Musician Gear & Tech Discussion

Ok...got another Question for you guys, I'm using a vintage Ampeg V-4B head, It has the old 2 prong cord and gives a slight shock when I touch the microphone, The Polarity switch doesn't seem to do anything, the guy that usually fixes the tough fixes to my gear is in the hospital indefinitely, so I was wondering if using a 3 prong cord and somehow disabling that polarity switch would fix my mild "shocking problem"
and if so, is there any instructions???
I did a search but got a bunch of message boards that didn't seem to tell you how to change the cable and the wiring for the polarity switch

Ok...got another Question for you guys, I'm using a vintage Ampeg V-4B head, It has the old 2 prong cord and gives a slight shock when I touch the microphone, The Polarity switch doesn't seem to do anything, the guy that usually fixes the tough fixes to my gear is in the hospital indefinitely, so I was wondering if using a 3 prong cord and somehow disabling that polarity switch would fix my mild "shocking problem"
and if so, is there any instructions???
I did a search but got a bunch of message boards that didn't seem to tell you how to change the cable and the wiring for the polarity switch

The third prong is a grounding wire, it SHOULD deguass the built up static.
If it doesnt its not the wire's fault.

I would never use a piece of equipment that gave me shocks.
No matter what you have to do - fix it, whatever it takes . . .

Yeah, was hoping someone knew how to rewire it so I can make the fix myself, I called a couple electronics places and music stores this afternoon , some didn't answer their phone, some said they wouldn't touch it and one said it might be a month until they can get to it! Ahh the joy of living in upstate NY!!

That's why I spent a little time learning how to fix my own.....no reliable techs in the area. Many years ago I took a 70's Sound city L120 head to a local guitar shop because it wasn't sounding right. They shipped it to a tech in NYC who didn't even bother listening to the symptoms I described. He just replaced my 6 ORIGINAL MULLARDS with some cheep Chinese "Jolita" tubes and called it fixed. By the way, he kept the Mullards for himself. I had to figure out for myself that what it really needed was a cap job. I found a forum that has a lot of technical guys on it and they talked me into taking a wack at it myself. It was a great learning experience and it turned out fine. I also learned how to bias my own amps and modded my SF Vibrochamp by bypassing it's tone controls. (It sounds completely badass now) One of the things I did on the L120 was replace the old 2 prong cord with a grounded one. Connecting the ground wire to the chassis should take care of the shocking issue. There's a schematic of your amp here if you are interested: http://www.schematicheaven.com/ The circuit in question is in the V4 power amp schem. You just have to make sure you are careful because the filter caps hold an electrical charge like a battery....even after you unplug it.

Caution: Even though unplugged, capcitors can KILL !!

I was just adding that to my post when you put yours in sanshouheil. You really have to learn a little about discharging them and a few basic safety things before attempting to work on an amp. I could direct you to a few websites if you are interested in learning a little. Changing the cord should be a pretty basic job but you still need to know what to stay away from under the hood. It's also a good idea to always keep one hand out of the chassis. You wouldn't want to ride the lightning, 450 volts is a rough ride. A good discharging trick to use that works on a lot of old tube amps is to turn the amp on, plug in your guitar then pull the power cord out of the wall while strumming the guitar. As the volume fades to nothing it will be bleeding the voltage out of the caps. Just make sure to check with a multi meter before touching anything. You take the meter and set it to read DC volts, connect one probe to the chassis (this is best done with aligator clips) and touch the other to one of the lugs on one of the filter caps (the big round tubular things that aren't tubes). Do it to all of them, there may be a few. Make sure they are all completely discharged. You can also make a tool to discharge them with a large power resistor, a few short lengths of wire and some aligator clips. One end connects to the chassis the other to the caps...one at a time until they are all discharged. I usually use the pull the plug trick and follw with the power resistor tool then check with a DMM. These are all things you can learn about on the web.

I had the same problem with my '74 Gibson amp. Just go to Home Depot, and buy a new plug with 3 prongs and convert it. That's all it took for my amp. $7 and the problem was fixed.

I used something similar to this.

http://www.agoraquest.com/editor/upload ... eef4bd.jpg

Worked great.

But you will need to replace the cable too because it needs to have a ground leg (3-conductor). This will do just fine: http://search.digikey.com/scripts/DkSea ... me=Q113-ND Hook the Blue wire to one of the places where the old cable was connected, the brown to the place the other wire was connected and the green/yellow to ground (the chassis). You can use one of the screws that is holding the transformer to the chassis.

Thanks for the tips, I've replaced cords on Hammond organs before and know about the dangers but the tip of playing while turning it off is new, thanks.
Mainly just wanted to ask on what to do with that polarity switch? Do I disable it? This model is the one with the 3 metal toggle switches with the Polarity switch in the middle, not the version with the 2 rocker switches, the schematics seem to be be for the rocker switch version with the 3 prong cord and mention nothing of the polarity switch.

I'll do some more searching, just want to make sure and double check before messing with my beloved vintage Ampeg Head.

When I was 14 and playing at junior high rec's my guitar player had this old Ampeg that used to shock the hell of of me when I touched him...not wishing to revisit those days again.

Thanks again

It's marked "gnd sw" on the bottom right hand side of this schematic: http://www.schematicheaven.com/ampegamps/v4poweramp.pdf You should be able to but there seems to be a pot also tied to that switch through a .047 uf capacitor. It's marked "hum balance", is that a control on the amp that lets you dial down 60 cycle hum? I've never seen that before. Then again it could be the bias adjustment because I don't see that anywhere else. I don't know enough about reading schematics to know what should be done with that. You should consult someone with a little more electronic knowledge. My best guess would be to connect that to ground but you better ask someone with some experience. Here's a vintage amp forum that has a lot of amp repair wizards on it. I'm sure someone here will be able to give you quick easy to follow instructions to remove that suicide switch. http://vintageamps.com/plexiboard/ It's a great forum with some cool people on it and there's an Ampeg section so I'm sure you'll find the answer there.

This topic reminds me of the sad fate of one of my favorite rock singers of the 60's. Keith Relf , late of The Yardbirds ' who was electrocuted due to faulty grounding of his rock&Roll gear!
Goes to show ya', don't frig around with those electrons if ya' don't know what you're doin'!

Rockin' can sometimes be fatal.

Howlin'

But you will need to replace the cable too because it needs to have a ground leg (3-conductor). This will do just fine: http://search.digikey.com/scripts/DkSea ... me=Q113-ND Hook the Blue wire to one of the places where the old cable was connected, the brown to the place the other wire was connected and the green/yellow to ground (the chassis). You can use one of the screws that is holding the transformer to the chassis.

Actually, believe it or not, you don't. It should still work with the same cord. Common logic would make you believe it won't work, but it does.

After another look I think that is your bias control. It's connected to the heater coil on the PT. The 6.3 volts is feeding the heaters in the tubes. The "hum" reference on the schematic is a little confusing. Check that forum out, you'll find what you are looking for there.

Actually you do need to have a 3 conductor cord Shredd6, otherwise that ground pin on the plug is connected to absolutely nothing and therefore doing absolutely nothing. Your not grounded if you're not connected to ground. All you've done is replaced the two blades from the original plug with the two blades on your replacement plug. If something in that amp fails and shorts to the chassis you can become the quickest path to ground and get fried.

I'll give that forum a try, the last post on the Ampeg section was over 4 months ago, hopefully someone watches it, now you got me all freaked out my head is going to fry me!

Thanks

John