Kay 703 Guitar amp

[raysolinski]

Any guitar amp gurus here? I bought one of these little guys and it hummed so I ...recapped, re tubed (nos tubes) and added a grounded power plug and it still hums considerably..works great..has a cool tone..it hums the same with a guitar plugged in and with nothing plugged in....in various locations around the house..there isn't a diode in the whole thing so it isdefiantely running off ac for the whole shebang..

Cheers,
Ray

 

[pucho812]

Check your caps. Probably needs a recap job due to age..

 

[CJ]

grid leak, nature of the beast, but that compression!

move to the sticks if you want to play grid leak.

my cardboard box sears silvertone, same thing.

 

[pstamler]

There must be a rectifier diode someplace in the circuit; tube plates run off DC, not AC.

Is your amp a 703 or a 703-C? The 703 has a 50L6 output tube, the 703-C has a 60FX5.

The 703 uses a transformer for the plate voltage, but only has a half-wave supply. This makes the ripple 60Hz (plus harmonics) and means the filter caps are less effective. The filaments, meanwhile, are hooked directly to the wall AC.

The 703-C, on the other hand, takes the plate supply from the wall AC. This could kill you real dead.

Peace,
Paul

 

[raysolinski]

Mine is the 703 C...I just noticed I did not replace 2 of the three sections of the multi section cap! A guy over at harmony central said this almost totally eliminated the hum...so I will get 2 caps and FINISH the re-cap and report back to you tube gods!

Cheers,
Ray

 

[CroatianSensation]

Is this why they refer to some older tube amps as "widowmakers?" :shock:

 

[emrr]

Believe it or not I had an NOS one still in the box around 1998, and it was quiet as a mouse (when not playing) when fired up. Sounded great for many things. Should be achievable.

 

[pstamler]

[quote author="raysolinski"]Mine is the 703 C...I just noticed I did not replace 2 of the three sections of the multi section cap! A guy over at harmony central said this almost totally eliminated the hum...so I will get 2 caps and FINISH the re-cap and report back to you tube gods![/quote]

Sounds reasonable...but while you're in there, put in a 120:120V isolation transformer, in the interests of not getting killed.

Peace,
Paul

 

[raysolinski]

My unit does have an isolation transformer ..both on the schematic and in the unit but not being overly familiar with tube circuits..could it be improved upon? Should I add one between the wall and the heaters or rectifier? The switch on the tone control kinda has me confused a bit (not hard to do at all!)

Cheers,
Ray

 

[pstamler]

If the schematic I have (Tube Amp Book, 4th Ed.) matches your amp, the isolation transformer only feeds the preamp tube's filament. The rectifier and output tube filament both need isolation. A single 1:1 tranformer feeding into where the wall AC now comes in would work fine.

Peace,
Paul

 

[raysolinski]

Thanks Paul,
I will order a 120:120 and install it...

Cheers,
Ray

 

[bradb]

Hey guys,

I also just picked up a Kay 703C...

http://www.freeinfosociety.com/electronics/schematics/audio/kay703c.pdf

I recapped the multi-section capacitor and replaced the non ceramics with orange drops. I also added a grounded plug and a fuse. The fuse and power switch are coming off the black wire in my black-white-green plug. The green wire is soldered to my chassis (scraped off paint and used the monster iron)

When plugged into one of the outlets in my house, the amp sounds quiet and insanely great when playing guitar.

When plugged into other outlets it hums INSANELY loudly, no audio can come through. I mean, this is CRAZY HUM, its unplayable.

What's the deal?

Is the neutral and hot in my house wired differently and because of the weirdo way the heaters are heated causing this hum?

Will that 1:1 transformer help me out aside from making it safer?

 

[pstamler]

[quote author="bradb"]When plugged into one of the outlets in my house, the amp sounds quiet and insanely great when playing guitar.

When plugged into other outlets it hums INSANELY loudly, no audio can come through. I mean, this is CRAZY HUM, its unplayable.

What's the deal?

Is the neutral and hot in my house wired differently and because of the weirdo way the heaters are heated causing this hum?[/quote]

Possibly. What you need to do is go to the hardware store and get an outlet tester (little doohickey with neon bulbs on it). It'll tell you if your household outlets are miswired.

Will that 1:1 transformer help me out aside from making it safer?

Probably. In any case, it could save your life, and that's worthwhile whether it fixes the hum or not.

Peace,
Paul

 

[bradb]

I know that this outlet where the amp works is not grounded now!

the outlets where it doesn't work (HUMS!) are grounded...



but what does it mean?

 

[mrphotodude]

an isolation transformer would be a great idea!
yet your power transformer output has one side running through the chassis.
schematic: http://img407.imageshack.us/img407/2723/kay703dw1.jpg

Hot chassis are not fun if you like playing guitar, singing and having lips on your face.
If your wiring in your place are messed up that could cause issues. That would change the hot to neutral on the chassis...
you will inherently get hum out of it from the random grounding points, how much has different factors to do with it.
I personally would rebuild it into a bit different amp, but that is some work and it would be different now wouldn't it  :

looks like there is a few different types of 703 by kay.
http://www.schematicheaven.com/bargainbin.htm
I know alot of old fenders used to route one side of the heater through the chassis and you could undo that and fix it..
but since one side of the mains secondary and heater and all signal grounds are going to chassis(most likely) i am not sure where to start!

 

[bradb]

Yeah, that schematic (http://img407.imageshack.us/img407/2723/kay703dw1.jpg) is the one I have.

I'm not seeing the point of the isolation transformer when the output of the power transformer is tied to the chassis. The isolation transformer was only for the heaters right?

However, now I do understand why the amp works when the outlet isn't grounded!!!  (THANKS!)



Ideally, I need to get an isolation transformer for the heaters, thankfully the heaters are not also run thru the chassis....

To deal with the hum in different outets:  i need to untie those chassis grounds and build a ground bus bar separate from the chassis?  I think so! 



after all, i like having lips on my face.

 

[bradb]

I'll have to have isolated input jacks now...

hmm

 

[MartyT]

Hi, I came across this post on a google search. I recently bought a Kay 703 in SUPERB (like new) condition. I have determined it is not a C model, but the original 1963 703. I love the way the amp sounds, but after receiving quite a shock last night after playing through it, I started researching what I now know is dubbed a "widowmaker" of an amp...ouch! I know very little of amp/electric stuff, but have shared what I learned through this post and others with a fellow electrician, who says he can make it safe with $75 labor + parts.

My question is: is it worth it to make the mods to this cool little amp to make it safe to play through, or is it more "collectable" if I leave it deadly and not play through it? In which case, I'd probably repost on Ebay...your thoughts??? Much appreciated!!!

 

[pstamler]

Think about it this way: do you want to be the guy who sold an amp to somebody who subsequently was killed by it?

Peace,
Paul

 

[MartyT]

Think about it this way: do you want to be the guy who sold an amp to somebody who subsequently was killed by it?

Peace,
Paul

Well, there would obviously have to be full disclosure, but NO...! 

I'm gonna have it fixed.  this thing rectifies the ac mains minus an actual "power transformer", so It will need the Isolation xfmr mod