Would you replace them?

[BluegrassDan]

Pretty much have completed restoration of a client's original 1959 Bassman. All NOS tubes from Brent Jessee, in-line voltage attenuator, recapped, biased, bypassed the selenium rectifier with a new diode, all four original speakers reconed by Gordon Kjellberg.

Last decision. Screen resistors.

The 470 ohm carbon comps look healthy. No signs of overheating. Both measure within 5%.

Whaddya say? Keep em or swap em out for wireounds?

[sluckey]

I would keep them. If they ever need replacing I would use 470Ω 3 watt metal oxide.

[thetragichero]

my own personaly opinion: i would say it depends on how the client intends to use it. at home or in the studio? leave em. if this will be gigged quite a bit, i would put more modern and rugged. having had amps crap out in the past in the middle of a gig or during soundcheck, i like to be as cautious as possible (i also now keep an outlet tester with my cables to ensure i'm plugging into properly-wired outlets)

[Soulfetish]

Personally, I like replacing those when I can. If it were my amp, I would replace them.
But... at the same time, I'm on an amp tech's salary. If I were to write myself a bill for all the things I would do if it were my amp, I couldn't afford it.
Sluckey's advice is prudent. Unless the values have drifted up near 500Ω(or higher), or there's some visual indication of heat damage, we're okay with leaving them. If it comes down to a matter of preventative maintenance, as it seems to be with the amp you are working on, I go over the repairs that have been done already with the customer. I give them my recommendation, but tell them it's not critical right now and let them make the call.
There are two ways of looking at a repair like yours;
On one hand, if they've already made the investment in the recones, Brent Jesse tubes ($$), recap, etc.,.. does it make sense not to replace those?
On the other hand, this already seems like a major overhaul and expensive repair. They might be strapped, and adding non essential parts and labor may cause problems and make the customer feel taken advantage of.
So, that's why I would touch base with them see what they think.

[pdf64]

When they fail, CCs in such applications can catch fire and cause collateral damage, eg to nearby wiring, which may end being significant.
MOs with a a flame retardant coating won’t.
For me it’s a default, do it unless instructed otherwise.
I suggest to retain the 1W rating, in order that they fuse in response  to a screen grid short. I only uprate them if also fitting a HT fuse, which would be a good idea, in view of the value of its transformers.

[Soulfetish]

I understand your reasoning of why you use underrated screen resistors, pdf, but tubes and fuses are removable for a reason.
A screen going open is better than a dead short across the secondary for sure. But, a tube or failure in the power supply likely the cause of a screen resistor burning up, so Why go through the trouble of making more work for you or the customer when more than likely it’s a bad tube.
Id rather the screen resistor survive the overload and allow the fuse to do its thing. Then all you need to do is swap the fuse and tube and  your on your way.
I think its wise to fuse secondaries. But, i will say, I’ve worked on countless old Fenders and I can count on 1 hand how many of their power transformers gave up  the ghost. The single mains fuse seems to protect them from catastrophe for the most part.
That’s my 2 cents, but you have a solid case, pdf

[mresistor]

If the original screen resistors were one watt then replacing them with 1w mo resistors is not underrating. 

[Soulfetish]

Right. certainly not underrated if he’s using them as fuses

[pdf64]

…
Id rather the screen resistor survive the overload and allow the fuse to do its thing. Then all you need to do is swap the fuse and tube and  your on your way…

My understanding is that the ‘reason for being’ of the equipment’s mains fuse is to protect the building and its inhabitants from injury / damage resulting from the equipment failing, not to protect components within the equipment. If the latter occurs, great, that’s a free benefit, but it’s not why the fuse is there.

My perception is that the 1W rating of the screen grid resistors was a protective measure implemented by the amp’s designer.  Uprating the screen grid resistors seems to be a change to the design, one that may be seen to work counter to the original designer’s intention?

I’ve worked on countless old Fenders and I can count on 1 hand how many of their power transformers gave up  the ghost. The single mains fuse seems to protect them from catastrophe for the most part…

Surely that should read ‘the single mains fuse in conjunction with 1W screen grid resistors’?
My view is that original transformers are the most valuable parts of the amp; even on a copy, transformers are the most costly item. The value of the original transformers in a vintage amp surely makes their preservation a priority?
Fault current incidents will only be accommodated so many times before damage occurs. And vintage transformers may be more fragile, perhaps having already been exposed to fault current incidents, and with age potentially having a degrading effect on the winding insulation.
Hence my view is that the protective measures around the transformers should be at least maintained. If anything, changes should increase the protection, rather than remove elements of it that may be perceived to be inconvenient.
If a screen grid short unfortunately results in a transformer failing before the mains fuse blows, the owner won’t be best pleased with the redesign.

My view is that uprating the screen grid resistors is fine, so long as the protective function of the 1W rating is moved over to a suitable HT fuse.
I find that the ground switch hole makes a handy place to site a HT fuseholder 

[Soulfetish]

I’ve worked on countless old Fenders and I can count on 1 hand how many of their power transformers gave up  the ghost. The single mains fuse seems to protect them from catastrophe for the most part…

Surely that should read ‘the single mains fuse in conjunction with 1W screen grid resistors’?
My view is that original transformers are the most valuable parts of the amp; even on a copy, transformers are the most costly item. The value of the original transformers in a vintage amp surely makes their preservation a priority?

[Soulfetish]

My understanding is that the ‘reason for being’ of the equipment’s mains fuse is to protect the building and its inhabitants from injury / damage resulting from the equipment failing, not to protect components within the equipment. If the latter occurs, great, that’s a free benefit, but it’s not why the fuse is there.

It is. I was just expressing my experience. It wasn't necessarily supposed to be prescriptive.

My perception is that the 1W rating of the screen grid resistors was a protective measure implemented by the amp’s designer.  Uprating the screen grid resistors seems to be a change to the design, one that may be seen to work counter to the original designer’s intention?

I've seen 2W screen resistors used as well. Either way, I think the original designer's intention was to use the most economical part so as not to fail. Like Fender did with most of their other decisions. 

If a screen grid short unfortunately results in a transformer failing before the mains fuse blows, the owner won’t be best pleased with the redesign.

Screen shorts are only one example of failures in the secondary. My thought is that if protecting the transformer is the goal, implement something that protects against most modes of failure if possible.
Like I said, I respect why you take the approach you do. You have a good case for it.

My view is that uprating the screen grid resistors is fine, so long as the protective function of the 1W rating is moved over to a suitable HT fuse.
I find that the ground switch hole makes a handy place to site a HT fuseholder 

I agree with this. Plus, I Love idea of repurposing the placeholder of the polarity switch!

[saxetc]

Restoring a Showman considering an additional fuse to protect the amp from a screen grid short in the ground switch hole. Has anyone done this?