Is there any advantage in substituting a 1/2 watt or even 1 watt rated resistor for a position requiring only 1/4 watt?
I don't think there are advantages in doing that. Perhaps an tolerance of +-certain % was more critical than that. Correct me if I'm wrong.
Of course there is an advantage - the resistor will be operating at a much smaller percentage of it's thermal maximum - thermal derating is always a good thing.
and more accurate - all R have temperature coefficients - some material/construction have smaller TC but unless it is 0 you can often reduce the effect of heating on accuracy by increasing the size of the resistor so that it stays cooler with the same power
mechanical strength on point to point, temperature drift, and operating voltage are the common reasons
Larger resistors also tend to introduce less noise than smaller resistors of the same value and construction.
Assuming that the higher wattage resistor has the same temperature coefficient and precision, then what everyone said about them points to the higher wattage resistor, space permitting, is correct. It's often the case that a lower temp coefficient resistor is physically larger anyway.
Voltage rating is the biggest issue for valve circuits. Get that wrong and the resistor will fail after a couple of months, perhaps becoming noisy for a few weeks before going open circuit.
Only downsides are size and cost both not much unless you go way out there . Every thing else is a positive including useful life and for most people it a win win as they say.
This is a tube forum. 2 and 3 watt metal film resistors can resist 500 v. Modern 1/4 watters are too short, look at the voltage rating. Imagine a coating of dust (tobacco smoke, dander, bug parts, organics,) and a foggy evening at a beach bar in Galveston or Savannah. Zot, your plate voltage jumped right across the resistor. Yeah, if your amp is never going to leave Idaho, you can get away with less. I'm buying the 2 or 3 watt ones for high voltage in tube amps - like my four Hammond organs. The metal film plate resistors in ampfication service have noticeably less hiss than the (still correct value) 1961 carbon comp resistors I replaced in my Dynaco ST70 and PAS2. The H100 organs, burn up a 1/2 watter in the power amp, going from 10k to 11k in 2 organs so far. I put some old RadioShack bargain bag Dale RN65 metal film resistors there. The others, I'm buying Vishay and multicomp (farnell house brand) metal film 2 and 3 watters, which come in manufactured in India or Thailand.
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heat causes the resistor to fail
Hello,
Perhaps you fellow members will expand on your thoughts a little more about voltage in valve amps and the voltage rating of the resistor being the limiting parameter.
I get the picture of dirt and humidity bridging across a physically small RN55D resistor. My cognitive problem is the voltage rating being problematic. I think of it as selecting the proper wattage resistor for the application at hand. As the voltage across the resistor increases so does the current, this means more watts. I see it as I^2*R = watts and the resulting heat that causes the resistor to fail.
Is there a rule of thumb that should apply to the selection of valve plate and or cathode resistors, 25%, 50% of the manufacturer’s labeled wattage rating? The RN50, RN55, RN60, RN65 Vishay Dale series already has a conservative power specification, double the civilian labeled resistor of identical construction.
www.vishay.com/doc?31027
www.vishay.com/docs/31018/cmfind.pdf
DT
Hello,
Perhaps you fellow members will expand on your thoughts a little more about voltage in valve amps and the voltage rating of the resistor being the limiting parameter.
I get the picture of dirt and humidity bridging across a physically small RN55D resistor. My cognitive problem is the voltage rating being problematic. I think of it as selecting the proper wattage resistor for the application at hand. As the voltage across the resistor increases so does the current, this means more watts. I see it as I^2*R = watts and the resulting heat that causes the resistor to fail.
Is there a rule of thumb that should apply to the selection of valve plate and or cathode resistors, 25%, 50% of the manufacturer’s labeled wattage rating? The RN50, RN55, RN60, RN65 Vishay Dale series already has a conservative power specification, double the civilian labeled resistor of identical construction.
www.vishay.com/doc?31027
www.vishay.com/docs/31018/cmfind.pdf
DT
Ed Simon, I think he is Simon7000 here (iirc) published a paper on this subject.
