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Replicating (dogbone) resistors what method have you used ? - radiorich - 02-07-2021

I am restoring my Zenith model 908 and there are few dogbone resistors that I would like to replicate .

So my Question is what method have you used ?  
I am needing some 1/4 and 1/2 watt ones to make .
Sincerely Richard

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-07-2021

Several ways - probably the best/hardest/most time consuming - but still good:

https://www.russoldradios.com/blog/scott-phantom


You would need bring the leads out at 90cdeg for a DB. I can probably find some more examples after the SB.

Russ

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 02-07-2021

Hello russ ,

Here is what the Dog Bone resistors look like in my Zenith model 908 from like 1934

Sincerely Rich

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-08-2021

Ok - There are a lot of different dog bone resistors. I have made molds for most of them. Here are a few:

   

The top mold is what you need for a Zenith. It molds 1/4 W of the style Zenith used as well as other manufactures. I use as high wattage resistor as I can get into the mold, usually a 1 or2 watt modern resistor from Mouser. The 3 near the top mold are ones I have made.

   

The lower mold is for earlier resistors as are often found in late 20s early 30s radios such as Atwater Kent used. On these, the ends must be painted silver to match the lead ends used in the originals - pictured here.

   

Above is a "kit" that I made using the original lead ends with a center made of Pyrex glass, painted white. A new resistor is inserted into the glass. The ends have been drilled so that the leads pass through them. The ends are pressed back onto the tube. The new leads are then trimmed and carefully soldered to the old lead ends. Care must be used to not overheat the original ends so as to avoid melting them.

The advantage in the last style is in its ability to handle heat. With the molded vinyl types extreme heat can melt the vinyl - though I have only had one such - melt-down - in a 1/2W (4 watt inside) that I mistakenly connected from B+ to ground. This is out of the thousands I have made and used.

P.S. For future readers - I do not make these for resale.

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-08-2021

Also:

Molding materials are discussed Here:

https://www.russoldradios.com/blog/grilling-my-saba

Russ

RE: Replicating (dogbone) resistors what method have you used ? - klondike98 - 02-08-2021

Steve Davis describes another approach to replicating these. Found in the Philco Library information:
https://philcoradio.com/library/index.php/shop-talk/techniques/re-creating-dogbone-resistors/

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 02-08-2021

Hello Russ,
thank you for all the wonderful information so it seems red silicon mold works really well and that is what I should go with then ?
Then go with like 1 watt resistors

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-08-2021

The hi-temp red is a little expensive but it will stand up to a lot of wear and tear and temperature if you decide to use a epoxy-plastic. The cheap purple stuff will only last for a few times and it is soft which can allow it to distort.

You probably wont need a mold-release agent for such small parts with the stronger , red silicon. But, you might need it to release your new (red) mold from its wooden form (again, see the blog post on Grilling MY Saba).EDIT: You might use the mold release when making the top-half of your mold, to release it from the bottom half.

I generally use he highest wattage resistor that will fit in the mold or in the case of the glass-tube units, inside of the tube. I don't even stock 1/4 W resistors. New 1 or 2 watt resistors are the size of the old 1/4W units. I would use the highest wattage that fit will allow.


Russ

RE: Replicating (dogbone) resistors what method have you used ? - Arran - 02-09-2021

I might try a variation on the method Steve Davis uses, but probably with something other then the cases of ball point pens. The one pictured is made of polyethylene/polypropylene, and I don't think the paint will adhere too well., though they are more heat resistant then poystyrene, which is what the clear plastic ones are likely made of.
Regards
Arran

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-09-2021

The clear tubes that I use are Pyrex.

The problem with the pens is that unlike the glass, it only has one size. And for the pens and the Pyrex, most of the dogbones were not entirely round. The ones needed here have a flat side so molding them is the only way to get an authentic look. I have actually tried plastic. It tends to melt when the component is soldered in place. You must be careful with heat. Also, use flat paint on the plastic or glass or it comes out way-shiny. You can also rough-up the plastic and the glass for adhesion and a better look.

Russ

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 02-09-2021

Hello Russ,
thank you for all the great information !
Far as the The hi-temp red is that the plain automotive grade RTV that use ?
Sincerely Rich

RE: Replicating (dogbone) resistors what method have you used ? - Phlogiston - 02-10-2021

I am not sure if it is the same but I doubt it. See the materials list (picture) at the bottom of the blog link that I posted. The stuff, in the bucket, is a 2-part mix.

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 03-31-2021

Hello Russ,
thank you for the information .
I have not ordered the stuff yet but I have all next week off spring break !
Sincerely Rich
P.S. if i have any more questions I will for sure ask for your help.

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 11-15-2021

Hello Everyone,
I was looking at Alumilite Amazing Mold maker to use to produce  molds for my Dogbone resistors.
I wonder how well this would work it says it is Great for replicating knobs, handles and ornamental filigree.
Cures flexible for easy demolding of intricate pieces
Sincerely Richard

RE: Replicating (dogbone) resistors what method have you used ? - radiorich - 11-15-2021

Hello Everyone ,
I will be having some time off for the holidays so I thought it would be a good time to try my hand at molding .
Sincerely Richard