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Author Topic: Death cap  (Read 27755 times)

Art Welter

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Re: Death cap
« Reply #40 on: December 18, 2014, 08:37:36 AM »

CTS pots are the only way to go. Gibson historic are CTS, but they get a premium for them.

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.
Bob,

I must have been lucky with vintage tube amps, only one out of six needed caps replaced, and that was one of the "newer" ones, probably mid 1960s.

Bourns makes pots that are rated for a half million cycles, what are the CTS pots rated for?

Art
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Art Welter

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Re: Death cap
« Reply #41 on: December 18, 2014, 08:43:51 AM »

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. 

Of course, some guys would call a capacitor circuit that cuts lows a high pass filter  ;)
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Mike Sokol

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Re: Death cap
« Reply #42 on: December 18, 2014, 09:10:54 AM »

Of course, some guys would call a capacitor circuit that cuts lows a high pass filter  ;)
I guess it's technically a high-pass shunting filter since it shorts the higher frequency energy to earth. But the final effect on the audio signal is like a low-pass/high-cut filter since it makes the high frequency buzz go away in the speaker.

This is a tough room, isn't it?  :o
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Mike Sokol
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John Roberts {JR}

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Re: Death cap
« Reply #43 on: December 18, 2014, 09:34:44 AM »

I wouldn't call it a filter at all. It has a capacitor but no characteristic resistance or inductance to form a filter pole with.

It is a cap coupled shunt.

JR
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Mike Sokol

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Re: Death cap
« Reply #44 on: December 18, 2014, 11:02:02 AM »

I wouldn't call it a filter at all. It has a capacitor but no characteristic resistance or inductance to form a filter pole with.

It is a cap coupled shunt.

JR

I would argue that there IS an internal resistance/inductance to form a filter pole with, but it's a completely unpredictable value because it's formed by all the various parallel resistance leakage paths inside a guitar amp itself.

However, if a human makes contact with an external hot chassis, then they're introducing perhaps 1,500 ohms of their own body resistance into the fault circuit and it becomes a true single-order filter.

But to avoid confusion and for practical reasons I agree with your definition of a capacitor coupled shunt. That's really what it is...  ;D
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Ned Ward

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Re: Death cap
« Reply #45 on: December 18, 2014, 11:57:03 AM »

Bob,

I must have been lucky with vintage tube amps, only one out of six needed caps replaced, and that was one of the "newer" ones, probably mid 1960s.

Bourns makes pots that are rated for a half million cycles, what are the CTS pots rated for?

Art


Art - on my 3 1965 Fenders, I had the filter caps replaced on all. While the Tremolux caps appeared fine and had no bulging or bubbling, I felt safer with F&T caps under the doghouse. If those vintage filter caps blow, they can take out a lot more important mojo downstream, including the transformers. Better safe than sorry...
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Jonathan Johnson

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Re: Death cap
« Reply #46 on: December 18, 2014, 03:01:37 PM »

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.   

So what you're saying is I should put a capacitor in my reverse polarity bootleg ground?  ;D
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Jonathan Johnson

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Re: Death cap
« Reply #47 on: December 18, 2014, 03:23:19 PM »

2. The amplifier can have the death cap removed, but can not be altered in any other way.

Given that there are musicians who refuse to alter their amplifiers in any way that affects either tone or appearance, I assume that you mean that replacing the two-wire cord with a three-wire cord is not an option.

How about making up a simple adapter, kind of like a 3-prong to 2-prong cheater plug, but going the other way?

You would have a 3-prong male plug, out of which would be a ground wire, a hot, and a neutral. The hot and neutral would go into a 2-prong female cord-end receptacle, while the ground wire would be long enough to attach to some convenient point on the amplifier chassis. The amplifier would plug into the 2-prong receptacle, and the 3-prong plug would plug into the power source.

This would give the amplifier chassis a solid ground connection without modification, thereby preserving historical accuracy and monetary value. However, I don't know how that would affect the tone; that is outside of my scope of experience.

Some potential issues if the death cap is not removed:
  • If the non-polarized plug is oriented correctly (cap between neutral and chassis), you have a bond of sorts between the neutral and ground. This could result in some return current on the ground wire, which could induce hum and also cause a GFCI to trip. If the cap is shorted, the current and noise will likely be greater.
  • If the non-polarized plug is inverted (cap between hot and chassis), you have a bond of sorts between hot and ground. You may induce current in the ground wire, creating noise and imbalancing the load on a GFCI. If the cap has shorted, sparks might fly and your guitar player may never allow you to touch his stuff ever again.
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Mike Sokol

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Re: Death cap
« Reply #48 on: December 18, 2014, 03:47:08 PM »

So what you're saying is I should put a capacitor in my reverse polarity bootleg ground?  ;D

Actually, that is one possible shock-blocker possibility. JR and I have discussed what would happen if you used a GFCI outlet but instead of bonding the ground screw to the incoming EGC, you put the "stinger cap" in series with the earthing path. Since the UL and NEC allow you to install a GFCI on a non-grounded power feed, then maybe they won't care if you put a capacitor in the ground line. I've also suggested putting a 47K resistor in parallel with the cap just to pull the chassis voltage close to earth potential so it doesn't set off a NCVT during a stage check.
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Mike Sokol
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Bob Leonard

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Re: Death cap
« Reply #49 on: December 19, 2014, 07:37:00 PM »

Given that there are musicians who refuse to alter their amplifiers in any way that affects either tone or appearance, I assume that you mean that replacing the two-wire cord with a three-wire cord is not an option.

How about making up a simple adapter, kind of like a 3-prong to 2-prong cheater plug, but going the other way?

You would have a 3-prong male plug, out of which would be a ground wire, a hot, and a neutral. The hot and neutral would go into a 2-prong female cord-end receptacle, while the ground wire would be long enough to attach to some convenient point on the amplifier chassis. The amplifier would plug into the 2-prong receptacle, and the 3-prong plug would plug into the power source.

This would give the amplifier chassis a solid ground connection without modification, thereby preserving historical accuracy and monetary value. However, I don't know how that would affect the tone; that is outside of my scope of experience.

Some potential issues if the death cap is not removed:
  • If the non-polarized plug is oriented correctly (cap between neutral and chassis), you have a bond of sorts between the neutral and ground. This could result in some return current on the ground wire, which could induce hum and also cause a GFCI to trip. If the cap is shorted, the current and noise will likely be greater.
  • If the non-polarized plug is inverted (cap between hot and chassis), you have a bond of sorts between hot and ground. You may induce current in the ground wire, creating noise and imbalancing the load on a GFCI. If the cap has shorted, sparks might fly and your guitar player may never allow you to touch his stuff ever again.


I should have been more precise. Removing the death will also include the replacement of the 2 prong plug with a correctly wired and grounded 3 prong chord of suitable length and gauge.

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Re: Death cap
« Reply #49 on: December 19, 2014, 07:37:00 PM »


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