Tube amps and death cap

[ajh]

Went with a buddy a couple weeks back to pick up an old Fender Vibro Champ Tube amp. We came back to my place and I played some electric mando through it and loved the sound. So now I am shopping around for old tube amps......and along comes the question of the "death cap".

Have dug around for info on the net and am fairly comfortable with understanding how replacing the old two pin plug and cutting out the cap works (along with making sure the cap is drained before cutting), then replacing with a modern three pin plug (round pin to amp chassis).

Via the usual method the conversion involves just cutting out and discarding the cap....not replacing it with a modern "safety" cap.

What I question (and have not found any info on) is that when you convert the amp to a three pin without the cap replaced, then go to a venue/house/etc that either still only has a two prong outlet (must use an adaptor on the amp plug going back to two pin) or has a three wire outlet that is not wired up to code (ground pin unused), are you then running a greater risk of shock since the new chassis ground is NOT connected to the house ground?

I am thinking you would want to replace the cap in addition to the three pin plug for protection from that situation.

Any clarification would be appreciated.
Tony

[EdHanrahan]

Just a question and an observation:

Qst: I'm guessing that you're referring to a "capacitor"? "Safety cap" is, more usually, that which stops folks with arthritis from opening their arthritis medicine bottle.

Obs: My actual experience with such electrical stuff is from the mid-'6os, when grounded outlets were rare and 3-prong plugs were never seen on Fender, Gibson, Premier, etc. Playing electric while singing into a mike that's plugged into a different amp could give a nice shock to the lips - not fun! A common solution (or OUR solution!) was 4-foot wires with alligator clips to ground the various amp chassis together. No clue what it did to the electrical system but it did seem to work and, at least, never started a fire!

[pops1]

When your rewire the amp for a grounded cord there is more to it than that. Make sure for one that the hot lead is not on the chassis, two look how your fuse is wired, usually you have to rewire the fuse so when you want to check it you don't grab the end of the fuse, which may be good, and get the hot wire. If you have one of those little lights you plug into an outlet that lights up when there is power and hold one end and touch the chassis with the other and it lights up, reverse the plug in the wall and you won't get a shock. If you carry one of these and check it you won't need to rewire. If I want to leave an amp stock I sometimes cut a v in between the prongs, after I check polarity, and cut the v where the ground is. When plugging into and outlet I can then align the plug and if the outlet is wired right, no shock. I have gotten shocks from grounded PA's because the outlet was wired wrong so changing to a grounded cord is not a fail safe.

[Beanzy]

I would rewire the amp properly as described in many forums, no point repeating those here.
Then always bring in a plug checker to make sure you know what the house mains outlets are presenting.
To any gigging band for the eventuality that you encounter a premises without proper earthing or reversed polarity on an outlet, it is worth investing in an isolating transformer like you would use on a building site and an RCD you can plug in. That way you can protect yourself and your kit. But definitely buy a socket checker.

[almeriastrings]

Socket polarity and ground testers are very good things to have... this is what they look like (US version).



Some venues (and some extension cords) have very 'surprising' wiring....

[pops1]

I use the same little tool to check voltage with the two pigtails and light. If you hold one end in your hand and put the other into the small hole in the outlet. If the light lights then it is correct. If the light lights in the large hole then it is wrong and you will have a hot chassis.

[Brian Evans]

In the olde dayes, with two prong plugs, the chassis of the amp was not grounded. The two plug wires connected to each side of the power transformer, and the chassis floated, it wasn't tied to anything. Leakage from the power transformer, random energy from fluorescent lights, etc, would put hum on the chassis, which would get amplified. The ground switch let you flip which side of the plug went to which side of the power transformer, one setting would have less hum, and the "death cap" filtered the trace hum off the chassis. It got to be called a death cap because it could fail shorted instead of open, and connect the chassis to live power. Rarely happened, and it's not an issue when the cap works correctly - but as they got old, some did fail. In the new days, we connect one side of the power socket to ground (back at the breaker panel in the building), and call that Neutral. It's the white wire. We use a three prong cord, and connect ground to the chassis, with the green wire. The other wire is the hot wire, it's usually black, and it connects to the power transformer through an on/off switch. Chassis is tied firmly to ground so no hum can exist, so no need to filter it out with a cap, so no need for a "death cap".

[pops1]

I have had dozens of old amps that will have voltage on the chassis depending which way you plug the cord into the wall. The death cap was connected to one or the other leg of the AC (on the primary side of the power transformer) depending on the switch and then goes to ground (chassis) Since a capacitor will conduct AC that puts what ever side is switched in to ground. If that is hot, hot chassis and whooo hoooo time. Since most switches were on/on switches the cap was connected to one leg or another. Sometimes there would be an on/off/on, but mostly on/on. If everybody had their amp the same as the PA no shocks, but if it were different, look out. I have had blue flame hit my mouth from the mic and could be seen from the back of the room because of an improperly wired outlet. Are we having fun yet?

[rockies]

Being a really old guy and a electrician and electronics tech in my working life I was very aware of the dangers of tube equipment and also the low current leakages between equipment. These were the ones that tingled the lips etc when touching a mike and your guitar. So I made a few cords, a single wire connected to the shell (ground) of 1/4" plugs. These were then plugged into unused jacks on the amps, PA etc thus connecting all equipment grounds to one another. No more lip burns. Also IF there had been (never happened to us) a shorted grounding capacitor or other electrical problem the grounding would have tripped breakers in the A/C panel.
Dave
Dave

[Mandoplumb]

Pops1 you must have antique plugs to be able to turn them over. As I understand it a modern plug only plugs in one way and will not put hot to chassis assuming receptical is wired properly. Am I wrong or missing something? If I'm correct all you should have to do is install new plugs making sure that neutral goes to chassis ground and use tester to insure proper wiring.

