Vishay Dale RN65 resistor max voltage @ 350v? Any Danger to use in plate loads?

[JubileeMan 2555]

I was looking to experiment with some of these in my plate load resistors on the preamp and phase inverter posistions.

But I noticed in the specs that their max working voltage is 350v and my super reverb's preamp plates are giving off about 320-330v.

Am I playing with fire? Almost literally? if so, do you know if the RN70 has the same tonal characteristics as the RN65 but with 500v of maximum working voltage?

I'm guessing few know this answer, but figured I'd try.

[VacuumVoodoo]

The 350V rating is for total voltage across the resistor. If your preamp B+ is ca 300V the plates will be at perhaps 140-200V, taking signal swing into account you end up with maybe max 280V peak across plate resistor.
In short: generally should be no problem but read on:

One often forgotten thing is power rating of the resistor which takes precedence over the voltage rating. Knowing resistance R and it's power rating P you get max allowable voltage as V =sqrt(P*R).

Example:
R=100k, P=0.5W --->Vmax= 223V even though the rating is 350V
R=220K can work with 330V
etc.

[JubileeMan 2555]

The 350V rating is for total voltage across the resistor. If your preamp B+ is ca 300V the plates will be at perhaps 140-200V, taking signal swing into account you end up with maybe max 280V peak across plate resistor.
In short: generally should be no problem but read on:

One often forgotten thing is power rating of the resistor which takes precedence over the voltage rating. Knowing resistance R and it's power rating P you get max allowable voltage as V =sqrt(P*R).

Example:
R=100k, P=0.5W --->Vmax= 223V even though the rating is 350V
R=220K can work with 330V
etc.

Crap...so since its a Fender in question, the Plate load resistors are all 100K. I can't use this resistor then. Pooey

What about a 1watt version of the RN65?

[VacuumVoodoo]

Well, Fender uses 0.5W resistors, at least in the RIs. Why? Consider this: preamp tubes work in class A which means that you get maximum plate resistor power dissipation at idle with no signal. Sound signal averages over time to zero so power dissipation stays basically constant.

The instantaneous peak voltage across plate resitor should be lower than another resistor rating: dielectric isolation withstand, this IIRC is 500V for the RN65.

So all things considered 0.5W plate resitors with some 300V supply are fine.

Sorry for not being very clear on this in my first reply - a bad cold is clouding my gray matter.

[JubileeMan 2555]

Well, Fender uses 0.5W resistors, at least in the RIs. Why? Consider this: preamp tubes work in class A which means that you get maximum plate resistor power dissipation at idle with no signal. Sound signal averages over time to zero so power dissipation stays basically constant.

The instantaneous peak voltage across plate resitor should be lower than another resistor rating: dielectric isolation withstand, this IIRC is 500V for the RN65.

So all things considered 0.5W plate resitors with some 300V supply are fine.

Sorry for not being very clear on this in my first reply - a bad cold is clouding my gray matter.

LOL! Ok... so I should not worry about using these in my Fender's Plate positions? THANKS!!

[mooreamps]

The 350V rating is for total voltage across the resistor.


The 350V rating is for the total working voltage with respect to ground.

-g

[VacuumVoodoo]

The 350V rating is for the total working voltage with respect to ground.

-g

So I can't hook a shorted circuited resistor to a 1500V potential and just let it hang there?. Voltage is not same as potential. I can ground the B+ instead of 0. You're confusing things.

[reaiken]

The 350V rating is for the total working voltage with respect to ground.

-g

No, it isn't. It is the voltage across the resistor. The resistor doesn't "know" where ground is.

Randall Aiken

[BK-Amps]

I was looking to experiment with some of these in my plate load resistors on the preamp and phase inverter posistions.

But I noticed in the specs that their max working voltage is 350v and my super reverb's preamp plates are giving off about 320-330v.

Am I playing with fire? Almost literally? if so, do you know if the RN70 has the same tonal characteristics as the RN65 but with 500v of maximum working voltage?

I'm guessing few know this answer, but figured I'd try.

You'll be fine. Lots of manufacturers use the RN65 as plate loads. Even if your rail was above 350V and your voltage swing somehow pulled the anode down to zero your average working voltage across the resistor would be well below 350V.

BK

[donnyjaguar]

If in doubt, use 4 resistors instead of one. I you're not familiar with parallel-series, just solder 2 resistors in parallel (twice) and then connect these in series. Net result will be the same resistance. This will double the voltage rating and quadruple the power dissipation.

[scottl]

No problem. First of all, the RN65 is dirable milspec piece.

Secondly, I and my other amp building buddies have been using the 100K RN65 for years as plate loads. In fact, I have a bunch of pics of real Dumbles and Dumbleators, where you see RN65 written on the 100K plate load.... Seemed to work for him too.

[donnyjaguar]

Agreed, ScottL. Just tabling another option. :)

[soulsonic]

If you're worried, just use an RN70. It's rated for 500v.
In The Art of Electronics they do an experiment where they exceed the working voltage rating of a resistor intentionally to illustrate what happens. It doesn't burst into flames or explode; what happens is its value of resistance begins changing until it goes way off from where it was originally. I think they did the experiment with either a 1/4 watt or a 1/2 watt carbon comp, and at maximum, they put 1,000 volts across it - all that happened was that the resistance was way off. You have to get up into thousands of volts before you start having serious issues with the dielectric being compromised and you need to exceed 600 volts for there to be potentials for arching through the air.

[reaiken]

If you're worried, just use an RN70. It's rated for 500v.
In The Art of Electronics they do an experiment where they exceed the working voltage rating of a resistor intentionally to illustrate what happens. It doesn't burst into flames or explode; what happens is its value of resistance begins changing until it goes way off from where it was originally. I think they did the experiment with either a 1/4 watt or a 1/2 watt carbon comp, and at maximum, they put 1,000 volts across it - all that happened was that the resistance was way off. You have to get up into thousands of volts before you start having serious issues with the dielectric being compromised and you need to exceed 600 volts for there to be potentials for arching through the air.

That's not entirely accurate - carbon comps are much more forgiving of exceeding the voltage because of their construction. The "modern" film type resistors (carbon film and metal film) will flash over with much less provocation, because of their tight-gap spiral cut construction. I've seen it happen, and they can indeed burst into flames
(that's why they make flameproof metal oxide resistors).

Randall Aiken