Resistor max working voltage vs watt rating

[dinkyguitar]

Hi All,

I'm looking for a 1meg 1 watt resistor that will be used between pin 6 of 5881 power tubes to choke for elevated heaters in a slo clone.

Here's one builder's pic.
http://fc03.deviantart.net/fs71/f/2014/201/8/4/eh_mod_by_haftelm-d7rl6uv.jpg

Plans call for a 1 watt, but the VR68 1/2 watt that can handle that voltage and more.
https://www.digikey.com/product-detail/en/vishay-bc-components/VR68000001004JAC00/VR68J1-0MCT-ND/1995208

If plans call for a 1 watt, could I use 1/2 watt provided it can handle the voltage?

I'm not understanding this....

Also, others have used resistors with a max working voltage of 500 without issues...but I'm concerned about longevity.

Looking for some opinions.

dinky

[2deaf]

VR68 is 1W.

I have used 1M 1W for single-resistor bleeders and elevated heaters because my 1M 1/2W resistors didn't have a sufficient voltage rating. 

[Latole]

Why don't you use 2 Meg 1/2 watts in parallel ? Or 5 watts cement resistors ? They are so cheap.

[pdf64]

Why don't you use 2 Meg 1/2 watts in parallel ? Or 5 watts cement resistors ? They are so cheap.

In regard of their voltage rating, it's not good to lump product types together like that as a generic whole, as if all are all akin.
eg of those types mentioned, they may be only be rated for 250V, hence they may have a short operational life if subjecting them to 500V.

[Latole]

Vintage amps wich many are working over 450 volts may not use special resistor ( rated for 500 V) or I'm wrong ?

[dinkyguitar]

I have read suggestion about using 2 in parallel/series as well, but I'm try to stick to a cleaner look like in the pic.

I'm just looking for correct way it should be done for reliability.

Also, I know VR68 in that link is 1watt, but they also have a 1/2watt (VR37) rated at 3500v.

I know it's overkill, but I only found resistors rated at 500v or these...I couldn't find any in between.

Can I use the VR68 or VR37? Metal Glazed Leaded Resistor

I don't have enough experience to know if these can be used in amplifiers.

dinky

[pdf64]

Vintage amps wich many are working over 450 volts may not use special resistor ( rated for 500 V) or I'm wrong ?

Vintage times were different to our current scenario.
There would be little point to make and sell low voltage parts in the 50s; the HT circuits of the time may have resulted in their early failure. Whereas now they are suitable for use in many, perhaps the vast majority, of applications; and it is HT circuits that are the oddball application, whose parts may require a non-typical specification.

[vampwizzard]

I try and buy as many of my components from here as a way to say thank you for hosting a great forum. That being said, if theres an oddball part that I need there are many other folks providing parts.

I dont want to sound rude but this is a 0.15 problem. Plans call for 1 watt 1meg? https://tubedepot.com/products/1-watt-carbon-film-resistor?gclid=Cj0KCQjwnv71BRCOARIsAIkxW9FVvfPcX8OUnXY7Je1mPg9RupqyWTenz85Ciyr9FX7QfNczzuBkiCoaAh3eEALw_wcB

Dont substitute power requirements on known circuits. Values for tone tweaking? all for it.

[Latole]

I try and buy as many of my components from here as a way to say thank you for hosting a great forum. That being said, if theres an oddball part that I need there are many other folks providing parts.

I dont want to sound rude but this is a 0.15 problem. Plans call for 1 watt 1meg? https://tubedepot.com/products/1-watt-carbon-film-resistor?gclid=Cj0KCQjwnv71BRCOARIsAIkxW9FVvfPcX8OUnXY7Je1mPg9RupqyWTenz85Ciyr9FX7QfNczzuBkiCoaAh3eEALw_wcB

Dont substitute power requirements on known circuits. Values for tone tweaking? all for it.

That is the resistors I always use

[labb]

I have kicked this resistor max. voltage rating around several times. Generally I will always come back to "there are thousands of guitar amps running that use 1/2 watt 300 volt resistors..I really think that if the layout is such that none of the resistors are in contact with metal or other high voltage components you are ok....What about the wire you use? 300 volt is pretty standard in most amps..I use 600 volt wire because I am comfortable with it.