[barney 59]

Pops1 you must have antique plugs to be able to turn them over. As I understand it a modern plug only plugs in one way and will not put hot to chassis assuming receptical is wired properly. Am I wrong or missing something? If I'm correct all you should have to do is install new plugs making sure that neutral goes to chassis ground and use tester to insure proper wiring.

Assuming of course that the receptacle you are plugging into is properly wired, that is often not the case. White to white,black to black, should be pretty simple right? It's still fairly common to find neutral and lead wires reversed!
A simple solution would be to make up an extension cord using a ground fault interruptor as a receptacle. If there is any imbalance then the gfi will trip saving you from a nasty shock. Sometimes breakers in the panel don't do their job or what we think their job is. Just recently I was running my jointer for who knows how long and it worked just fine, I have a rubber floor in my shop, but I moved the jointer and the casting on the jointer was touching the edge of my tablesaw top and when I switched on the jointer it literally welded itself to the saw! There was some motor short that didn't trip the breaker for some reason . I and the jointer were insulated by the floor but when it found ground on the tablesaw -Zap! I've been knocked flat a couple of times by faulty amps and microphones and/ or faulty wiring. I can hardly stand to watch a performance when the performer,as I have sometimes seen, is up there with no shoes!

[pops1]

Old amps have two prong plugs that are both the same size. I will usually rewire to a 3 prong grounded plug, rewiring the fuse also. The transformer wires are both black for the primary most of the time so either can go to neutral or hot. If i want to keep the original cord for someone that doesn't want it rewired (huh) then I mark the plug so they can put the mark to the ground in an outlet and keep them somewhat safe. Ground fault doesn't always work especially when you have a two wire appliance.

[ajh]

Thanks everyone. Looks like just clipping off the old cap and replacing the cord with the modern three pin, then checking sockets with the shown tester is the way to go for normal useage.

I have always been "respectful" of electricity. More so since I got zapped a few years ago when replacing a garbage disposal. I killed the circuit at the breaker, then went to disconnect the leads under the sink box...and "came to" about 5 feet away with blackened hands. Turns out the guy that wired the house had put two different circuits into the same connector box. Blew me completely out from under the cupboard. Very bad language.

Pops....we had picked up that Fender amp from Dave's Guitars in Lacrosse. Near you I would suppose. They let me play their Loar....and I was not too impressed. Now the upstairs at Dave's is really something else.

Tony

[pops1]

Tony, Yea Dave's is something else, he just started carrying decent mandolins not too long ago. My friend Todd is the electronic whizz there, he would have also been a good one to talk to.

I was also unimpressed with the Loar, tho it wasn't set up well, too much neck relief and it hadn't seen much play time for a long time. Mostly it sits in the case. I played it last year for 20 minutes or so. It was a treat to play one tho.

[Jeff Mando]

pops1 is right, old amps had two prongs the same exact size. To change the polarity, you unplugged it and turned the plug 180 degrees and plugged it back in. Problem solved. Many larger amps had a polarity switch built in, so you just had to flip the switch the other way to change it. In actual practice (been playing in bands since 1966) nobody got electrocuted. Sure, you got a tickle now and then or zapped your lips on the mic, then reached over and changed the polarity and everything was fine. No biggee.

Kinda bugs me when otherwise 100 percent original 60 year old amps have changed power cords, especially when the seller was being well-intentioned , but still in the dark as to how it affects collectors value. Kinda like putting Grovers on a Les Paul -- it's a modification!!!! OK? Don't do it..........please. If you are worried about such things -- simply buy a modern amp for your gigs.

[pops1]

Jeff, you are right, but I look at it like a violin, you can change all the black parts and not devalue the instrument as they are considered ware parts. Those old cords are brittle and most need to be change as the insulation is cracking off of a lot of them, or if you bend it it cracks and exposes the wires. I would not want old caps, tubes, or other parts in an amp unless it is a collector and never played or turned on. So doing a cap job, and changing the cord to me is no biggie. You wouldn't want to play an old Gibson mandolin with the original strings on now would you. I like keeping things original to some extent, but more than that I like keeping things playable.

[almeriastrings]

Safety is a major issue, too. Many of these old designs would fail current appliance safety tests, and if something went wrong, and someone (apart from yourself, possibly) was killed or injured, or a fire was caused (say a badly deteriorated cord), legal issues of liability would arise and negligence could be argued. Some of them are pretty dangerous by modern standards and age has not helped. Some venues do insist that all equipment used does comply with various safety rules (it is in the small print) and if you use something that doesn't - it is down to you...

[Beanzy]

It's no great shakes to take an old polarity switch out of circuit or swap the capacitor out for a modern one. In my book a safe working amp is worth a whole lot more, as you can use it to get that sound and the look without compromising on safety. The main thing when maintaining kit is to keep things safe and the last person letting it out of their care is the one responsible for that. Relying on someone needing specialist knowledge for a plug-in amp is not a safe option. Because we bang out 230v (or 115 peaks) this side of the pond we have always tended to be a bit more cagey about this than in the US where you were getting a 55v peak.

Also I should say that when we were being trained in electrical maintenance & servicing it was always pointed out that hazards from a shock aren't just whether you get your heart stopped by the volts, or get a burn from the contact. How you react to a shock needs to be taken into consideration too. If you get a zap from something and jump back / fall off stage, trip smash into something that topples over, those can have far more disastrous consequences than the initial shock.

For something as simple as sorting out the amps capacitor and polarity switch it really isn't worth doing it half measures or relying on the end user having specialist knowledge before they plug in an amp.