Help Vintage Guitar Hummer

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Hi Guys,

Im Stumped on trying to repair/restore a vintage {Guessing} 1960-1963 Kay Practice Amp Model #703.
I was at an auction this saturday on a 150 year old farm. Saw this little baby sitting there and just had to have it. Its exactly the same model I had as a kid when I got my first guitar.

Figureing the worst, I tested the tubes, All RCA originals and found them well within good usable condition. No shorts, about 85% out of 100 possible on my tester.

Everything inside looked to be clean, Nothing looked changed or burnt so I plugged her in. 10 or so seconds Its Humming very loud!! I quick shut her down figuring a cap job is in order. SO, I replace The paper caps with nice nos spragues orange drops of correct values. Then move on to the power supply muliticap.

40,40,30 - 150V
I just happen to have a brand new in box NOS sprague atoms in my vintage supply! SWEEET.

So, I proceed to swap out the old Ajax multicap with the NOS Sprague.
Plug her in, About 10 seconds HUMMMMMMMMMMMMMM again.
In Fact no change at all.

It is a VERY Loud hum, And Sounds like its at about Full volume when you touch the end of a open guitar cord !

I Have the schematic as it was pasted inside the unit, And Have also found it online.

(schematic removed by Moderators for safety reasons)

Heres A Review from Harmony central on this amp, I remember it as being exactly as this reviewer describes:

Sound Quality: 9
'93 American Std. Tele.
For clean to slightly dirty blues/jazz and rock this little gem shines.
It had a pretty loud buzz until I replaced a multi-stage electrolytic capacitor and put in a three prong power cord. Now, it's very quiet up till about 8 on the volume at which point a very quiet buzz manifests. Probably my unshielded guitar and not the amp.
I'm continually impressed with the gorgeous, snappy clean sounds I get out of this amp.
The distortion is mild, but I suppose it would respond well to a tube screamer. I don't own a distortion pedal so...
Still using the original speaker, but I might upgrade in the future.

Right Now Mines apart on the bench, So I will post more pictures later. Prime objective right now is to figure out what I must Be Missing.
Any Input would Be GREAT!!
Frank Berry said:
That is a dangerous amplifier. It has no isolation from the mains.

The very first thing that you should do is use an isolation transformer on the powerline.

Yes, this is very True.

BUT, for the sake of originality I wish just to restore it to working condition. Seeing as its never going to be used for anything but occasionally rare Mic'd recording and conversation piece.

Right now, I have just completed testing the resistors only to find no issues. Yet I still only get a constant Hum.
This is not affected by volume or tone control. Nor Instrument plugged in or not plugged in. The only thing left would be the ceramic caps. Or Possibly a tube that even though its testing correctly, Has an issue. I am really stumped.
Joined 2003
:cop: :cop: :cop: :cop: :cop:

If this amplifier does not have an isolation transformer then it is extremely dangerous and you do not have the necessary knowledge to be fiddling with it. Cut the mains lead off and keep it as a conversation piece. It doesn't matter how rarely you propose to use it. Lethal only needs to happen once.

:cop: :cop: :cop: :cop: :cop:
Here is what I would do with that. Rewire the power supply to include a 1:1 isolation xfmr, full wave SS bridge (you could use SiC Schotky's for an even quieter PS) and keep the 36AM3 as heater ballast, leaving the plate and cathode disconnected. That way, you preserve the aesthetics of the thing, remove the "suicide box" aspect of the original design, and have a much quieter PS, in that the filter capacitors will be twice as effective with double the ripple frequency.

That kind of xfmr-less PS is a disaster waiting to happen. If you connect the hot side of the main to the chassis ground, your guitar will also be at 120V(ac) to ground. :dead:

An utterly irresponsible, dumb-@553d design, if you ask me.
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