Kay 752 amp - does this sound normal?

[Steenamaroo]

Yeah, I don't know anything about US specifics or these amps in particular but I can echo the above.
In UK that third pin is simply to ensure that you are not the path to ground in the event of equipment failure.
If the chassis becomes live for any reason, that third pin is the path of least resistance and, depending on current flow, your fuse will blow to save the gear.

Ever hear the phrase, 'a little bit of knowledge can be a dangerous thing'?

I received a hotrod 2x12 for repair from a guy who said he powered it up to test...played for 5 minutes, went to make a cup of tea, and came back to a room full of smoke.

Some super-logical previous owner had thought well...the amp hums so I'll cut the chassis mains earth connection.
It also seems to blow fuses once in a while so I'll just stick in a 5amp fuse instead of the 1 amp or whatever it was.

One of the valves had shorted internally allowing large current to flow across a resistor. That should probably have blown the fuse and saved the amp but didn't.
That resistor terminated to ground/chassis so the chassis would have been live for the duration.

The damage pictured, was probably worse than it looks with several traces being charred through.
Swings and roundabouts, though. I cut away all the char and point-to-point repaired it, but after explaining all this to the man he asked me to keep it!
His intention was to sell the amp and although my repair is sound, neither of us was comfortable sending it into the wild!

 

[famous beagle]

Thanks much for the info y'all. It's helping. So am I right to assume that this statement:

"The plug was non-polarized. Meaning, every time the amp was plugged in, there was a 50% chance the chassis would have 120 volts on it."

is not entirely true on its own? It also requires something that's live within the amp to fail and touch the chassis?

And Steen, yes I know that you should never replace a fuse with anything other than what the amp calls for. If you keep blowing fuses, it's not because you have defective fuses; it's because you have a defective amp. I know that much for sure.

But I'm still confused a bit about when and why someone would get shocked.

For example, I've read that if you were falling and you grabbed a live power line to stop your fall, you would not get shocked unless you were to touch something else (such as the pole or, if you were hanging low enough, the ground) at the same time. This is, I assume, the reason that birds don't get shocked when they perch on power lines.

Now I don't entirely understand why that is yet, but if I take that with a grain of salt for now, I can't seem to reconcile it with our amp example.

In the power line example, you're touching (with both hands) a live wire. Now, you're not touching ground or anything else that's touching ground, so I guess that's why you don't get shocked?

Now take the amp or toaster example. Something breaks in the amp and shorts to the chassis, meaning that it becomes live. The chassis is connected to your guitar via the sleeve of the guitar cord, and your strings are connected to that. So you touch the strings with both hands.

So you're essentially holding on to a live wire with both hands, just as in the power line example. (Please correct me if I'm wrong.)

But, assuming you're in a carpeted house wearing shoes, you're hardly "grounded," right? Again, correct me if I'm wrong.

I could see maybe if you were in an unfinished basement with a wet concrete floor, and that floor was touching something that like a grounding rod.

But ... if you're in the second story of a house wearing shoes on a carpeted floor, how are you grounded?

 

[CoolCat]

from what I gather there is the two prong plugs that have one prong larger than the other. the larger side is neutral.
neutral is generally ground because its often the center tap of a transformer,but again generally the center tap/neutral is tied to earth ground too.....somewhere down the line.
thats why the third prong is often the same thing eventually earth ground.

this leave one prong of the wire with the voltage. like on motors you can make them spin another direction by reversing the polarity....so its probably in audio not as important to be polarized, because its AC (alternating current.)

disclaimer: I think. ..Im not a amp repair guy by any means.

so if you make sure your chasis/case is ground and the guitar cable ground is true to ground...and you power up and it blows a fuse or wall circuit breaker.....you found a short!

then the hard part is finding where exactly is the short...not so easy. very difficult actually.

but at least with good grounding you'll be safer.

 

[arcaxis]

from what I gather there is the two prong plugs that have one prong larger than the other. the larger side is neutral.
neutral is generally ground because its often the center tap of a transformer,but again generally the center tap/neutral is tied to earth ground too.....somewhere down the line.
thats why the third prong is often the same thing eventually earth ground.

The old amps like this Kay would have a plug that both prongs were the same size, thus could be plugged in such that the chassis and the instrument cord plugged into it could become 'hot', 'Why do I get a shock when my lips touch the microphone grill and my hand is on the guitar strings? ' Not sure when the wider prong came about.

 

[famous beagle]

Ok ... I'm still not clear on how you'd get shocked. Take this paragraph, for example, from this link:

Electrical Grounding Made Simple

Dangers of an Ungrounded Appliance
An ungrounded electrical box, appliance, power tool, or extension cord could become a danger if there is no path to ground, except through you. You see, without a ground wire, your body may complete the ground path and you may be shocked or electrocuted. In older homes with cloth wrapped wire or in homes with knob and tube wiring, this is the case. As you know, newer appliances and some tools come equipped with a three-pronged cord, incorporating a ground for protection. Remember, any contact with a metal box, appliance, or electrical panel that has no ground can potentially make you the ground connection, so be extra careful!

