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-   -   Metal Oxide Screen Stoppers (http://www.diyaudio.com/forums/tubes-valves/138268-metal-oxide-screen-stoppers.html)

SpreadSpectrum 10th February 2009 01:10 AM

Metal Oxide Screen Stoppers
 
I read somewhere on the internet that Metal Oxide(not metal film) resistors are similar in manufacture to Carbon Composition resistors just that the resistive material is different. Can't find the reference right now, but I'll keep looking.

I was looking for Carbon Comps for screen stopper duty, but they seem to be harder to find (or ridiculously expensive) at power ratings over 1/2 watt new. Metal oxides are much cheaper and available in higher ratings than carbon comps. I don't really want to use a dingy old one, either. Would a Metal Oxide have similarly low inductance and work as well in this application?

Ty_Bower 10th February 2009 01:58 AM

I thought I saw 100 ohm, 2 watt, carbon composition parts not that long ago. I can't seem to find them now.

These guys are rated 1 watt. I don't know if that's enough for your application. Still at little pricey at $0.90 each, but better than some others...
http://www.newark.com/multicomp/mcrc...tor/dp/72K6210

llwhtt 10th February 2009 02:07 AM

Digikey has Ohmite brand carbon comps. 1/2 and 1 Watts.

Craig

SpreadSpectrum 10th February 2009 02:21 AM

Still, if the metal oxides are non-inductive like the carbon comps, they are much cheaper and much higher power ratings are available.

I wanted to purchase a selection for experimentation, thus the desire to buy the cheaper metal oxides. If they work as well as carbon comps, I'd rather go that route. Do they? Anyone know of anyone who has done inductance measurements on these?

I guess I could always buy comparable parts and measure the inductance myself.

Ty_Bower 10th February 2009 02:25 AM

I thought the big problem with film resistors on the screen grids was that the resistors could catch fire and burn. Screens can draw a lot of current on the peaks. Of course, low inductance can only be a good thing, too.

anatech 10th February 2009 02:44 AM

Metal Oxide resistors have pretty good surge ratings. Choose your power rating for it's maximum voltage rating. I haven't used Metal Oxides types for screen resistors, but I'll bet they work fine.

BTW, they don't really burn. They smoke a bit, then go open. No flames from what I have seen so far.

The way they are constructed is much like a carbon film resistor. They are deposited on a ceramic (or similar) core. Then they are trimmed using a helical cut. Compared to the resistance, I wouldn't call these inductive.

Quote:

I guess I could always buy comparable parts and measure the inductance myself.
I can and have done so. They were unremarkable in that regard. That means that there was nothing I needed to remember (not a problem). I can always measure again if needed.

-Chris

tubelab.com 10th February 2009 02:50 AM

I have been using metal oxide resistors as screen stoppers in many of my amps including the Simple SE. If you chip the coating off of some of these resistors you find that they have been laser trimmed in a spiral pattern. This has not been an issue with any of the common power tubes that I have tried.

Quote:

I thought the big problem with film resistors on the screen grids was that the resistors could catch fire and burn.
YES! I found this out early in the SimpleSE design. Most experts say to use a 1/2 watt screen stopper. After I blew one and set another on fire, I will say to use at least a 1 watt, maybe 2 watts especially with an EL34 that gets pushed to clipping. Of course I tend to "test" things a bit more strenuously than the average user does. All of my amplifiers must be able to live with me playing my guitar through it with a guitar preamp set on kill. That takes out the wimpy designs, including the two previously mentioned screen resistors.

Beware some "carbon comps" aren't what they look like. Crush one with vice grips and see whats inside. If it's full of black charcoal looking stuf that crumbles. they are real. I have seen some that have a carbon film resistor inside!

SpreadSpectrum 10th February 2009 02:52 AM

http://www.koaspeer.com/pdfs/res52.pdf

The above data sheet shows the type of resistor I had in mind. Unfortunately, the picture seems to imply that there is some helical cutting (which I am assuming would increase inductance).

The data sheet also implies some flame resistant properties.

Unless a resistor guru weighs in and clears up all of my confusion, I may just have figure this out the old fashioned way.

edit: I'm a slow poster, it seems.

SpreadSpectrum 10th February 2009 02:59 AM

Okay so to sum it up: carbon comps are still best, but metal oxides are probably good enough.

anatech 10th February 2009 03:10 AM

Hi SpreadSpectrum,
Quote:

Okay so to sum it up: carbon comps are still best, but metal oxides are probably good enough.
I wouldn't say so. Carbon composition resistors are best in high frequency RF work. With the frequencies you are working with, the metal oxide parts are not considered inductive.

Hi tubelab,
Quote:

I have been using metal oxide resistors as screen stoppers
Me too, on rebuilds and some playing around.

Quote:

Most experts say to use a 1/2 watt screen stopper. After I blew one and set another on fire, I will say to use at least a 1 watt, maybe 2 watts especially with an EL34 that gets pushed to clipping.
The voltage rating on metal oxide types may have been your problem there. I often use 2 watt parts in tube amps for the voltage breakdown rating alone. Some 1/2 watt parts may only be good to around 200 VDC from memory, maybe 250 VDC? If it arcs over, you will see some cool effects!

-Chris


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