Ampeg Jet J-12 power switch question

[Heemis]

Hey guys,

Sorry, non pedal question, but it's really bugging me so I was hoping I could find some help here.  I'm working on a 69 Ampeg Jet J-12, just finished replacing the cap can, and now I'm trying to figure out the proper way to wire up the mains, but it's a bit confusing. The switch has a neon attached and I'm not familiar with how the switch is wired. Of course I'm looking to run the hot side to the fuse first then the switch and wire the neutral straight to the primary.  In looking at the switch, it appears that the neutral wire is run through the fuse and then is always connected to the primary of the transformer.  The hot side seems to be switched between connecting directly to the primary, and being connected to the primary through the .05 cap.  Would it be ok to move the hot wire to the fuse and reverse the leads on the switch, or would the neon need to be reversed as well?

Here's the schematic:



Any help would be appreciated!

[Ben N]

Sorry, I don't have your answer, but where'd you get the can?

[Heemis]

http://www.fliptops.net/  Although there are plenty of sites which sell multicaps... I recommend the ones made by CE manufacturing, used them in a few amps now.

[UKToecutter]

The neon is not polarity sensitive.
Common sense would indicate that the live (hot) is switched although if I was building something like this from scratch I'd definitely be using a double pole switch and switching live and neutral.

[R.G.]

I'm trying to figure out the proper way to wire up the mains, but it's a bit confusing. The switch has a neon attached and I'm not familiar with how the switch is wired. Of course I'm looking to run the hot side to the fuse first then the switch and wire the neutral straight to the primary.  In looking at the switch, it appears that the neutral wire is run through the fuse and then is always connected to the primary of the transformer.  The hot side seems to be switched between connecting directly to the primary, and being connected to the primary through the .05 cap.  Would it be ok to move the hot wire to the fuse and reverse the leads on the switch, or would the neon need to be reversed as well?

This was intended to be a two-wire setup. If you're already messing with rewiring the primary AC power, and if it were me instead of you, I would do it this way:

1. Ideally, switch both sides, line and neutral, with a DP switch. Make sure the switch has UL, CE, TUV, etc. listing embossed on the switch body. This is safer in a world where sockets are installed and wired by the lowest-bidding electrical contractor.
2. Line cord MUST be strain relieved. Use a proper plastic strain relief, or better, install a new IEC socket.
3. Safety ground wire goes to a bolt on the chassis that isn't used for anything else. A bolt on the mains transformer is not good enough. Sand a spot on the metal chassis to bare, shiny metal, then stack on the bolt: external toothed washer, ring terminal that's crimped to the safety ground wire, external toothed washer, and nut. Tighten it down. The ground wire must have more slack in it that either line or neutral, so if the line cord is pulled out of the chassis, the ground breaks last.
4. Hot/Line to the fuse first. Then to the mains switch.
5. Cold/neutral to the mains switch.
6. From the switch to any AC in the chassis; PT and neon indicator at least. The neon power indicator is an AC part and does not have a polarity as UKT noted.
7. The 0.05 cap is extraneous. If you think you have to use it, first make sure it's rated for use as an "X" capacitor - that is, directly across the AC power line. It is probably a 1kV ceramic disk given the age of the unit, so toss it and get one that's correctly rated.
8. I personally would install a MOV of a suitably high rating across the hot/line after the fuse and switch to suppress transients. This should be rated for at least 150Vac in 120Vac countries, and at least 300Vac, maybe 350Vac in 240V countries.

As I always caution, this is just a sketchy set of suggestions and does not contain enough information to do this all correctly with the right spacings, insulation, terminal connections, and so on. There is a whole lot of lower level skills not even mentioned to allow you to wire AC power safely. Don't go doing this unless you know those skills already. Betting your life that you can guess right the first time is not a good bet.

[Heemis]

Thanks for the replies!

Already installed a 3-prong cord, using the original strain relief bushing, and wired the safety ground to its own bolt (with extra slack of course!).

I guess my real question is... what is that .05uf cap doing there?  I was really mostly wondering why it was wired that way.  Since you say it's extraneous I'll probably just end up cutting it out and leaving the rest of the switch as is (except for switching the hot and neutral wires so that the hot goes to the fuse and then gets switched.)

Unless this would be a suitable way of connecting it:

[R.G.]

I guess my real question is... what is that .05uf cap doing there?  I was really mostly wondering why it was wired that way.  Since you say it's extraneous I'll probably just end up cutting it out and leaving the rest of the switch as is (except for switching the hot and neutral wires so that the hot goes to the fuse and then gets switched.)

It is possible that it was intended to suppress arcing on the main power switch when it opened.

[Heemis]

Well, I removed the .05 cap from the switch and lo and behold, a nice loud POP every time I powered the amp off.  I replaced it with a new production cap and wired the switch back up the way it is shown, except moving the hot wire coming from the fuse to the switched position.

