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[Ned Ward]

Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........

I do need a Pro Reverb, and a Deluxe Reverb, and a Dr. Z Maz 18 or 38, an orange Gretsch, and a QSC KW181... all in good time. With 5 Fenders in the stable, it's a challenge just to get them all used on a regular basis! We're playing at Saint Rocke Thursday (150 person bar/club) so the Tremolux will do for that show, but we have another outdoor show in April, and for that, nothing but the Showman will do.

[John Roberts {JR}]

Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?

What the stinger cap is "supposed" to do is ground the chassis to neutral which is nominally 0V to reduce hum pick-up by the circuitry, and connected guitar.

A relatively safe 3 wire conversion is to bond the mains cord ground wire directly to the chassis. Most (all) modern UL approved guitar amps are wired this way.

I know of at least one musician who was killed when he got between two modern UL approved ground bonded guitar amps. One of those two amps was plugged into a miswired (RPBG) outlet and he was killed by the electrical shock when he grabbed a guitar that had it's ground energized while still holding the other properly grounded guitar.

The stinger cap is sized so that if in the wrong switch position, putting hot on the chassis through the stinger cap, the shock hazard is still low enough to not be lethal (thus "stinger" cap, not "sure death" cap). A faulty shorted stinger cap could indeed present a deadly hazard and these caps are specified at 600V breakdown and often use special caps designed for that purpose.   

I am trying to take a step back from ground bonding the chassis and using a GFCI to protect against a typical mains power fault, then isolating the chassis through the old school stinger cap sized such that even a RPBG outlet is not a deadly hazard, or likewise from a hot mic coming from a rouge FOH with RPBG.

This is not a typical application so would be considered an exception to typical UL wiring practices. 

JR

[Steve M Smith]

an orange Gretsch

The orange Gretches are so last year. All the cool kids are playing them with amber maple finishes now!

Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?

It is the work of the devil when used with a two pin power plug.  Theoretically, the capacitor is not required with a normal grounded chassis but if I understand correctly, the purpose of this discussion is to see if there is a safer arrangement which would pose less of a danger to a guitarist simultaneously touching his guitar and a microphone which has become live.

EDIT:  Whilst editing my post to answer this, John has posted a much better answer.


Steve.

[Mike Sokol]

Just remember that while we're on the subject of 2-wire guitar amps "grounded" with a "death cap", there are also a few old tube amps that had AC-DC non-transformer power supplies. Just like old AC-DC tube radios, all the filament voltages added up to 120-volts, and one side of the incoming line was bonded to the chassis. So if you used a non-polarized power plug, there was a 50/50 chance that the strings of your guitar were now hard-bonded to the hot side of the power line. I have one of these amps in my closet which I've considered rebuilding with a 120-120 volt isolation input transformer and a proper 3-wire grounded power plug. But except for powering it up on the bench, I would NEVER use something like this on stage or in the studio. Way too much chance that something would go wrong and kill somebody, maybe even ME.   

[John Roberts {JR}]

Just remember that while we're on the subject of 2-wire guitar amps "grounded" with a "death cap", there are also a few old tube amps that had AC-DC non-transformer power supplies. Just like old AC-DC tube radios, all the filament voltages added up to 120-volts, and one side of the incoming line was bonded to the chassis. So if you used a non-polarized power plug, there was a 50/50 chance that the strings of your guitar were now hard-bonded to the hot side of the power line. I have one of these amps in my closet which I've considered rebuilding with a 120-120 volt isolation input transformer and a proper 3-wire grounded power plug. But except for powering it up on the bench, I would NEVER use something like this on stage or in the studio. Way too much chance that something would go wrong and kill somebody, maybe even ME. 

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    .

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. 

 

[Steve M Smith]

Just like old AC-DC tube radios, all the filament voltages added up to 120-volts

We also had AC/DC radios and TVs here in the UK.  Our heater voltages had to add up to 240v (or about 200v with a series resistor).

I vaguely recall it had an electromagnet in the speaker... yes it hummed.

I had a radio like that.  The speaker electromagnet was wired in place of a smoothing choke.


Steve.

[Mike Sokol]

I had a radio like that.  The speaker electromagnet was wired in place of a smoothing choke.

Yeah, I swapped a permanent magnet speaker into one of these old radios and simply connected the electromagnet wires together. Hummed like crazy until I figured out the speaker's electromagnet was also the power supply choke. But hey, I was probably 14 years at the time so it was a good learning experience. 

The point is, anytime you're playing with old/tube gear, make sure you understand how it works before you start modifying or using it. Things can get dangerous in a hurry if you do something wrong.

[Ned Ward]

Amen to that. That's why I leave any amp work to competent folks like Bob or my amp tech at Harbor Music.

[John Roberts {JR}]

My dad was a recording engineer for RCA records when he died in the 1950's so we always had a decent sound system in the house but only mono at the time he died. One large speaker mounted at floor level in a corner of the living room angled up to fill the whole house from only a few watts. Before he worked at RCA he was chief engineer for Muzak, so I was the only house on the block with a Muzak receiver. :-)

 I found some early demo stereo 1/4" magnetic tapes in my did's stuff but his collection was mostly mono and '78s with only a few very early 33 1/3 LP. By the time I started messing with hifi I built a cheapo turntable based on a lafayette platter mechanism bolted to a cut out piece of wood and a plastic tone arm. I recall my first stereo amp, a small 4 tube job (probably a couple watts), and recall seeing the gas inside the tubes light up and dance around to the music.

Dad's  idea of an old amp was a 10W (?) western electric 19" rack unit in the basement using several sections and about 6' tall...It may have powered the home system at one time, but by the time I can recall he had a more modern tube unit, smaller than a bead box, but not by much.

Old amps were really big and heavy, The CS800 was a high powered light weight amp compared to truly old iron.

JR   

[Dennis Wiggins]

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

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

-Dennis

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