Again ... I can see this if you're outside using a drill or something and you're barefoot. But even so, if you're in socks and shoes, aren't you pretty much insulated from the ground?

And again, if you're upstairs in a house on carpet, how would "an electrical panel that has no ground potentially make you the ground connection?" How are you grounded?

I'm starting to think I'm never going to understand this because, no matter how many times I ask these questions, it seems I never get an answer that clears it up for me. I'm sure the issue is on my end, but for some reason, I just can't wrap my head around it.

 

[garww]

It's a circuit. Now not much flows against high resistance

 

[famous beagle]

It's a circuit. Now not much flows against high resistance

Thanks for the response. I'm afraid I don't know what you mean though.

I understand it's a circuit, and I understand that, the higher the resistance, the less the current (ohm's law E = IR, or I = E/R).

But I'm not sure which part of my question you were addressing.

 

[garww]

Keep your free hand in your pocket. hahah

 

[einstein magoo]

Maybe...



Famous, don't know if this will help or hurt but I thought I might be able to explain the two prong plug thing.

Hopefully you can blow up the extremely elementary drawing and read my writing.

Don't use anything I say here as advice or accept it as 100% accurate, it's only an effort to shed some light onto the subject.

In the US, 120VAC power comes into the house from a transformer either on a utility pole or mounted on the ground. Either way, doesn't matter.

To simplify, we will just talk about two of the wires, hot and ground. Hot (120VAC potential) and ground enter your breaker box or fuse box. the hot side feeds the breakers, and the ground side is connected to earth ground through a ground rod driven into the earth near your service entrance. The ground side is also connected to the neutral bar in the breaker panel. Neutral and ground are at the same electrical potential.

An individual breaker feeds 120V potential to a circuit containing household receptacles, lighting, etc. The wire from the breaker carrying the 120V hot connects to one of the receptacle slots, and the neutral is connected to the other. There is 120VAC potential at the receptacle. An appliance (guitar amp) then completes the circuit from hot to ground through the electronics in the amplifier. The input jack on the amp shares the same path to ground through the sleeve connection. Amplifier ground, neutral, and the ground wire from the input jack are all at the same potential. Being an alternating current device, the amplifier doesn't care where the hot or neutral connects to the circuitry. Just plug in your two prong plug and the amplifier is powered and ready for use. The problem with inserting the plug upside down, or the other way comes from the fact that you have a applied the hot potential to the amps ground circuit. Since the guitar input jack on the amplifier shares the same path to ground as the amp does, the sleeve of the guitar cable now has 120VAC (potential) on it. Then through the ground wire in your guitar connected from the guitar's controls to the bridge, you apply 120VAC potential to the strings.

If you are "floating" above earth, or not directly touching anything that is (bare feet on lawn, lips on mic, etc.) the amp works, you play your bit, and nobody cares. But when you create a path to ground like from your fingers on the strings, through your body, to your lips touching a properly grounded microphone. you have just created a parallel path for 120 volts to flow current through you = shock or electrocution!

When the electrical industry went to having three prong plugs for safety, a standard was created where whenever you plug your three wire device into a receptacle, the receptacles hot, ground and neutral are automatically made to the appropriate connections in the device.

DISCLAIMER: I am not an expert and nothing stated above should be taken for gospel truth. Always observe all electrical safety concerns and consult an expert (not me) before servicing or rewiring any appliance. This is my humble attempt to clarify a potentially dangerous path to ground when an appliance is powered improperly.

Hope this helps and does not create more confusion. Just trying to help[!

 

[famous beagle]

Einstein, thank you sooo much for this! This is exactly what I need, and it answers my question(s) perfectly.

Like I said, people would always make claims like:

"The plug was non-polarized. Meaning, every time the amp was plugged in, there was a 50% chance the chassis would have 120 volts on it."

And they would leave it at that, not the adding the remaining important information like: "So if you played your guitar when the amp was plugged in the wrong way ... and then you touched something else at the same time that was grounded, like a mic or the ground with your bare feet, you would get a shock."

Since they didn't say that, I assumed they were implying that, if you plugged the amp in the wrong way, you'd get a shock just by playing the amp regardless.

Your answer makes great sense, and also thank you so much for the illustration. It really helped!

Now .. if I could trouble you for one more question:

Why is it that people say things like "replacing the two-prong cord with a 3-prong (which I have done now) is safer, but in order to be really safe, you should add an isolation transformer."

If the three-prong outlet is standardized, and one side is always hot and the other always neutral, what is the danger once you've installed a 3-prong cord?

Thanks again so much!

Oh, and by the way, it seems incredibly odd to me that they didn't have a standardized system worked out when they were only using a two-prong cord. In other words, something like: "The hot receptacle is always the right, and the hot plug is always indicated on the plug."

