[Amps] Alpha low drive mod resistors

[Colin Lamb]

"Doing a low drive mod on Alpha 76CA. Saw five 470 ohm 2W composition carbon as recommended, hard to find. Will modern carbon film or metal film resistors work or are the inductive enough to screw things up?"

Carbon comp resistors are not good long term choices.  They change resistance just sitting on the shelf (years), absorb moisture and more importantly change resistance when they are run near their rating.  Many years ago, I wanted to use some 2 watts resistors in a commercial product and made tests to see if I could "age" them to become more stable.  In open air, I took 2 watt resistors and ran them at 2 watts for 1 minute, then cooled them down.  The resistance changed permanently.  So, I ran the same test again, with the resistor changing to a new value after it had cooled down.  I kept running the test and the resistor kept changing value, although at some point it started going in the opposite direction.  

Many of the resistors made today have the resistance on the surface and material is removed in a spiral manner, so they can have inductance.  However, it is often very slight and will vary depending upon value.  The manufacturer will have the information available.

In the value you need for the Alpha, you can make up the required assembly then check it on something like the MFJ vhf bridge.  See if the resistance changes with frequency.  A network analyzer or more sophisticated device is better, but may not be necessary.

73,  Colin  K7FM