I guess that’s one way to add a fuse…

[Whatizitman]

Picked up a little Kay 503 (I think) widowmaker on the cheap cuz it’s not powering up. PO said they had a shop add fuse and 3-prong, and that it was working when they picked it up. I tried it on my light bulb limiter and got nothing. Filaments dead. Fuse had heat shrink, so I had to peel it off to see. Looks OK. Well, except for the manner of wiring. But I’ll attend to that later.

The ground side of the main is soldered to the ground tab on the cap can. If the amp ever had an iso transformer it ain’t there no more. I haven’t done any further testing, given the potential angry nature of a widowmaker Kay.

I will more than likely get an iso transformer, change caps, etc… But I want to know all that I have before I buy parts. I get that a 1:1 transformer won’t cost much. But what do I do with the filament series? Ok to keep it direct from mains?

What should I check next, and how to do it safely? My current limiter is a high wattage bulb, so I’m not even sure it would show a short at mains voltage(?).

Any ideas are helpful. Even the inevitable “don’t walk away. Run” responses.

 

[Peegoo]

@Whatizitman

If that fuse is anything more than a 1A rating, it's not going to do its job, and some other component in the amp will serve as the "backup fuse."

The three-wire mains cable is a great idea, but it's a life saver only when all the AC outlets in use are properly wired for correct polarity. Always test the outlets using a $10 neon tester. It really is the cheapest life insurance you can buy.

If you're not going to run an isolation transformer, swap out the 'death cap' for a class Y cap because they always fail open--not closed.

The death cap is not a safety feature to prevent death. It's included to suppress AC line noise, and is called the death cap because it will kill you if it fails closed.

 

[Jon Snell]

The 503 has an isolation transformer, in fact all of the 5 series appears to as well.
A photo of it will help to its correct identity.
Either way, the new isolation transformer goes between the amplifier and the mains. Not just the secondary side.

 

[jhundt]

I can understand the hard-wired fuse, after seeing so many amps with aluminum foil wrapped around a blown-out fuse in the fuse-holder....

at least this way, somebody has to open upo the amp and take a look.

 

[Whatizitman]

The 503 has an isolation transformer, in fact all of the 5 series appears to as well.
A photo of it will help to its correct identity.
Either way, the new isolation transformer goes between the amplifier and the mains. Not just the secondary side.

Yeah, there’s a bare spot on the chassis where it should be. Either it was never there, or was “pilfered”. I love that word. In any case, I have read about 503s that didn’t have one. Seems inconsistent.

 

[Whatizitman]

I can understand the hard-wired fuse, after seeing so many amps with aluminum foil wrapped around a blown-out fuse in the fuse-holder....

at least this way, somebody has to open upo the amp and take a look.

Got it. But it’s like $5 for an inline fuse holder with a threaded cap end. It would be nice to not have to heat up iron to check and/or change the fuse.

 

[Whatizitman]

Nope. I was wrong. Fuse is dead. I checked it for continuity. I can’t tell the rating. But the innards must have been very thin, implying lower amperage. The little black specks on the counter in the shadow

are actually reflections of the obliterated wire in the glass.

 

[Whatizitman]

So something caused it to blow…

I’ll do some more continuity testing on the grounds, but I’m not plugging it in until I replace the iso tx and better fuse mounting.

@Peegoo , what amp rating do you recommend? Anything under 1A ok? Seems like whatever was there did its job.

I can post a 503 schem pic if anyone would find it helpful.

EDIT: here's a thread from a couple years ago on essentially the same type of amp I have. @Wally posted a schem.

 

[Peegoo]

So something caused it to blow…

I’ll do some more continuity testing on the grounds, but I’m not plugging it in until I replace the iso tx and better fuse mounting.

@Peegoo , what amp rating do you recommend? Anything under 1A ok? Seems like whatever was there did its job.

I can post a 503 schem pic if anyone would find it helpful.

Go with 1A. You can get an inline fuse block at any auto parts shop for a few dollars. It makes it easy to replace a blown fuse because it's not soldered directly in series.

I've read in multiple places that a fuse is not necessary on these amps because the tube filaments will serve as fuses and pop at 150mA or so. This seems completely goofy to me based on the following.

The live side of the AC mains runs to the switched pot. What happens if the switch fails shorted to ground? The filaments are not in that circuit so they will provide no protection. There could be a fire or a shock hazard.

If I am off in the rhubarb on his, please jump in here and set me straight because I am not an expert on any of this stuff.

 

[Whatizitman]

Go with 1A. You can get an inline fuse block at any auto parts shop for a few dollars. It makes it easy to replace a blown fuse because it's not soldered directly in series.

I've read in multiple places that a fuse is not necessary on these amps because the tube filaments will serve as fuses and pop at 150mA or so. This seems completely goofy to me based on the following.

The live side of the AC mains runs to the switched pot. What happens if the switch fails shorted to ground? The filaments are not in that circuit so they will provide no protection. There could be a fire or a shock hazard.

If I am off in the rhubarb on his, please jump in here and set me straight because I am not an expert on any of this stuff.