 

[garww]

.."standardized system worked out when they were only using a two-prong cord".

Going back to the ribbed side of the power cord, and then, they made one plug prong larger, so it would only go in one way. Obviously, they couldn't rib one side of the old twisted pair in a cloth sleeve, but modern cords are still ribbed if they are regular two-wire lamp cord

 

[famous beagle]

.."standardized system worked out when they were only using a two-prong cord".

Going back to the ribbed side of the power cord, and then, they made one plug prong larger, so it would only go in one way. Obviously, they couldn't rib one side of the old twisted pair in a cloth sleeve, but modern cords are still ribbed if they are regular two-wire lamp cord

Thanks! Apparently, this system didn't catch on too well, as I've never known what it meant.

So, on all two-prong devices produced nowadays, if they don't have a polarized plug (one bigger than the other), do they always have this ribbing system?

 

[garww]

Got me hangin'. If stuff was made in the USA, maybe - UL approved. I can see the power for the old Toshiba Laptop is ribbed

 

[Steenamaroo]

And Steen, yes I know that you should never replace a fuse with anything other than what the amp calls for. If you keep blowing fuses, it's not because you have defective fuses; it's because you have a defective amp. I know that much for sure.

Ha! No....that wasn't directed at you.
Just relaying the story.

The main point, really, was that without chassis third-pin ground, the player would have been the path of least resistance.
That was a modern amplifier with a known history of light use and good care. I sure as shit wouldn't play any amp now without a third pin.

 

[famous beagle]

Update 4

Ok, well, I guess things were going too smoothly; it was time for me to screw something up! And I did not disappoint!

So ...last night I replaced the 2-prong cord with a 3-prong. Not knowing (at the time) the "ribbed cord" system that garww has just explained to me, I decided to wire the cord similar to my Champ clone, which has the hot wire going to the switch.

So tonight, I zipped it back up and wanted to try it with an 8-inch speaker. So I wired in the speaker from my Champ and was set to go.

Turned on the amp and immediately saw a spark, heard a pop, and saw some smoke. Turned off the amp and, after determining with my meter that everything was safe, took a closer look.

Long story short, I'm pretty certain that I fried the output transformer, because that's the side from which the smoke emanated, and it smelled burnt. That smell has faded since then, but something was smoking, and I can't see any other signs of charring. So I'm assuming it has to be that.

Then I remembered the ribbing system garww mentioned and took a look at the old 2-prong cord. I noticed that it was ribbed, and the ribbed lead (i.e., the neutral wire) was wired to the switch --- not the hot wire.

So it looks as though I had the power cord wired the wrong way. Would this have done it? I'm 99.99% sure I had the speaker (which is 4 ohms, just like the Kay speaker) wired correctly, and I didn't touch anything else in the amp.

Also ... does anyone know how I can determine what a suitable replacement for the OT would be?

 

[Steenamaroo]

Ok, so I learned a good bit tonight. I got the top and bottom chassis separated and got pictures of that.

Not sure if it's important to you now or not but I can see better how the board is mounted from those pictures.
The circled part is just like a washer with internal teeth. The teeth bend on fitting and act like barbs.

They will come off with force (lever with something thin/flat) and they will refit and grip again no problem.

Hope that's useful.




Sorry to hear about the setback. Your OP transformer has a part number printed on it but I can't see it all.
What is it?

It would be worth disconnecting it and testing primaries to see if you burned it or not.
It may also be a good idea to fit a suitable fuse before moving on.

 

[famous beagle]

Not sure if it's important to you now or not but I can see better how the board is mounted from those pictures.
The circled part is just like a washer with internal teeth. The teeth bend on fitting and act like barbs.

They will come off with force (lever with something thin/flat) and they will refit and grip again no problem.

Hope that's useful.

View attachment 100044


Sorry to hear about the setback. Your OP transformer has a part number printed on it but I can't see it all.
What is it?

It would be worth disconnecting it and testing primaries to see if you burned it or not.
It may also be a good idea to fit a suitable fuse before moving on.

Thanks for the tip on the washers. That's good to know.

I've never tested an OT before. How would I do that?

 

[Steenamaroo]

Thanks for the tip on the washers. That's good to know.

I've never tested an OT before. How would I do that?

I only know the basics myself but checking impedance across each winding, when disconnected from the circuit, will tell you if a winding burned through.
Sure, you don't know what it's meant to read, but if it reads infinite you've proven that it's totally stuffed.

 

[garww]

Just as general info, it is not just guitar amps that shock people. I've notice a lot of 3-prong how to toob vids on radios & recorders and the like

 

[famous beagle]

I only know the basics myself but checking impedance across each winding, when disconnected from the circuit, will tell you if a winding burned through.
Sure, you don't know what it's meant to read, but if it reads infinite you've proven that it's totally stuffed.

Ok thanks. I'll do that soon.