Knox 603 3-Wire Power Cable

[bgavin]

I have an ancient Knox tube guitar amp on the bench because it is shocking my guitar player.

The power cable has been replaced by somebody else.

Link to Schematic

Please note the bottom left of the schematic.
The On/Off switched leg for 120VAC goes to the 0.05 cap and chassis ground.

Q: which leg is Hot, which is Neutral?

It is my understanding the switched leg is normally Hot, but that would apply 120VAC to chassis through the 0.05 cab when On.

I would think the leg going to chassis should be Neutral.
Those with more understanding than me, please post a comment.

[PlainBill]

Re: Knox 603 3-Wire Power Cable

I have an ancient Knox tube guitar amp on the bench because it is shocking my guitar player.

The power cable has been replaced by somebody else.

Link to Schematic

Please note the bottom left of the schematic.
The On/Off switched leg for 120VAC goes to the 0.05 cap and chassis ground.

Q: which leg is Hot, which is Neutral?

It is my understanding the switched leg is normally Hot, but that would apply 120VAC to chassis through the 0.05 cab when On.

I would think the leg going to chassis should be Neutral.
Those with more understanding than me, please post a comment.

WhooEE!!! That is one funky design! I would guess it was designed by a fan of classical music.

A conventional design would have the switch attached to the live side of the AC line. In this case that would allow about 3 ma peak current through the 100K resistor and .05 MFD cap. Not enough to kill anyone, but enough to make him reluctant to touch the chassis.

Use a three wire cord. Hook the Green lead to the chassis. Hook the black lead to pin 4 of the 50C5 and the white lead to the power switch. If anyone cuts off the ground pin on the plug, give the guitar player permission to wrap a G string about the idiot's neck.

By the way, a good design would use a power transformer. Or use a solid state amp.

PlainBill

[bgavin]

Re: Knox 603 3-Wire Power Cable

The Death Cap is wired to the Neutral side, which is the switched leg.
Evidently, this is was the common method of doing this.

Further digging shows I cannot ground the chassis, because it is part of the circuit, i.e. no power transformer is present.

I checked the continuity and found Neutral on the 3-wire plug is wired to the switched leg, as per the schematic. I know the guy who installed the cord, so I think it was 50/50 blind luck.

Basically, this is just a scary and dangerous design.
I'll head to RS to get a replacement 0.05uF/400VDC death cap, just to be safe. I have no idea how long these have been installed.

I'll do the 0.1uF/400VDC tone cap also. It is a wax type, and probably leaky as hell.

Hi-Res Photos are Here

[japlytic]

Re: Knox 603 3-Wire Power Cable

I recommend replacing all of the paper/wax capacitors with polyester units. The unit between the floating and chassis grounds should be replaced with five 0.01uF Y-class units in parallel.
Also, I recommend replacing (if electrolytic) the capacitor between Pin 1 (cathode) of the 50C5 valve and the floating ground too.

[PCBONEZ]

Re: Knox 603 3-Wire Power Cable

http://www.bgavinsound.com/reference...9.07.03-01.jpg
Thinkin' maybe the green wire that's cut off just past the ball of electric tape used to go somewhere.

[PCBONEZ]

Re: Knox 603 3-Wire Power Cable

Something that old probably didn't originally have a polarized plug.

[bgavin]

Re: Knox 603 3-Wire Power Cable

Good catch, but that green wire belongs to the non-original cable that was installed sometime later. This amp doesn't have a chassis ground, nor does it it have ground lift/reverse switch. The death cap is wired to the Neutral side, where it produces minimal hum.

The web site parent directory has a hi-res closeup photo of the PTP wiring.
There is only one wax cap, and that is the 0.1/400vdc in the tone control.
The death cap is some sort of ceramic (or ??) type.

I ordered replacements for both from Hoffman Amplifiers.
Bought Mallory caps, but still paid $7 shipping for $2 in parts. Shit.

Q: why the parallel equivalent using 5 caps?

If any one of them shorts to chassis, the others won't matter.
Please clarify, so I can understand.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Why not just wire up and install an isolation transformer inside the chassis?

It would provide a double barrier of protection after you replace the cap that bit the dust.

This thing couldn't draw too much current so the transformer should be pretty light.

I agree in the tube days transformerless chassis were the bottom of the barrel price wise. The good more expensive amps had transformers.

I have a Kodak 16mm optical sound projector with an unisolated mono tube audio amp the only thing between me and a shock are the plastic volume knobs .

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Here is an example of a tiny isolation transformer.

Just wire up the primary to the cord and the secondary to the chassis input.

I'm sure there is plenty of space to fasten it somewhere.

http://www.newark.com/triad-magnetic...mer/dp/78K8611

This is an example of course I have no idea how much current this thing consumes you'd have to match that yourself.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Oh ok I looked around a bit more and saw the sticker on that back the amp consumes 55 watts divide that by 120 volts and we have 458ma.

