Screen grid resistors - do they have to be 5W

[s2]

Fender used 1W screen resistors on all their 6L6GC amps. I imagine those things get warm...

[CathodeBiased]

IHMO, 5W screen resistors are extreme overkill. An EL34, at full bore, may pull 25ma through a 1K Ohm resistor, which translates to .025 * 1K = 25V drop = 25 * .025 = .635 Watts of power, in the form of heat, that needs to be dissipated (beam tubes draw about half of that amount). Hence, even a 2W power resistor is derated by a factor of three in this application. If it makes one feel better to use 5W resistors, then by all means do it. However, IMHO, having to support the extra weight of the resistor outweighs any heat dissipation advantage.

[Tdale]

How exactely do you see the weight as a problem?

Tommy

[mhuss]

...= .0635 Watts of power

I think you're off by a factor of 10 there ( = .635), but as you point out, 2w is still almost a factor of three margin.

I'm also curious why weight is a factor.

--mark h

[VacuumVoodoo]

When you consider derating due to selfheating and temperature of surrounding air inside the amp chassie you'll find that 0.65W power dissipation puts a 2W resistor on the low side. 5W resistors derate to ca 3 at 60 degC ambient. Go 5W for the safety margin.

['67_Plexi]

6L6GC's don't pull same level of screen current as EL34s hence the lower power rating.

[Darkbluemurder]

6L6GC's don't pull same level of screen current as EL34s hence the lower power rating.

And of course don't forget how the tubes were actually used. Marshalls (EL 34) are mostly played cranked. Fenders (6L6) usually aren't. As a result EL 34s tend to get more heat from the application. IIRC the early Marshalls didn't have screen resistors and must have eaten a lot of tubes when cranked - hence 1k 5W.
When I replace screen resistors I always use 5W. They are flameproof which the carbon comps aren't.

[CathodeBiased]

...= .0635 Watts of power

I think you're off by a factor of 10 there ( = .635), but as you point out, 2w is still almost a factor of three margin.

Yes, the leading zero was a typo (some would call it a brain fart ).

I'm also curious why weight is a factor.

Component weight is a problem in guitar amps because of vibration. Over time, vibration can cause stress cracks to occur in solder joints. The heavier an unsupported part is the more stress it places on its solder joints, and heat just compounds the problem.

With the above said, to put things into proper perspective, no resistor takes a beating like a cathode bias resistor. Tweed Deluxes were/are run deep into saturation long before Jim Marshall and Ken Bran built their first prototype, and these amps used 5W cathode resistors to bias the output stage. Pulling 80ma, at idle, through a 250 Ohm resistor creates 1.6 Watts of heat. That figure derates a 5W resistor by approximately a factor of three (without respect for ambient temperature), and I have never seen or heard of a Tweed Deluxe failing because of a cathode resistor failure

[CathodeBiased]

When you consider derating due to selfheating and temperature of surrounding air inside the amp chassie you'll find that 0.65W power dissipation puts a 2W resistor on the low side. 5W resistors derate to ca 3 at 60 degC ambient. Go 5W for the safety margin.

Xicon MO Series power resistors are not derated for heat until the ambient temperature exceeds 70C/140F (see http://www.mouser.com/catalog/specsheets/XC-600043.pdf). At 140C/284F (an ambient temperature that is darn unhealthy for the entire amp), a 2W MO Series resistor is still capable of safely dissipating 1.2W of heat. That figure still puts the resistor close to being application-derated by a factor two when the ambient temperature is so high that component failure within the amp is just a matter of time.