The previous tech installed the fuse before the switch, which makes sense. But the blown fuse doesn't give any indication for where a short might be. There is no isolation tx in this one (missing or never there), and the heaters are after the switch, of course. The schem shows the heaters are tapped off the mains before the TX. I imagine that's an easy fix with a replacement iso TX.

 

[Whatizitman]

OK, since the schem calls for an iso transformer, and this doesn’t have one (but was apparently working after the fuse and grounded plug installed), this amp was getting mains AC to the filaments and rectifier. Without the iso transformer the filament and plate/grid supplies are not isolated from each other? Is that correct? If, say, the rectifier (35Z5, diode) blows, could that blow the fuse? If, OTOH, under the original design with iso tx and no fuse, could the filament supply survive a blown rectifier or tube?

 

[NTC]

I am not touching the fuse question. With the isolation transformer, the filaments are still in series. Rectifier (35Z5 here?) Is usually at the "hot" side and the preamp tubes at the ground side. You simply put the transformer between the cord (and switch and fuse) and the rest of the circuit. There is no filament winding.

 

[Whatizitman]

Confirming some 503s made it into the wild without iso trannies, here’s a pic off the interwebs of one such. Compared to mine below it. Mine has quite a bit of real estate on the rectifier side. Two what I assume to be mounting holes, with no evidence of any prior mounting. Not sure of year difference, but they are otherwise identical looking on the outside.

On the bright side, my chassis would accommodate a proper PT better for a Champ build.

 

[NTC]

What tubes are in it? If it were mine, I would rebuild it. It is probably good for 1, MAYBE 2 watts with the present circuit. Not a bad thing nowadays

 

[Whatizitman]

What tubes are in it? If it were mine, I would rebuild it. It is probably good for 1, MAYBE 2 watts with the present circuit. Not a bad thing nowadays

Similar design to an old small radio. 35Z5 rectifier, 12AU6 pre, 50L6 power. The pre-amp tube may be reusable in a more traditional guitar amp circuit. The others not so much. 12AU6 is a pentode, but it can be heated at 6v, if I read that correctly somewhere.

Here's that schem pic.

https://el34world.com/charts/Schematics/files/Kay/Kay503a_1962.jpg

Good suggestions from @robrob on repairing/upgrading widowmakers:

Widowmakers

Widowmaker Guitar Amplifiers

robrobinette.com

My rebuild options are thus:

1. Fix and keep as is. Cost of a few fuses and holder, and maybe caps and/or a tube or more.
2. All of the above and add in small iso transformer. Following the original circuit of iso on the plates/screens. Heaters on mains. Cost $20-30+ beyond the above.
3. Follow suggestion by @robrob and get an iso transformer big enough to handle entire circuit. Say, 70VA tranny costs upwards of 3x of smaller tranny or more*.
4. Gut and build something like a tweed Champ. The cab, speaker, chassis and some parts make it a good donor prospect. Regardless, this is the costliest option.

*Hardly worth the money. May as well build a Champ.

 

[Whatizitman]

OK, I installed a proper inline fuse holder to make it easier to do some troubleshooting. The the B+ wire to the 50L6 plate had snapped off at some point. Reconnected it, but it still blew a fuse. I rechecked the grounds, and sure enough there was no ground continuity where the reported tech connected the safety ground, which was at the ground tab of the filter cap can. Nada. I connected the safety directly to the chassis, and it fired up. Fuse holding.

It sounds horrible. Noisy, like a radio with no reception. Very tiny input sound that is muffled and distorted in a bad way. Bad filter caps? Certainly having no iso trans is not helping. I'm definitely curious as to how it ever worked with the safety ground where the tech put it. But even a bad cap can still ground to the chassis. But this is clearly not the case here.

Soooo.... either way, some mo' money is gonna have to be spent to get this playable. I paid $40 for it. At the least I have a nice little plywood cab and old alnico speaker to use for something. For what, I'm not sure. Certainly not worth buying an iso transformer for less than mediocre results. I think a champ build will be in the future.

 

[Whatizitman]

Oohhh wait! A Champ Micro! That's the ticket. Maybe I could get by with the original output tranny on a micro champ build. It's quite a bit smaller than a champ OT. I doubt it could take 5 watts.

 

[Peegoo]

On a crusty/dusty thing like that, it's generally good practice to replace all the caps that aren't ceramic disks or micas. But even so, it's also good practice to test every component for proper spec. This usually entails lifting a few leads to prevent stuff like reading two resistors instead of the one you intend to test (use the schematic).

Very tiny input sound that is muffled and distorted in a bad way.

Remember to test the speaker and that choke that's on it.

 

[Whatizitman]

Revisiting this thing. I’m trying to get a sense on the previous work. Can someone help me figure out what’s going on here with the AC neutral? It’s connected to the rectifier anode. Why? If it ain’t right, where should I move it to? I can’t see any reason why the neutral would go there. Seems backwards(?)

 

[Whatizitman]

Ok I think I got it. There’s no transformer to connect neutral, so I think the last tech just swapped the neutral from the switch to the plate of the rectifier. It’s connected to the heater through the 150r, making the rectifier at the end of the heater series (wall AC). I guess that would work. But now I’m finding other issues that need to be fixed. The filter caps weren’t grounded because the tech cut the wire from the ground tab to the “star” ground to chassis. Why is anyone’s guess.