10 ohm R's going high - problem.

[Diabolical Artificer]

I've had several (5) Vishay 10 ohm 2w 1% metal film resistors go high - up to 200k. They're not being run hard either, part # is CPF210R000FKE14. also one go low 4 ohms.

I'm using them as sense resistors in the the cathodes of EL34's. Maximum voltage is 1v at 100mA or 100mW. But this is hardly ever, only for testing. A safety circuit cuts power at 1v.

So am I using a too low a wattage? I would have thought 2w an adequate safety margin. they are diddy though in comparison to your average 2w R, IE 8mm by 3.5mm.

To replace them with 5w resistors will be problematic, 5w 1% R's being uncommon. Of course I could use 5% R's and match them.

Lastly I've found lower value R's tend go go drastically high as opposed to your higher values.

Andy.

[Terry]

In the days when carbon composition resistors ruled the roost, the mantra was : low resistors go low; high resistors go high!

[Refugee]

I would be looking for a spike at power up or power down.

[Murphyv310]

Bypass them with a suitable capacitor. Could be spikes or any AC Audio content putting them over their dissipation.

[ppppenguin]

This is pretty extreme. Vishay resistors are excellent quality. About the only thing that might kill them apart from gross sustained overload is pulsed overload. That could blow them open circuit, as with any film resistor. I don't understand how they might go low.

Why do you need 1% parts? Valves just aren't that accurately matched or specified.

For ultimate bomb proofing use small wirewounds.

[Crackle]

What you need Andy are some of these slim little resistors that you find in KB radios from the early to mid 1930's.  
Even after 85 years or so they are still spot on, well almost.
The lower value ones are normally wire wound like this 300 ohm one, still spot on at 300 ohms
   
The higher value ones are what look like a spiral wound carbon element on a ceramic former.
This 100k resistor still comes out at 102k ohms. What is that 2% out?
   
They only ever seem to need replacing if they have burnt out due to excessive current caused by leaky capacitors.
It is a shame because they started to be replaced with the more standard fat dog bone ones from about 1934, and they are hopeless, I have changed every one of them in the KB 381 I have been restoring over the last couple of weeks, the only original resistors are these two seen here.

Mike

[Diabolical Artificer]

Can't see that there'd be any spikes at power on, one end of the resistor is at ground, tother on the cathode of an EL34, so there's zero current till the heaters warm up. Spikes at turn off? Again HT falls in about 5 seconds and current through the R falls steadily, but I'll scope em anyway. The only other thing I can think of is the protection circuit where a thryristor fires to switch on a relay, that cuts neutral mains.

Someone over on UKVRR has also had trouble with these particular Vishay resistors. I still can't see how a tiny R can dissipate heat as well as a big bodied one.

I've got a handful of those type of resistors Mike, they are pretty damn good as regards being in spec. Sure I read somewhere they were designed to handle very high voltage and other extremes.

I'll get some 5% jobbies and see how I go and check for spikes/pulses.

A.

[ppppenguin]

I find it hard to see how cathode resistors will suffer pulse overloads. If it was a long term overload you'd know about it.

If these Vishay film parts are really no longer trustworthy then it's time to look at wirewound. By their nature they will either be the correct value or open circuit.

To my surprise you can get 1% wirewounds for not too much money:
http://uk.farnell.com/vishay-dale/rs02b1...dp/1167848

Or the usual 5% parts such as: https://uk.rs-online.com/web/p/through-h...s/4851717/

[Crackle]

Watch out for the excessive delivery charge if it is US STOCK.

Mike

[ppppenguin]

Whoops, didn't notice the US stock.

RS have a selection of 1% WW parts in stock such as: https://uk.rs-online.com/web/p/through-h...s/8506792/