making dogbone resistors

[mike hadley]

I was just going through some old photographs of the restoration of my allwave fifteen, I decided to update the post I placed a while back about dogbone resistors,

so here is the process, I don't claim to have invented this as it's already on the net, the process works surprisingly well.

you will need some metal filled epoxy putty, an old resistor to copy (thanks Kent) and a mould release agent, I used homemade beeswax paste polish.

first mix a chunk of epoxy putty, then rub wax into the "donor" resistor, this resistor is then pressed into the putty about half way,

using a flat blade knife or similar push the putty around the resistor to close up any gaps left by pushing the resistor in to the putty,

once the putty has set, lift out the resistor to make sure that it won't get stuck later,

re coat with wax and replace the resistor.

Coat the whole surface with paste wax taking care to remove any excess,

mix up some more epoxy putty,

this is then pressed over the top of the embedded resistor,

when the top layer of putty has set, separate the two halves, and remove the donor resistor.

This is your resistor mould, again coat with paste wax or your preferred release agent and select the resistor values of your choice,

in the picture I have used two resistors, I reasoned that as the finished dogbone will be used on the set ht and most modern resistors don't have a high enough working voltage I would need to address this, here the the examples are 1 watt, these are well over rated.

Remove these resistors and mix up some more putty,

I use stuff called Milliput, it is a model makers putty, good fine texture and slow setting (about 24 hours), I did not want to use metal filled putty as I was not sure about the insulation properties.

the trick here is only put enough putty in the mould to fill half way.

This is then covered with more putty before the waxed top half of the mould is pressed on, too much putty and the finished resistor will be fat and mis-shaped, not enough and you will have voids,

after about 5 hours when the putty is set like hard rubber, the mould can be separated,

and the still soft resistor pulled out,after cleaning off the flash and other small imperfections the resistor will look like this,

if you look at the center of the resistor there is a small round blob, this is where the mould has picked up the paint marking from the original, the process is really that good, I was surprised at this myself.

Once fully hardened the resistor can be painted with the correct body tip spot color code, the only problem I had was matching the old paint colors,

can you spot the fake resistors

the paint I used was hobby shop acrylic type as this has a matt finish I gave the finished resistor a quick coat of varnish.

in the above photo the genuine dogbone resistors are the ones with short legs.

[Jon Ketron]

Excellent write up Mike.  Thank you for sharing a detailed explanation of this process.  I am going to try this very exact method.  Your reproductions look to be very accurate.

Jon