Transformerless Silvertone 1389 amp: a question (and schematic wanted)

[slidincharlie (Carlo P)]

The only time I played this little thing was right after the delivery with a stepdown autotransformer. I'm still alive... but maybe I've been lucky!
I will have to search further for an isolation stepdown transformer...
but - since I have several stepdown autotransformers at home - I still wonder if a 1:1 isolation transformer exists, and if it's a possible way to go.

[Enzo]

Of course 1:1 isos exist. That would be the most common form really. The order is not really an issue as long as th direct connection between the unit and the mains is broken.

[slidincharlie (Carlo P)]

Thank you Enzo,
I'll search for an 1:1 isolation tranny.
BTW, I checked today with a small cheap digital VAC tester: there WAS 110V on the chassis! (only if I inserted the plug into the stepdown transformer in one way; the chassis was 'cold' with the plug inserted the other way round).

[Rob Mercure]

Carlo,

Think about it - you've been around here long enough and you and I've communicated enough for you to realize that this is exactly what you've expect if you've been running the thang with a step down auto transformer. And this is why so many of us continually stress out safety concern with line connected equipment and stress the need for the isolation tranny. Think what would happen if you touched something grounded while your guitar was plugged into the amp with the plug in the position where the chassis is hot!

And since you can't find an isolation tranny why not make one out of two trannies as I've suggested. I've got several around the shop that I've built into a separate chasses with switches, fuses, power lights, and outlets - sometimes you have "regular" power transformers that have primary to frame shorts that leave the chassis hot. Get a 240 to 12 VAC and a 240 to 24 VAC tranny and wire the low voltage secondaries together. Then feed 240 into the one with the 24 volt secondary and you'll have isolated 120 VAC from the "old" primary of the tranny with the 12 VAC winding. These trannies should be readily available from dealers of heating and cooling equipment, door bells, and garage door openers as well as many other sources. Build it into a metal chassis with fuses, switches, etc. and then use the Sears amp safely. Otherwise get a small power transformer with a 6 volt and a 120 volt winding, replace the recfifier with solid state, replace the 12AU6 with a 6AU6, replace the 50C5 with a 6EH5 and enjoy your amp.

Rob

[slidincharlie (Carlo P)]

Rob,
you make me smarter than I actually am!
I've learned many things here, but the absence of any electronic background makes me dependent on detailed explanations about anything that sounds new to me...
Today I have located stepdown isolation transformers by an Italian web-based electronic store, and tomorrow I'll visit a couple more shops here in town. If I don't succeed I'll try the two-tranny trick.
Thanks a lot,

[slidincharlie (Carlo P)]

I have an isolation transformer now. I believe it's a unit normally used for industrial circuits, and surely is oversized for my needs (160W).
I attach a photo and the schem below.
This tranny is provided with two little metal brackets that can be used to bridge two adjacent lugs in the secondary.
It can be wired either 1:1 or 230>115 Vac. I would use the second option AND add a 3-pron cable to the amp.
How should I wire this tranny to feed my amp and to ground the chassis with a 3-prong cord?

[Rob Mercure]

Carlo,

Can't remember your specific amp but if there's room in the chassis simply mount the iso unit as far from the 12AU6 as possible and then wire a fuse and a switch in series in one side of the line to the iso while wiring the other side of the iso straight to the line - make sure the grounding wire continures straight to the chassis (I believe this is right but euro 240 volt may want the switch in one side and the fuse on the other but I doubt it). From the isolated side it doesn't matter which side is wired to which. Now I can't remember if you amp came with a power on/off light but I'd add one as well as also add a power light to the iso tranny so that you eliminate suprises. While you don't need a switch upstream of the iso I'd put one there and I'd also put a fuse upstream of the switch on the amp

While the isolation tranny won't draw much current when the amp's turned off it will draw some so if you put a switch upstream of it you'll save energy and if you make it an easy switch to use you'll cut down on wear and tear on the "vintage" Sears switch. The fuses you can mount under the deck so that you don't have to punch a hole and for indicator lights you can use either neo bulbes or LEDS which only require a 1/4 inch, or so, hole punched and a dab of hot melt glue.

Now if you don't have enought room for the iso then build it into a project box, add all the swictches, fuses, and lights, and then an output that matches your amps plug. You can then use this if you get any other USA equipment that you want to isolate. I can't think of the designation but I like the three prong female 'jacks" that computers use and recommend that you use one of these which either option works best. This way you can get your switch and fuse already hooked up w/o having to punch another hole while also having a power cord that you can remove and get out of the way. On all of my "valuable but not vintage gear" I've converted to these outlets making taking the amps over the road so much easire - the on the road power cord bag carries several from three feet up to 18 feet so that I've got as much cord as I need on stage and not an ince more - I love it (I've now started wiring watch batteries to LEDs and taping them at each end of my guitar and speaker cords. I can take down in the dark in minutes and the rest of the guys want me to start making them sets (but I'm limited by the colors of LEDs, dang),

Rob

[Steve Conner]

That 3-prong female power connector is an "IEC 320 C14". Everyone just calls it an IEC.

The one important thing in the hookup is to connect the ground wire to the chassis. Then if you get anything else wrong, it'll just pop fuses instead of killing you.

[slidincharlie (Carlo P)]

I'm furiously scratching my head now...
As you can see in the photo of the iso tranny attached to my post above, there is only a ground lug on the secondary side, but none on the primary side. So where do I attach the mains' ground on the primary side?
Or is it enough to run a ground wire from the amp chassis to the iso secondary via a 3-prong cord?
Further, a guy that I have contacted in an Italian electronics forum told me that such the amp must NOT be grounded when used with an isolation tranny...

[Enzo]

You don't ground the primary side. The point of the iso is to isolate the mains itself from the mains wiring in the amp. The ground wire is a separate issue. The ground wire connects the chassis to earth ground so if some voltage gets on the chassis it will be grounded off.

Wire the mains to the primary, wire the secondary to the amp, wire the ground from the mains outlet right to chassis. The whole idea is to be able to ground the chassis. You CAN'T ground a hot chassis.

[slidincharlie (Carlo P)]

Job finished

I have tested that stepdown isolation tranny that I bought. Everything works fine, the chassis is grounded and it doesn't carry AC anymore.
I'll install the tranny inside the cabinet if I can find a noiseless position.
Thank you all for the advice.