If you were building an AX84 Firefly..

[chptunes]

..would you use Metal Film Resistors, Carbon Film Resistors, or Carbon Comp Resistors?

I like to use 1% MF for stompboxes.. the Firefly is my first Tube Amp project.

I'm getting ready to order some parts.  I'd appreciate your input.

Link at AX84:  http://www.ax84.com/index.php/oldprojects.html?project_id=firefly

[rutabaga bob]

Some folks say to use metal film resistors in the plate supply and input jack, at least...but, to me, if you're going to buy metal film for that anyway, why not use them for everything?  Forget the old-style carbon comps.  Larry

[pappasmurfsharem]

This brings up a question.

Sure there are mojo advocates. I'm not one of them.

But has anyone made a comparison of the same circuits one with new stock items and another with mojo stuff (PIO caps, carbon comps etc...) just to put somewhat of an end to the debate.

Sure there would be some differences but I want to know if those are only based off the tolerances or if there is some snake oil hidden in there.

[psychedelicfish]

IMO the only thing that PIO caps, CC resistors etc. add to your sound is noise.

[mistahead]

IMO the only thing that PIO caps, CC resistors etc. add to your sound is noise.

Why, IYO, would CC resistors add noise?

I know there is MUCH mojo discussion already - but I'm just posting a query directly to you regarding your opinion.

[thelonious]

chptunes and anyone else who is interested---if you haven't read RG's article on carbon comp at geofex yet, it is definitely worth the read. http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm Basic idea is that there are only certain positions in the amp where CC resistors will add good harmonic distortion in percentages large enough to be audible.

[mistahead]

Sorry ignore my last post, was having a massive brain fart, Fishy you're all good I know why you have that opinion and share it...

Just a glitch in my neural matrix... probably too much carbon up there.   

[chptunes]

OK.. thanks for the input folks.

I read Mr. Keen's article (linked above), and I am persuaded to stick with MF.. mostly because of this statement in the article:

"5. as a corollary to the power guideline, we should be prepared to replace CC's every few years as the life at high temp makes them drift and get noisy(-er)."

I'd drive me self crazy wondering if it was time to update my Resistors.. I know me.

-Corey

[PRR]

> Why, IYO, would CC resistors add noise?

Carbon Composition is crushed coal mixed in clay and pressed in molds like dinner-rolls.

The carbon particles are not in good contact. It is a million "loose joints".

Carbon film, the carbon is roasted to a gas and then condensed on a form. It is molecule-by-molecule assembly. The carbon atoms are in intimate contact. There's no clay involved.

[mistahead]

Thanks PRR - thats exactly the brain glitch I had.

[chptunes]

Hey guys.. what about "Metal Oxide" Resistors.. I see that they are gray in color.. I've never used them.  What are your thoughts on these, in reference to amplifier construction?

-Corey

[thelonious]

Hey guys.. what about "Metal Oxide" Resistors

In guitar amp applications, I don't think you'll notice any difference between metal oxide and metal film. Either one will work fine. I researched it a while back and found poobah's answer in this thread helpful: http://www.diyaudio.com/forums/parts/82035-metal-oxide-vs-flamme-proof-vs-metal-film.html. I use them interchangeably... whatever's on sale when I'm buying.

[Morocotopo]

Search Geofex.com for a paper about carbon comp resistors and their effect on circuits/tube amps.