Resistor

[238]

My question is how do you know what resistor to use where. By that i mean you have carbon film/ metal film/ metal oxide. Ever thing i work on is mine. I do not do this for profit, so i probably use some in places where i could use cheaper one. I was told i should use carbon where i could, because they will pop sometimes before you have bigger trouble. 238

[Bluerunner]

Use solid carbon resistors for RF circuits, they are non-inductive. The others are coiled wire or coiled metal or carbon film and are inducters at RF frequencies.

[238]

So a parasitic choke should be a carbon, like the old resisters, not a metal oxide like i used? What about the bleeder resistor? Thanks 238

[Crusher]

Carbon composite for the parasitic choke is the preferred method.

[238]

I reckon i will have to order me some carbon for my chokes. I have some new ones that that are probably 30 years old. Do new ones die of old age? Thanks 238

[Bobcat]

Hopefully I don't stir up disagreement.....

But, with regard to bleeder resistors..... if they are across the bigger caps in the power supply they can be pretty much anything. No RF goes through them. They are only ever subjected to well filtered DC. Metal oxide is a good choice for them.

If you are intending to maintain "period correctness"... stick with some big carbon comps.

[238]

OK thanks. I am just trying to up grade these old Maco/ Brute 75s. Some builders used the lest expense stuff they co. Maco build good amps, just trying to add some of the things that would have made them better, but would have made them cost more. I am learning more about the resisters & where to put the different kinds. Thanks for all the help for ever one. 238

[Bluerunner]

Carbon resistors, especially ones that get hot, change their value over time and sometimes cause problems in older equipment. You can ohm the carbon resistors to see if it agrees with the color code.

Like everything else, everything has it's place. Big carbon comp resistors are expensive and not always easy to find. Gotta have them in a RF circuit to avoid unwanted inductance. Other stuff like audio frequency and power supply circuits don't care. The metal film etc. are cheaper, more stable over a long time and can handle more wattage for their size too.

And another thing......

I came across some specialty "non-inductive" wire wound resistors at 50 & 75 ohm (other values too) rated at 10 watts. Good power rating for their size. My plan was to parallel / series a few of them together to raise the wattage rating to make higher power but small 50 ohm dummy load. I got an SWR of 1.2 with one of the 50 ohm ones on a short piece of coax jumper. Not great but Ok. Thought they might be handy to replace carbon comp resistors in a circuit they heat up in.

Does anyone have experience using the specialty Non-inductive wire wound types?