. Yes, the internet has a wide variety of information. Some of it is actually good.

The problem is that he ran all of his tests within the amplifiers. At no point did he reference another ground outside the chassis, or did he reverse the power cable. Lastly, the purpose of the cap was not to reduce hum, but to provide a path to ground for the AC source. The switch on a Fender amplifier allows the EU to choose that path with the result being less hum if the path is correct. The amplifiers he showed do not have this option and the resulting hum will not change regardless of the cap being there or not UNLESS he turns the 2 prong plug 180 degrees. This is a bad explanation that I've seen before.
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Ned Ward

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Re: Death cap
« Reply #36 on: December 18, 2014, 12:02:43 AM »

Bob,

All good, and makes me realize I probably should have done your mod on the Nordmende Isabella amp I just sold yesterday.  The guy that bought it was very aware of the potential shock hazard problems with vintage tube gear. I had labeled the "hot" side of the non-polarized two pin AC plug, and he already knew "the little guy gets you", so I don't think he will have problems.

I'm sure any solder connection you made would be robust enough to last another 50 years, but having experienced too many "cold solder" joints that look OK, but have infinite resistance, my only addition to your instructions would be the ground lead should have a mechanical screw or bolt connection to the chassis in addition to solder, what my dad (R.I.P.) would have called a "belt and suspenders" approach to safety. In addition, the ground lead should have a strain relief within a few inches of the chassis connection so there is no possibility of it flopping around in transit, which can eventually lead to the wire breaking at the connection point.

All that said, I still feel a moral ambiguity about altering classic gear, fortunately I sold my last ungrounded tube amp before seeing your post so won't have to struggle with my conscience  ;) .

P.S. I would also suggest modifying point #3 to read "Tone must not be affected.

On a semi-related subject, what brand and value volume and tone potentiometers for Gibson humbuckers,  P90s, and Melody Maker lipstick style pickups are least prone to "scratchy pot" syndrome?

Art


Art -


While I agree that altering classic gear makes me not feel great, since I play these regularly, adding a grounded three prong cord and changing the filter caps are an absolute must for me. My amp tech used to save the parts for me (2 prong cord, original filter caps) in case I wanted to sell the amps with the originals to put back in, but I play these old '65 Fenders regularly and will be playing my '65 Tremolux tomorrow. Will probably take that Bogen CHA-33 PA head in over break to have him go over it and replace the power cord (frayed at strain relief) and ensure it's grounded properly.


Yeah, it takes away a little of the vintage value, but I'll be around longer to enjoy it...




[EDIT] Forgot to add - for those that want to keep the possibility of 100% authentic down the road, just store the parts and if you decide to part with the amp, the buyer can choose to put them back in. The filter caps and AC plug are some of the least destructive "mods" I can think of.[/EDIT]
« Last Edit: December 18, 2014, 11:54:55 AM by Ned Ward »
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Steve M Smith

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Re: Death cap
« Reply #37 on: December 18, 2014, 02:40:30 AM »

While I agree that altering classic gear makes me not feel great, since I play these regularly, adding a grounded three prong cord and changing the filter caps are an absolute must for me.
It would be a must for me too (although in the UK, there is no need).  These amplifiers were made to be used, not to sit on a shelf looking pretty, so a minor modification to the power input to allow them to continue to be used safely, as long as it is done properly, should not be a problem.

A collector might not be interested in an amplifier modified in that way but I don't care what collectors think and don't buy anything thinking about its future value, just its current usefulness.


Steve.
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Bob Leonard

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Re: Death cap
« Reply #38 on: December 18, 2014, 07:31:14 AM »

True vintage pieces that are collected and not used couldn't be used but in very few instances. Dry caps do not a good sound make, so these collectors aren't interested in tone as much as they are interested in having a 100% original amplifier. I really don't know anyone who collects old amps just to look at, and in the vintage market having new caps and a 3 prong cord doesn't lessen the value of the amp. That would be why a great sounding 65' or 66' Deluxe Reverb that is in great cosmetic shape, and that has been re-capped and overhauled, will have a resale value north of $3K.
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Mike Sokol

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Re: Death cap
« Reply #39 on: December 18, 2014, 08:29:51 AM »

Here's an attempt to explain the "Death Cap".  FYI only...
https://www.youtube.com/watch?v=MMTN3B-zClo
I finally took the time to watch the entire video and you're correct. It's a bad attempt. This guy really has NO idea what the line cap does or even how to use the switch. I think that part of the problem is that he doesn't understand the difference between hum and buzz. Hum is a 60 Hz (or 50 Hz) sine wave with little or no harmonics, and usually caused by ground loop currents that sneak into a gain stage. There's also a 120 Hz hum (yes, an octave up) with a bit more harmonics caused by open power supply capacitors, but that's a different thread. Buzz is a raspy sounding noise which is mostly  upper harmonics of a 60 (or 50) Hz line frequency.

The idea of the"death cap" was to provide a shunting path to earth for these high frequency harmonics that show up on your guitar just from being in the room full of power wiring. You can observe this harmonic energy by touching the tip of your guitar cable while plugged into a guitar amp. Of course, with a properly "grounded" amp using a 3-wire power cord that energy goes directly to earth. But old-school guitar amps with 2-wire power cords needed a way to establish this earthing connection without the 50-50 chance of death from a flipped power cord with a hard neutral-to-chassis connection. 

So this death cap with a switch allowed you to find the line side of the incoming power easily without flipping a non-polarized power plug. The switch position that hums/buzzes/shocks the least means your amp chassis is now connected to the neutral side of the power line via this cap, which of course forms a 6-dB/Octave low-pass filter. And even if you get the switch reversed and connected to the "hot" line, the impedance at 60 Hz is high enough that it only provides a few mA of source current, not enough to kill you. Of course, if this old capacitor shorts out, then that becomes a low-impedance connection with line current available to electrocute you if flipped to the hot side.

So these line caps weren't really dangerous UNLESS they got leaky or shorted out. Then they became VERY dangerous.     
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Mike Sokol
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Re: Death cap
« Reply #39 on: December 18, 2014, 08:29:51 AM »


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