How do I make this 'widowmaker' safe?

[Gaz]

[Mod's note: This thread has been pruned and edited for clarity- and closed until further notice. Steve A.]

I've read about transformerless 'widowmaker' amps before, but this is my first experience with an amp which has no power transformer. My friend who owns the amp complained of a buzz, and I said I'd take a look at it.

The amp in question is a Magnatone "Starlet" 107 (schematic below).



magnatone_107 Scarlett.pdf

My first question is about the .05/170K combo that separates the ground from earth. What does it's function? This particular model I have does not have the 170K resistor.

Before I had this amp, my friend told me that a shop added a 3-prong cord. From my research it seems like it was the wrong thing to do, and it turns out the added earth connection to the chassis via the 3rd prong was the cause of the hum.

I think I just need to add this isolation transformer and then I can add the earth connection again. Is that correct?



this isolation transformer

Is there anything else I need to do? What about the mysterious .05/170K?

I also plan on adding a fuse to the primary of the iso transformer.

Thanks for any clarification on this process.

Gaz

[R.G.]

My first question is about the .05/170K combo that separates the ground from earth. What does it's function? This particular model I have does not have the 170K resistor.

Wow. That's impressively dangerous. It relies on the voltage insulation of the *knob* on the volume control and the isolation of the ground symbol from the negative side of the power supply. I know what the .05 is there for - it's to ground the input jack and first tube to the negative side of the power for RF. I can't fathom how the 170K works. It looks to me like it's between the cathode resistor of the first half of the 12AX7 and the power supply minus for DC purposes.

I guess this means the first half-tube floats between V+ and V- with the plate resistor and the 170K for DC, and the input jack is floating on the grid-to-cathode resistor connection and "ground" wobbles around in the middle.

Before I had this amp, my friend told me that a shop added a 3-prong cord. From my research it seems like it was the wrong thing to do, and it turns out the added earth connection to the chassis via the 3rd prong was the cause of the hum.

Oh, yeah. That will NOT work on this amp as shown.

I think I just need to addthis isolation transformer, then I can add the earth connection again. Is that correct?

Yes.

Is there anything else I need to do? What about the mysterious .05/170K?

I would tie the "ground" end of the 3.3K cathode resistor and the input jack ground to the V-, now that V- is no longer one side of the AC power line. You *might* need to put the 170K between the V+ and the 270K plate resistor, and decouple that point to the (now-real!) V-/ground with a 1uF to 10uF electro cap. This would make the thing quieter as a side effect. But I bet the .05/170K can simply be removed.

I also plan on adding a fuse to the primary of the iso transformer.

Good plan!

[loudthud]

That schematic has the UL logo on it. That means it passed the UL safety standards at the time it was made. Since UL does not publish what those standards are or were, we can only guess. My thinking is that the 270K and .05uF cap limited the current that a person touching the input ground would see if the line cord was installed with the hot side to the power supply ground.

If an isolation transformer was installed, you could just install a wire between the two grounds. The schematic says 21 Watts. You should use at least a 50 VA transformer because of the half wave rectifier. The one you linked above should work.

[Prairie Dawg]

The series string amps were an outgrowth of series string radios, what was known as the All American Five. They were fine sitting on a shelf but when you're intimately connected to the chassis they are full of shock potential. Arvin, a/k/a Arvin Meritor, known then and now for their metal stamping expertise made metal cased radios in this fashion but relied on a baked enamel finish for insulation. I cringe when I see them and someone has stripped the paint and chrome plated them...

[Enzo]

That is a precedent. There are some Crown power amp models that have an extra winding on the fan motor to power the low voltage circuits.

[Gaz]

R.G. and others, thanks so much for the help! All my questions were answered, and am going to get going on bring this thing up to code

[Gaz]

Hi, Just wanted to post an update. I installed the isolation transformer, and simply removed the .05 cap, and connected the V- directly to the chassis. I did not make any other changes, and the amp is very quiet now. There is also a proper safety ground now with its own dedicated ground lug.



Here's a picture of my handy-work. Not exactly how I would have ideally done it, but the transformer was way too big and heavy to mount on the inside or outside of the tiny flimsy steel chassis.

I also added a primary fuse you can see in the photo, but am not sure how to calculate the proper value. Any ideas?

[g1]

The rear panel says "117 volts 21 watts", this would be power consumption of the amp, (not power output for this design ). A 1/2 amp fuse would be good for around 60 watts, a 1/4 amp would be good for about 30. Use slow-blow fuses. If you can get them, I would try the 1/4 amp with volume at loudest setting you will be using. If it blows, try the 1/2 amp.
I don't think you will find any sizes in between. If you can't get the 1/4 to try, the 1/2 amp should be fine.

[Enzo]

You know, I think I'd just use a 1A fast and never look back. The slow blows are used mainly to cover starting surge. The 1A fast is large enough to handle that, but small enough to pop if something goes pfffft. And the blow time of a 1/2 slow and a 1A fast, I don't think are all that different on something like this.


But I don't disagree with g-one either.

[tedmich]

this might help...



Isolation transformer upgrade for old guitar amps

or this



Remember the Alamo - Telecaster Guitar Forum

[g1]

I see that GAZ simply added a decent sized 1:1 transformer inside his amp to isolate it from the power line. Can someone post a stock number for a tranformer like that?

It really depends on the power requirement of the particular amp. The VA rating of the iso. trans. is all important. The one you linked is 300VA which translates to around 2.5amps at 120V. So it would be suitable for amps that are fused up to 2.5Amps. Seems a reasonable price.
The one GAZ linked to has a stock number for the Triad part, more here: (US price will be lower, but still more than the JAMECO)
Triad Magnetics Power Transformers | Mouser

[bluto]

I see that GAZ simply added a decent sized 1:1 transformer inside his amp to isolate it from the power line. Can someone post a stock number for a tranformer like that? I looked over at Jameco and all I could find was this one for $49.99

ITR300: JAMECO VALUEPRO: Power Supplies & Wall Adapters

Mouser carries a bunch with various power ratings and they start at about $11...FWIW I used the $11 item on a series filament amp I built and it worked fine.

Mouser Electronics - Catalog Viewer