Electrical discharges with my old Ampeg B25B

[ricothetroll]

Hi,

I measured a voltage of about 10Vac on the case of my old Ampeg B25B. I (painfully) noticed it when I touched both the strings of my bass and a microphone (luckily I wasn't singing into it !).

The amp isn't earthed. I also power it through a 220 to 110V transformer, that's not earthed either.

My first thought was to earth the case but then I though that if Ampeg did it that way there's reason ! So I considered a faulty cap, as the amp is nearly 40 years old that would be no surprise. Even if changing them would be a good thing anyway, could it solve that particular problem ?

Schematics are here (Schematic Heaven seems to be down) :

http://www.wuala.com/ricothetroll/public/b25.pdf
http://www.wuala.com/ricothetroll/public/b25b.pdf (that one is shown earthed but my one is marked "B25B" and isn't)

I though that maybe some excessive leakage in C25 (filter cap for 500V node) could do that, but that's just the idea of some inexperienced guy 

Best regards.

Eric

[nick d]

                    I am by no means an expert , but any mains-powered gear should be earthed. ( unless you enjoy dying !)

[wavley]

1. Have the two pronged changed to a grounded plug and the death cap removed.

2. For now, flip the ground switch, it should stop.  If it doesn't have a ground switch, then turn the plug the other way in the receptacle.

Right now, it is most likely that your hot and neutral are reversed and that is why you are getting shocked.

[ricothetroll]

Thanx for your answers !

I must admit I never understood the purpose of that "ground" or "polarity" switch. That made me think about something I read in N. Boscorelli's "Stombox Cookbook", about hum issues : "If earth-grounding cured hum, all consumer audio products would use three prong plugs, which practically none of them do. Guitar amps came late to earth ground, and then out of safety, rather than hum. The polarity switch on vintage amps tied to a high voltage 0.047uF cap from circuit ground to either side of the AC line, selected through a SPDT switch. That cap's 56k reactance at 60Hz was low enough to reduce hum; but if the cap ever shorted, the full line voltage appeared to the chassis." (that last sentence is scary, isn't it ?).

So if C24 (and not C25 as I first though) is kind of shorted (huge leakage) maybe that's what's happening.

Why should reversing the polarity help ? I'm not doubting your answer's aptness, just trying to understand and learn.

Anyway, I'll definitely earth ground and recap this amp soon !

Best regards.

Eric

[nick d]

             Let your motto be :- If in doubt  , GROUND IT!!! . You might get the odd hum etc. , but you'll still be around to talk about it! 
                      Stay well , stay LOUD , be happy!

[ricothetroll]

I definitely will ! Dying sucks ! 

[wavley]

That cap and switch is what decides which side of the ac the chassis uses as ground, if it's connected to the hot side there is a larger difference in potential so you get shocked, if it's connected to the neutral then there is a lower difference in potential to ground and that is why the hum diminishes.

The old guy guitar player trick is to brush your guitar strings against the mic and see if it sparks, if it does, flip the ground, that way you don't have to find out with your lips.

If that amp has never been re-capped, it's time, a supply cap may be the problem, but grounded ac is always the way to go especially if you are using other equipment that is earth grounded.

[ricothetroll]

Hi Wavley,

I'm quite ashamed but I must admit that I never read much about mains characteristics. Actually, I did last night (after a few compressors, mic preamps, power supply... all connected to mains, and also a bunch of stompboxes, I think it was just about time to do it) and read that the two mains prongs aren't the same  . Better late than never, I guess...

Then I noticed something weird : when measuring the hot side and neutral at home with my dmm, voltage between those and earth is about 115Vac on BOTH. Shouldn't it be 220V on the hot side and 0V on neutral ? Or maybe there's something else I didn't understand ! 

Best regards.

Eric

[wavley]

This is what I would expect, but I have no idea how a euro wall socket should be wired.  You might go buy the equivalent of one of these http://www.amazon.com/Electrical-Receptacle-Outlet-Ground-Tester/dp/B0012DHVQ0 I carry one to every gig to make sure everything is right.  Here in the states we should see 120v hot to neutral and hot to earth and 0v neutral to earth and as I understand (I'm not a household wiring expert, I'm an electronics guy) there is no neutral on our 240v lines, it's two hots and an earth, somebody please correct me if I'm wrong on this.

Hopefully someone here with more experience with 220v 50Hz power can jump in and help you.

[ricothetroll]

That looks like a nice tool to have at work ! Makes me think Rat Sound's Sniffer/Sender :
http://www.ratsoundsales.com/snif_send_pak.html

I'll create a topic dedicated to that subject, an I never saw any and I think it deserves it ! How can we go on working with electronics if we are not even 100% sure of how the mains work... 

Best regards.

Eric

[PRR]

HARD-GROUND the chassis!! Please.

Put a modern US-style THREE-pin power cord on the Ampeg. Use a 220/110 transformer which passes wall-outlet ground to the 110V outlet.

You are caught between bad USA history and a 220/110 conversion. 

> Why should reversing the polarity help ?

Because in the USA, past 1960, wall outlets were two =equal= slots. There was NO ground hole, and we could not know which of the two holes was more-groundy.

Lamps don't care (except the screw-base lamp is a shock hazard).

Audio amps would not care, except the PT has 500pFd stray capacitance to -both- sides of the line. Since the line is normally unbalanced (one side more groundy than the other), this injects 60V AC through 500pFd onto the chassis. We could drain this leakage with a connection to ground... but grounded outlets were just being widely introduced, we mostly played in old buildings, so most amps stuck with 2-pin plugs.

If you connect 50,000pFd (0.05uFd) from chassis to the more-groundy side of the line, hum gets about 100 times less. To the other side, hum gets worse. Since we didn't know which side was more-groundy, we turned the plug over for best hum. Or in fancy amps, a switch let you try both ways in a second.

3-pin grounded wall outlets became required in new construction some time after 1960. My 1830 house has 1950s 2-pin upstairs but 1980s 3-pin in the kitchen (but wired WRONG). By the late-1980s, the advantages of 3-pin plugs and the availability of 3-pin outlets in many venues urged most amp-makers to use 3-pin power cords. (We had to carry 2-pin/3-pin adapters when we worked in older venues.) For about the last 10 years, when an old amp comes in for significant service, most US amp technicians will strongly advise a 3-pin power cord upgrade.

[ricothetroll]

Hi PRR ! Thanx for your answer.

Audio amps would not care, except the PT has 500pFd stray capacitance to -both- sides of the line. Since the line is normally unbalanced (one side more groundy than the other), this injects 60V AC through 500pFd onto the chassis.

I found that figure (from the radio designer's handbook) that characterizes the imperfections of practical transformers :
http://www.wuala.com/ricothetroll/public/Transformer.jpg
"C1, C2 the primary and secondary equivalent lumped capacitances
Cw the equivalent capacitance between windings"

Which of those is the stray cap ?

Best regards.

Eric