My only question... say the cap that prevents the pop fails and shorts, now my amp will be turned on whenever it is plugged in, but without the neon indicator being turned on... any way to safeguard against this, or do I just leave the amp unplugged when i'm not playing it?

[R.G.]

My only question... say the cap that prevents the pop fails and shorts, now my amp will be turned on whenever it is plugged in, but without the neon indicator being turned on... any way to safeguard against this, or do I just leave the amp unplugged when i'm not playing it?

Two things.
(1) Make sure that cap is specified as an "X" cap - intended for connection across the AC power line. This rating also implies certain self-healing properties in addition to a voltage rating.
(2)If the neon indicator is connected after the switch/capacitor, so that it gets AC if the cap shorts, then it will light up if the cap shorts.

It is possible to set things up so the switch doesn't cause a pop when the switch is opened. The cap may well have been a last minute bandaid for a wiring/routing problem at the original manufacturer.

[Heemis]

I replaced the .05 cap on the AC line with a 600v .047 sprauge orange drop.  If I'm reading it correctly, the datasheet for this cap says it can withstand up to 200v AC at 60hz:  http://www.mouser.com/catalog/specsheets/715p.pdf

This did kill the popping.  As of right now the switch is wired like so: 



If this seems ok, I'll probably just leave it be, as the amp is otherwise functioning perfectly.

[PRR]

Don't leave it plugged-in but off 24/7 for years. I'd unplug when I went to bed.

While the 600V Orangedrop will stand 120V AC for a very long time, it is not rated to stand that "forever", and it is not rated to die "gracefully". The "X" caps R.G. mentioned *are* made to live across-line forever.

There must be something else odd in that amp, because they bothered to short-out the Neon when "off". Normally we don't have to do that. However a Neon will dim-glow on quite small leakage. Normally not from the leakage of an open switch, so it is odd.

[PRR]

> If I'm reading it correctly, the datasheet for this cap says it can withstand up to 200v AC at 60hz:

500 hours at + 85 deg C.
Rated DC Voltage -- 600
Maximum 60Hz Voltage -- 200
Not more than one failure allowed in 12 units tested.

Yes... for 500 Hours. Hardly 3 weeks total.

And 1 in 12 may fail. Russian Roulette with a 12-shot revolver.... do you feel lucky?

However this is at 85C, very hot. Much hotter than an "off" amp should be. What is Arrnehaus, double every 10 deg C? So about 64 times better, over 3 years. (Which is still "nothing" to a 50 year old amp!)

And 125VAC is short of the 200V rating, so there's a little more slack.

And when the cap fails, if failed-short, it just forces the amp ON. If that is all which is wrong, it sits there and hums quietly. (But if other old parts are on the last-leg, un-watched ON may lead to fire etc.)

[R.G.]

Paul's right again. The AC power line is a sneaky, dirty place. It's not just a power-line level sine wave. There are spikes, transients, and nearby lighting strikes. They used to think it was safe to test the AC power line components for flashover at 1kV. Then they made the standard, 1.5kV, 2kV, and it's now at 4kV.

The untimely death of even 600V or 1kV caps across the AC line is the reason that people dreamed up specialized X caps. These fail too, but their failures are deliberately intended to be "soft" failures, not a flash of light, a cloud of plastic vapor, and a puddle of molten copper.

[Heemis]

Thanks so much for all the help guys!

I just wish there was a more elegant solution to this problem.  Is there no way to wire this switch so it doesn't pop when opened?

The amp is completely original otherwise and I aimed to keep it that way, but maybe I just need to drill a hole and wire up a 6.3v lamp off the heater supply.

[PRR]

> Is there no way

Get an XY or "Safety Film" Capacitor.

http://www.mouser.com/search/refine.aspx?Keyword=PHE850EA4470MA03

Mouser Part #    Mfr. Part #    
80-PHE850EA4470MA03R
PHE850EA4470MA03R17
Kemet    Film Capacitors 300vac 0.0047uF 20%
28,694 In Stock
1:    $0.49
10:    $0.354
Safety Film Capacitors    Polypropylene (PP) 4700 pF 300 V 20 % Radial 10 mm - 55 C + 110 C
Kemet PHE850EA4470MA03R17T0

data: http://www.mouser.com/ds/2/212/PHE850-194909.pdf

Yes, the test spec is just 1,000 hours, but at over-voltage (on top of the higher spec than OrangeDrop), with 1000V spikes. On top of a 100% quick test at many thousand volts. And these things are used by the BOAT-LOAD in every digital box, passing UL CSA ENEC consumer safety certification.

For all the reasons above, personally I *might* use the OD for some weeks until I get an order in to one of the Big Houses which carry XY caps. Especially since I'd un-plug before bed. But the 50-cent investment (even plus shipping) is justified for an amp which is still valid and mostly happy after 50 years, to cover the next 50 years.

The neon is odd but I do not think that is your pop.