I think he found lower distortion (generally speaking) with same resistor types of higher power.
Perhaps someone might ping him and/or post up his paper... originally he put out his stuff in the John Curl Blowtorch thread, iirc.
_-_-
I think he found lower distortion (generally speaking) with same resistor types of higher power.
Perhaps someone might ping him and/or post up his paper... originally he put out his stuff in the John Curl Blowtorch thread, iirc.
_-_-
Hi,
The power rating is to prevent overheating, which requires time for the heat to accumulate in the resistor and the temperature to climb.
But the voltage rating is instantaneous (flash-over will occur immediately the voltage across the resistor goes too high).
The shape of the waveform (from flat to spikey) will determine which rating is effective. A spikey waveform could exceed the voltage rating but be well within the power rating.
Malcolm
The power rating is to prevent overheating, which requires time for the heat to accumulate in the resistor and the temperature to climb.
But the voltage rating is instantaneous (flash-over will occur immediately the voltage across the resistor goes too high).
The shape of the waveform (from flat to spikey) will determine which rating is effective. A spikey waveform could exceed the voltage rating but be well within the power rating.
Malcolm
You may see it like that. The resistors don't. As Malcolm says, you need to be within both the power and voltage rating. In valve circuits you are likely to hit the voltage rating first for resistors in the anode circuit of a valve.DualTriode said:
You can always spray the final result with conformal coating, then the disgusting smokers wont have to worry.
As far as mechanical strength goes, I always put a kink in the leads to allow a little bit of movement.
Interesting not the heat but the voltage!
Hello,
I agree that the voltage parameter is important when specifying a resistor. We can configure a test circuit where the resistor will fail due to a voltage spike before it will fuse due to heat.
Even in a vale amplifier power supply the resistor voltage spec may be more critical than the wattage spec when the start relay kicks in. However many builders include a soft start feature to protect sensitive things like the valve heaters.
What conditions exist to cause the voltage spikes referenced for the valve anode resistor?
I am still leaning towards the wattage rating and the resulting heat over time as more important. This leads to the thought that if the proper wattage resistor is selected for long term survival, the voltage rating will take care of its self.
DT
Hello,
I agree that the voltage parameter is important when specifying a resistor. We can configure a test circuit where the resistor will fail due to a voltage spike before it will fuse due to heat.
Even in a vale amplifier power supply the resistor voltage spec may be more critical than the wattage spec when the start relay kicks in. However many builders include a soft start feature to protect sensitive things like the valve heaters.
What conditions exist to cause the voltage spikes referenced for the valve anode resistor?
I am still leaning towards the wattage rating and the resulting heat over time as more important. This leads to the thought that if the proper wattage resistor is selected for long term survival, the voltage rating will take care of its self.
DT
The only thing that I have to add is that if you're connecting these resistors directly to tube sockets, then you need to make sure that the wattage is a lot higher than you think you need because one of the main heat transfer modes for tubes is conduction through their sockets. This is why it's generally a bad idea to install tube sockets onto circuit boards without fan cooling. Circuit boards are insulating material, so they will not dissipate heat very well. So, it's left to the circuit board copper traces as well as the components installed thereon to dissipate the heat. My advice is to either always mount tubes on heatsinks and use a fan. If you must use a circuit board, make sure it has forced air cooling.
The fact that tubes survive while installed on circuit boards is more a testament to their ruggedness and durability than to good design. Tubes would last far longer if provided with proper cooling. If you had a solid state device and it was dissipating 2 watts at idle, you would put a heatsink on it. If so, then why not on a tube?
Tube shields can serve as good thermal conduction pathways as well.
The fact that tubes survive while installed on circuit boards is more a testament to their ruggedness and durability than to good design. Tubes would last far longer if provided with proper cooling. If you had a solid state device and it was dissipating 2 watts at idle, you would put a heatsink on it. If so, then why not on a tube?
Tube shields can serve as good thermal conduction pathways as well.
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