Screen Resistor Wattage

[Mars Amp Repair]

Hey folks,
This is probably a rather rudimentary design question, but can someone explain why the mfg's use such high wattage screen resistors, when the actual wattage calculated is waaaayyyyy less? Like in some cases only 1 - 2.5watts and their using 5 to 10watters? Just a thought...Glen

[Justin Thomas]

I always thought it was to prevent them from blowing or getting cooked from heat... Or, it's marketing - it "looks good" & not "cheap" to see big beefy components in your new $4000 booteek amp.

Iremember years ago there was a long discussion here about using the closer-to-calculated wattage so they could act as a "ghetto fuse," & I'm kind of that idea in vintage amps. Otherwise I'll just fuse the B+ if I'm building new. Though I guess I'd still rather kill a resistor than something else more expensive.

Sorry I don't have anything more to offer here...

Jusrin

[Helmholtz]

Actual screen resistor dissipation depends on resistor value and tube type (acc.to P=I²R).
E.g. a 1.5k resistor with an EL34 dissipates around 17 times the power of a 470R resistor with a 6L6 in a 55W PP amp at full output.
I think a 1.5k EL34 screen resistor should not be rated lower than 3W.
But taking into account an ambient temperature of > 60°C in a closed amp chassis (especially when mounted to the power tube socket), it would be wise to use a 5W type.

[pdf64]

Hey folks,
This is probably a rather rudimentary design question, but can someone explain why the mfg's use such high wattage screen resistors, when the actual wattage calculated is waaaayyyyy less? Like in some cases only 1 - 2.5watts and their using 5 to 10watters? Just a thought...Glen

Try rechecking actual wattage when the amp is being heavily overdriven.

[g1]

As to Helmholtz comment about EL34 having much higher screen current than 6L6, I'm generally used to seeing 5W screen resistors for EL34 type amps, and 1 or 2W for 6l6 amps.

Try rechecking actual wattage when the amp is being heavily overdriven.

Here's some numbers I took recently dealing with the same question: https://music-electronics-forum.com/...256#post998256

[Helmholtz]

.. can someone explain why the mfg's use such high wattage screen resistors, when the actual wattage calculated is waaaayyyyy less? Like in some cases only 1 - 2.5watts and their using 5 to 10watters?

Maybe you only considered DC power?

Also resistors must be de-rated above some ambient temperature (often 70°C with carbon film but sometimes only 40° with wirewounds).
Another limit is the max. surface temperature, e.g. 125°C.
Best check the datasheet.

[Mars Amp Repair]

All interesting information. I guess to be safe in most cases 3-5watts non-carbon (for fire hazard reasons) acceptable. Thanx all...Glen

[g1]

All interesting information. I guess to be safe in most cases 3-5watts non-carbon (for fire hazard reasons) acceptable.

Depends how you look at it. There is the old conundrum: when a tube fails and a screen R that Fender spec'd 1 or 2W does not blow because there is a 3 or 5W there, are there possible repercussions?

[Justin Thomas]

Personally, if I have to sacrifice a resistor or even a tube in hopes of saving a transformer, I'll do it. But I also try to make a point of not running my amps borderline.

Jusrin

[Chuck H]

I don't have the repair experience that many here can brag about. My own experience is that the most common power tube failure is the screens. I agree with Justin that if there's any chance a blown screen resistor, in the event of that mode of power tube failure, can save more expensive components like a transformer then I'm all for it. The idea of using marginally rated screen resistors has been both poo pooed and endorsed here on the forum. I vote for a design that is likely to take out the screen grid resistor and shut down the tube. Many decades of that design philosophy have proven the merit IMO. In fact I've never seen a case where a marginally rated screen resistor needed to be replaced as the only repair. There's always a bad power tube involved. I think the margin between what takes out that resistor if it's rating is spec'd such as to blow with a screen short, and what it typically takes to blow an output transformer offers plenty of margin. So yeah, I like screen resistors spec'd just a little over their expected duty in circuit. It just seems to work out for the best most of the time.

[The Dude]

I think it's unlikely overcurrent through an oversized screen resistor would take out an OT since screen current doesn't flow through the OT. That said, it might indicate some other type of tube failure that would. If an amp is properly fused, excessive tube current should cause the mains or HT fuse to blow, but we've all seen 20A car fuses in amps.

[Justin Thomas]

I think it's unlikely overcurrent through an oversized screen resistor would take out an OT since screen current doesn't flow through the OT.

What if I have one of those fancy hated late-70s/early-80s "UL" Fenders? A 75 to be precise... I guess I could just intentionally try to blow it up in the name of "sCiEnCe," but I think getting a replacement OT will cost me as much as the amp!

Jusrin (Local Resident Doofus)

[pdf64]

Whether pentode or UL / distributed load, if the screen grid overheats, sags and shorts (eg to the beam plate = cathode), it should still blow its 1W 470R resistor.

[Helmholtz]

Whether pentode or UL / distributed load, if the screen grid overheats, sags and shorts (eg to the beam plate = cathode), it should still blow its 1W 470R resistor.

Screen resistors are not common with UL wiring.

[Chuck H]

Screen resistors are not common with UL wiring.

But not uncommon either. In fact the UL "guitar amp" we're most familiar with here, the 135W Twin Reverb, does have 1W 470R individual screen grid resistors.