Metal Film or Metal Oxide resistor?

Mark Herrick · Post by **Mark Herrick** » 20 May 2005 1:16 pm

What's the difference, if any, as applies to using as a plate load resistor or screen grid resistor?

If the schematic calls for a 1-Watt resistor is there any reason I could not use a 2-Watt or 3-Watt resistor?

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Jack Stoner · Post by **Jack Stoner** » 20 May 2005 1:29 pm

The only reason is a higher wattage than what is specified could cause other component failure if excessive current flowed and the 1 watt resistor didn't fail.

The 470 Ohm screen grid resistors on 6l6's are the same way. On those the screen grid resistor sort of acts as a safety device and will overheat and open up. I've replaced a lot of 470 ohm resistors in Twin Reverbs back when I was doing amp repair.

I've always taken the attitude I was a tech there to fix something, not to redesign what some engineer or enigneering department designed. [This message was edited by Jack Stoner on 20 May 2005 at 02:30 PM.]

Mark Herrick · Post by **Mark Herrick** » 20 May 2005 4:28 pm

Well, I just ordered a bunch of Metal Film and Carbon Composite resistors from DigiKey, so I guess I'll know soon how they work out...

Ray Minich · Post by **Ray Minich** » 20 May 2005 10:21 pm

A metal film resistor can be made by taking a small rod shaped piece of glass or ceramic, onto which has been deposited a metal coating, and carving a spiral thru the coating at a certain pitch, and for a certain length, as to "spiral" the resistor blank to the required resistance. Temperature coefficient, stability, accuracy, and fire resistance are quite good. A "composition" resistor is made by molding a housing around a glob of goop.

(15 Months at Corning Glass Works, Electronic Products Division, making the d*&m things) Typical spirallers used a sharp carbide wheel. The neat spirallers used a Carbon Dioxide laser. One of the first lessons learned was that the "ware" (resistor blank) for metal film resistors that were to undergo laser spiralling, should be made from an opaque blank. Else the laser just wiped the whole coating from the rod.

Typical tin-oxide resistors were made by drawing a glass rod from the bottom of a white hot glass tank about 4 stories in the air. Once the glass was molten you kept the tank hot by passing an electric current thru the material. You pulled this taffy like string from the die at the bottom of the "draw" tank as it was known, and then passed the string thru chambers containing tin-oxide in a gaseous state, which deposited the metal film on the rod. At the bottom (floor level) was the cutting head that cut the string into 3 foot blanks. From there the rods got cut into itty bitty lengths and processed with a fair degree of difficulty into axial lead resistors. Some as good as 0.01% resistors I might add.[This message was edited by Ray Minich on 20 May 2005 at 11:39 PM.]

Michael Whitley · Post by **Michael Whitley** » 21 May 2005 9:09 am

Metal film is typically used nowadays for a plate load resistor, instead of the old carbon comp. Metal oxide is used for screen grid and power resistors. Carbon film is frowned on by the boutique amp dudes.

Mark Herrick · Post by **Mark Herrick** » 21 May 2005 12:15 pm

I did notice on the specs of the resistors I ordered from DigiKey that the metal film resistors are tiny! The body of the .5W resistors is only .134 inch long. That's just a hair over 1/8"

Is metal film (as opposed to metal oxide) not good for the screen grid resistor?
[This message was edited by Mark Herrick on 21 May 2005 at 01:15 PM.][This message was edited by Mark Herrick on 21 May 2005 at 01:18 PM.]

Michael Whitley · Post by **Michael Whitley** » 22 May 2005 12:00 pm

I think metal film is O.K. there. Metal oxide handles the heat best.

Brad Sarno · Post by **Brad Sarno** » 22 May 2005 4:42 pm

I've got metal oxide (flameproof) 2w plate resistors on my Twin and Deluxe. I also stand them off the tube socket about an inch to keep them away from the tube heat. The stock resistors are right there on the socket and they tended to burn more easily.

Brad

Doyle Huff · Post by **Doyle Huff** » 23 May 2005 6:52 am

Hi Ray, sounds like you and I have been down the same path. I worked for a company called Electra Mfg. back in the 60's. At one time I was group leader over the spirlling line. Metal film resistors produce lower noise than the other types. I have an old Fender amp that I am going to replace all the resistors with metal film.
Doyle

Ray Minich · Post by **Ray Minich** » 23 May 2005 7:37 am

Doyle, I bet you & me are the only ones that know what a "camel's foot" is. (For everyone, it's what a poorly welded leadwire looks like when attached to the end of the spiralled blank by welding, instead of a nice cone shaped weld, the weld looks like a camel's foot.)[This message was edited by Ray Minich on 23 May 2005 at 08:37 AM.]

Brad Sarno · Post by **Brad Sarno** » 23 May 2005 9:24 am

You may not want to entirely replace the Fender's resistors with metal film types. Many metal film types have a kind of glassy sounding top end. Although the resistors will make the amp quieter with less hiss, you may lose some of the warmth that makes a Fender a Fender. Many of the very high end botique guitar amp makers swear by the carbon comp type resistors. Personally I do like some metal film resistors. The Dale/Vishay 2% metal film resistors sound very musical. Some people say that in a Fender, it's ok to replace the resistors that don't have high voltage on them with film types, but in high voltage places like plates and slope resistors, you may want to keep the carbon comps in there. The idea is that the "tone" of a resistor is more dramatic when it has high voltage on it.

Just a thought.

Brad Sarno

Malcolm Leonard · Post by **Malcolm Leonard** » 25 May 2005 10:43 pm

I service home electronics entertainment devices of all kinds made between about 1935 and 1975 but I am not by any means a guitar amp guru or specialist.

If there is a difference in sound between carbon and film resitors,I can't hear it in any of the stuff I work on.

There is,of course,a whole highly controversial debate about how capcitors and resistors and even slightly different metallic compositions used hook in up wires can affect sound.
Most of it does not fit into accepted electronic engineering practice and theory as taught in the colleges and technical schools and is very subjective,since we all hear differently anyway.
The information about not using larger wattages so that the resistor could purposely act as a fuse,also does not usually apply in the gear I work on,so I found that interesting.It certainly does make good sense in the context given here.
Malcolm

[This message was edited by Malcolm Leonard on 25 May 2005 at 11:46 PM.]