[dinkyguitar]

I have kicked this resistor max. voltage rating around several times. Generally I will always come back to "there are thousands of guitar amps running that use 1/2 watt 300 volt resistors..I really think that if the layout is such that none of the resistors are in contact with metal or other high voltage components you are ok....What about the wire you use? 300 volt is pretty standard in most amps..I use 600 volt wire because I am comfortable with it.

The components won't touch as you can see if the fist pic...I'll be building it the same way.

As per the wires, they are all rated for 600v, 200 deg C, teflon milspec wire....it came with the kit.

dinky

[labb]

Don't fudge on the wattage rating required and you should be fine.

[dinkyguitar]

Got it...

[PRR]

> the VR68 1/2 watt that can handle that voltage

Most "1/2W" won't. If you are sure about the specific part you buy, that's cool.

[Latole]

Antique electronics ( tubes and more ) write in their old 2008 Catalog page 50.

1/2 watt Carbon Comp, Carbon Film, Metal Film resistors; 350 volts working voltage, 700 volt overload
Same for 1 and 2 watts Metal Oxyde  and Ceramic flame retardant 5 and 10 watts .

[pdf64]

Antique electronics ( tubes and more ) write in their old 2008 Catalog page 50...

Kudos to them for providing that info, some 'tube guitar amp' parts suppliers don't.
Hence the benefit of raising awareness of this rating.
The saving grace being that the number of applications within a tube guitar amp that actually subject a resistor to >350WV are vanishingly few, so the typical, popular 1/2W CF resistors are generally fine.
dinkyguitar has raised a particular instance where that is not the case.

..I really think that if the layout is such that none of the resistors are in contact with metal or other high voltage components you are ok....

The issue is that subjecting the resistor to voltage somewhat in excess of their rating may eventually cause the resistive element to fail somehow; not that it would arc through the coating across the adjacent stuff that was at lower potentials. Though, of course, as voltage was increased, at some point around a few kV, that would occur  [ Invalid YouTube link ]&feature=emb_logo

...What about the wire you use? 300 volt is pretty standard in most amps..I use 600 volt wire because I am comfortable with it.

Whereas resistor voltage rating is an issue when maintaining / building tube guitar amps, wire voltage rating absolutely isn't. Any decent undamaged plastic coated wire is good for many kV; arcing through previously undamaged insulation just doesn't happen. In this case the rating process probably subjects the wire to extreme environmental conditions, eg temperature, pressure, altitude, immersion in fluid; think of aerospace, marine etc. Far beyond what a tube guitar amp or any regular equipment could be exposed to and be expected to operate, or perhaps even ever work again.
eg the flying leads from vintage OTs made in the UK (including Drake / Dagnal in Marshalls) are incredibly wimpy, the look barely fit for >100V, yet I've never known of their insulation being a point of failure, despite the plate circuit of an overdriving tube amp handling kV spikes thousands of times per second.
Rather, if there's an event such as speaker cable pulled out or speaker fusing, the actual transformer windings or valve sockets will arcover. 

[labb]

From an SMI paper:

"Multiple voltage ratings are typically provided on a datasheet, but most often the primary concern is the maximum
working voltage. Maximum working voltage is the maximum amount of voltage the resistor can withstand constantly
without arcing. Maximum working voltage is often expressed as “Vrms.”"

As I posted have kicked this around several times. Same with the wire voltage rating. Why waste time and effort putting the rating on the wire if it is ignored.

[pdf64]

From an SMI paper:

"Multiple voltage ratings are typically provided on a datasheet, but most often the primary concern is the maximum
working voltage. Maximum working voltage is the maximum amount of voltage the resistor can withstand constantly
without arcing. Maximum working voltage is often expressed as “Vrms.”"
...

My interpretation here is that the arcing will be along the resistive element (thereby causing it to fail), rather than punching through the surface coating to nearby items.

...Same with the wire voltage rating. Why waste time and effort putting the rating on the wire if it is ignored.

Context / application is key here; if wiring up a high altitude plane etc, then the voltage rating of the wire used is significant, as the environmental conditions are harsh.
The same wire may have 2 or more voltage ratings according to the system and clause used. For our purposes, the voltage rating doesn't matter, as the environmental conditions it will be used in must be human friendly and so the insulation will be good for several kV.

In the same way that if we are building fx pedals, the voltage rating of the resistors used can reasonably be ignored, as whatever they are, they will be massively more than the demands of our application.