So a 500ma iso transformer would do.

Here is an example that would work I'm sure you could find one cheaper though.

http://www.newark.com/triad-magnetic...mer/dp/18C5406

Here it is for less without the pics.

http://www.mouser.com/search/Product...P2zKa3ZQ%3D%3D

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Sorry this edit limit is killing me the transformer above weighs only 4 lbs a very small price to pay for the protection it provides.

Ok I'm done now .

Now back to my obligatory 4th of July drinking .

[bgavin]

Re: Knox 603 3-Wire Power Cable

Thanks for the legwork!

This is new ground for me, so I assume the 3-wire input connects to a standard 3-wire plug. The isolated output is Hot + Neutral at 115v, 0.5a, which feeds the chassis input.

As I understand it, this isolates the user from a high current shock?

I assume the xformer is not mounted to the metal chassis, but screwed into a block, etc, inside the cab.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Yes unfortunately chassis mounting wouldn't be wise in this situtation. You could use nylon or rubber washers for isolation from the chassis but mounting it to the case would be safer.

The reason why people can get a shock by touching the hot lead in an outlet for example is because the neutral and the ground are tied together. At the service entrance and at the pole transformer (the center tap is neutral and tied to ground).

Since the neutral and ground are tied and at the same potential the ground you stand on is half the circuit. Touch a hot wire and you have a complete circuit. From the hot lead through you and into the ground from ground back into the neutral due to the neutral and ground being tied together.

Isolation transformers break this neutral ground connection.

Don't get me wrong if you grabbed the two secondary wires of the isolation transformer you will get a shock however in order to get a shock with an isolation transformer. You need to make 2 simultaneous connections on the secondary side at once to complete the circuit. That means no more shocks from the guitar since earth ground no longer is one half of the circuit.

Whoever put that ground lead there didn't know what they were doing. You can't ground hot chassis electronics unless you like equipment damage. Polarized plugs aren't necessary when using an isolation transformer they are AC and don't care which way they are plugged in and will isolate either way. A two wire plug is fine in this situation.

Unlike in an unisolated chassis where if you put an unpolarized plug the wrong way into an outlet you can electrify the chassis!

You could use a 3 wire plug anyways and ground the isolation transformers body if you want.

I hope I got the whole ground neutral thing right. I just learned this myself.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

You'll notice the transformer in question has 3 wires on 1 side and 2 on the other. The two wires on the other side are the secondary side which connect to the amp.

The 3 wires are for the primary the reason for this is this transformer is for both 120V or 230V AC. For 120V AC wire one side of the plug to the center wire (the center tap) and the other to either of the end wires it doesn't matter which.

For 230 volt service you would connect to both of the end wires ignoring the middle.

I guess if connected for 230 V service this would act as a 230 V to 115 V step down transformer maybe?

[bgavin]

Re: Knox 603 3-Wire Power Cable

I was out running errands this morning, and passed by the local electronic supply shop this AM. Too bad it was 9am, and they don't open until 11am. It will be awhile before I'm in that area again. So... I will mail order the xformer.

I have to upsell the client into spending the additional $30+ for the part and shipping. I think the safety issue alone is worth the money.

I'm recapping the tone and death caps, no matter what.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Don't you love the AC / DC chassis?

I guess it's ok in radios because you don't hook yourself up to the chassis when you're using them .

Don't you love what passed as "safety" in those days a ceramic cap or two ?

Nowadays we've got warnings for everything like this sign:

http://www.safetyimages.info/admin/u...rp%20edges.jpg

If the customers safety isn't worth 30 bucks dump that thing and get a another tube amp with a transformer is what I say.

[bgavin]

Re: Knox 603 3-Wire Power Cable

The replacement caps arrived yesterday, and I have the iso xfmr on order from Mouser. Thanks for the legwork.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

No problem.

Tube chassis IMO are fun to work on in moderation . I had a few faults drive me up the wall on a tube signal generator I had to get away for a day or so before I could come back and find the problem.

A bench ISO transfromer is a must if you intend to work on AC / DC (unisolated) sets like this in the future.

Much safer to work around the live chassis taking measurements knowing you won't get buzzed by just touching it.

Switching PSUs and TVs fall into this category too.

This is the one I use (an ISO transformer in a nice self contained case fused on both the input and output sides with a switch):

http://www.newark.com/jsp/search/pro...questid=117851

Post back and let us know everything goes.

[bgavin]

Re: Knox 603 3-Wire Power Cable

I'm a computer tech by trade, so the amp repair is just a favor for my bass player. I don't do amp repairs... wish I had the knowledge, but I do not.

I have a permanent picture site located on my web server for this project.
I'll mount the iso transformer to a block glued to the inside cabinet wall.
After it is done, I will add to the pictures already on the site.

[Krankshaft]

Re: Knox 603 3-Wire Power Cable

Any update on this project?