Vitreous Enamel vs Wirewound Resistor

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Greetings,

I'm gathering parts for my first tube project and most of them will be ordered from digikey.com.

In the spec that I have there is a high-power wirewound resistor for the output tube cathode. But I could only find "vitreous enamel" type with the suitable resistance/wattage value.

Are there any reasons not to use one or it can be freely substituted for a wirewound resistor?

Thanks!
 
Vitreous Enamel Resistors

It's a pity that Americans are somewhat ignorant about these devices from the other side of the pond!

WELWYN have been making these since time immorial. they cover wattage ratings fron 21/2 to 14 watt. Because of their construction are very quiet and low noise. Are relatively non magnetic (since no metal is absolutely non magnetic in absolute terms). Their reliability is very good and the inductance is low i.e, similar to other wire wounds. The purpose of the vitreous enamel was to protect the wire from oxidizing at elevated temperatures. they are rated to a maximum of 350 deg C.

Values from 0R1 to 100K depending on wattage. have a loook at the Farnell cataloge at www.farnell.com/uk

The TO220 style power resistors are thick film types and do not handle short term overloads well and are noisy. Vishey this and Vishey that come on. Just very expensive thats all.

Aluminium clad need heatsiking to realize their wattage rattings and the voltage rating between resistive element and case is only good for 160 volt AC. so no good on tube HT circuits.

Go forget about toilet humour.
 
I've heard that the aluminum power resistors also have hysteresis and eddy current issues - something to keep in mind if you are using them with any appreciable frequency (>100kHz I guess; don't forget about harmonics!).

And hey, I ain't ignerent of 'em... they're great. Although since I only ever got any from old tube-aged colletions, I thought they were an ancient thing... until I spotted them in Mouser that is ;)

Tim
 
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Sch3mat1c: There's negligible magnetic coupling between the resistive element and the aluminium case, so eddy currents don't cause additional losses. Open one up, and you'll see what I mean. Unfortunately, most use copper-clad steel tags.

Hacknet: The only thing against paralleling lots of 1/2W resistors is that you will need to keep a good air flow over each one, otherwise you won't get the composite power rating you expected.

AV8R: Perhaps they've changed recently, but all the Welwyn W20 series resistors I have use copper-clad steel wires and magnetic end caps.

The voltage rating between case and resistive element varies according to the component's power rating. The 50W types are fine for 1250Vac. Enough, I think.

Agreed, a thick film resistor will always be noisier than a wirewound, but this shouldn't be an issue at the cathode of an output valve!
 
Welwyn Resistors

I was discussing a generalization not a specific. The leads are copper clad iron. I am not going to go into the 'Whys and wherefores' of magnetic materials.Try to make an output transformer without magnetic materials?
For cathode resistors they are fine.
Yes the larger 50 watt aluminium cased resistors can handle larger voltages between element and case.
 
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It's speculation, but...

A wirewound resistor shouldn't produce significant excess noise compared to Johnson noise because the ratio of the circumference to cross-sectional area of the wire is sufficiently low to make surface irregularities insignificant. Of course, that assumes a noiseless connection to the resistive element. Vishay S102 metal foil resistors are available in 0.02% tolerance, and reducing thermocouple effects in order to meet that specification might require considerably more care in making the connection to the resistive element than would be justified for a 5% tolerance power resistor.

So, yes, it could be that a 0.02% S102 produces less noise power than a 5% W21. Unfortunately, the S102 is 10dB more expensive.

Can you expand on your information, Positron?
 
Info

Yes, I found my old source. Here is what is stated.

" Bulk Metal Foil Technology: -40db.
Resistors made of alloys of metal have their resistivity as a result of hte intergranular boundaries between conductive metallic crystals in the alloy. These boundaries are quite long and therefore mask any local site distortions. The signal-to-noise ratio with these resistors is the best available...."

"Wirewound Technology: -38db
Wirewounds are made of alloys similar to foils so the only noise insertion comes from the tabs used to connect the fine wire to the coarse external leads. "

"Metal Film Technology: -32db to -16db

Metal films are made by evaporating or sputtering a layer of nichel chromium onto a ceramic substrate. The thickness of the layer is value dependent and may be from 10 Angstrom to 500 Angstrom thick. The thicker it is the lless noise insertion...."
Unlike the foil resistors where the lines are generated by pricise photo etching, here the spiral of resistance is generated by either grinding, which leaves a ragged edge or laser adjusting which leaves a curl edge with eddy-current paths both of which are a source of noise."
 
Re: Info

Positron said:
Yes, I found my old source. Here is what is stated.

"Wirewound Technology: -38db
Wirewounds are made of alloys similar to foils so the only noise insertion comes from the tabs used to connect the fine wire to the coarse external leads. "

Thanks for your input!
Some wirewound make fanny noises as loud pops when they become hot as the connection with leads aren't usually very reliable...
 
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