Choking tone? Carbon Comp vs. Metal Oxide SCreen grid

[Jerry2013]

Resistors. I realize this is so basic , its like the old CD vs, Vinyl debate in the audio domain. Some of you are rolling your eyes I'm sure.
From most of my gandering on the web, it looks like resistance on the old stock carbon Comp resistors rises over time, and this appears to be the "tone " factor on some amps like mine that I am talikign about here; vinatge original blackface 65 Bassman.
So by most accounts, there is no difference in tone , all things being equal , between carbon Comp and Metal Oxide ( forget about carbon film and metal film, thats too deep for me).
My original carbon comps look good. It doesn't look like they are burnt at all. maybe only very slightly tarnished on the band colors.

Should I leave em? Should I do new carbon Comps? Should I just do new Metal oxide? I kinda want to restore this for posterity, so noone ever has to worry about the screen grid resistors, yet if carbon comps are going to give up the golden tone, then I would put em in.

I like wat Ive heard about the Marshall Dale wirewounds, yet it doesnt look like they are made in the values for the Fender Screen grid .

Advice gents?

[Cliff Schecht]

Sounds like you are getting BUMS (blind urge to modify syndrome) man. Carbon comp resistors are great in power supplies if they are properly rated for the duty they are handling. Also they happen to be somewhat flameproof, maybe not as much as metal oxide guys but I've never had or seen one catastrophically fail and actually catch fire.

Some people use these resistors (screen resistors specifically) as a fuse for when the screens on old worn out power tubes fail. The theory is that using a "properly rated" screen resistor, the resistor will blow before the power tube is able to do any permanent damage (take out a PT for example). This IMO (and in many others) is bad practice and not something that I do in my amplifiers. Fender did it, I don't think Marshall did, but regardless you are misusing a component essentially to save money. If this really needed to be fused, you would put a fuse on there. With all of that said, this is assuming that you know to change tubes regularly or if/when a pair starts going south. For new manufacture tubes I go with a once a year change (for power tubes) and for old stock, it just depends on how much life is left in the tubes (these tend to last a LOT longer even under pretty heavy abuse).

Back to the question at hand, if it ain't broke then don't fix it. When I'm working on other peoples old Fender's I will only change all of the electrolytic caps (including in the preamp unless the owner requests otherwise) but try to leave all resistors alone unless they are dead/dying. Signal caps remain untouched unless they are leaky (yellow Astrons leak more often than blue molded IME).

Also, as far as tone goes, you won't hear or feel a difference between a carbon comp or metal oxide resistor. The voltage drop across the resistor is too small to develop a significant change in resistance and furthermore, this coefficient of resistance vs. voltage was still minimized in carbon comp resistors by design. It's a side-effect of the technology that manufacturers aimed to minimize so any quality CC resistors only exhibit this phenomena in a small amount when the voltage drop is significant enough to cause a slight change in resistance.

[sliberty]

Carbon comps can be a little noisy in some positions in an amp circuit. Carbon films, metal oxoide,and metal film are a bit quieter. However, I wouldn't make the change unless noise is an issue, and even then, only the offending resistors (plates perhaps).

As for mojo, if the reason your amp sounds so cool is that the resistors have drifted in value, you can measure them note their values on your schem, and if it ever becomes necessary to replace one of them, you would replace them with the measured value instead of the original schematic value. The time to do this blue printing is when the amp is working well - don't wait until something fails to get those measurements. But remember, you have to lift one leg of each resistor before you measure it.

Bottom line - just enjoy your amp and don't "fix" anything unless it really needs fixing.

[Blackburn]

If you do end up applying new screen resistors, why not just go with the smartest and most cost effective method and use 5W ceramic resistors? They're cheap and you can just mount them on the socket. And I know they don't come in 470R.... Just use 500R. That area is not one where much cork sniffing should be done. You want your power section durable and over-spec'd for longevity.

[alvarezh]

Blackburn, you can buy 470 ohms 5 watters here in case you prefer them:

http://www.parts-express.com/pe/showdet ... er=015-470

All the best.

[Blackburn]

Very cool. I use CE Distribution and they don't stock that value. Good to know! Thanks.

[pdf64]

I've seen Fenders where the original carbon comp screen grid resistors have burnt out and made a nasty mess of nearby wiring, so my view is that it's best to use metal oxide flame retardant replacements.
The contribution to mojo that carbon comps could conceivably make in this application (using the geofex rationale) is negligible.
As for the power rating, I agree that a fuse with specified fusing characteristics would be better, however the designer didn't chose to go that route, rather using 1W resistors.
The good news is that 1W metal oxide resistors in this application (in my experience) fuse consistently with little collateral damage, when presented with the significant overload of a screen grid short.
So unless you intend to fill the amp with fuses, my view is that it's best to keep to the original design and use 1W resistors for this application.
To use higher wattage resistors without suitable additional fusing would expose the transformers to potentially prolonged fault currents of likely failure modes, which strikes me as being a bad idea.
Pete