Should you 'upgrade' screen resistors?

[Buddha's Guitar Tech]

Fender '65 Twin reissue, blown screen resistor, blown internal fuse.

replaced both - but all i had was a 2W 470R instead of the spec'd 1W.

The older i get, the more i doubt EVERYTHING i do ....

The old valve works alright in the amp, but i'm hemming and hawing over suggesting new valves - i don't like to spend unnecessary £$£$£.

Socket is clean, no signs of conductive residue. So i'm wondering....
- if there was a screen-filament short
- if 'upping' the screen's power handling is wise.

Any thoughts?

[jelle]

I would replace all four screen resistors with 2W ones. 1W is on the light side.

Jelle

[Buddha's Guitar Tech]

That's what my instinct told me.

But another part of me says it's better if a 1-watt burns up as opposed to letting the screen pull too much.

[Andy Le Blanc]

its a good opportunity to set the screen grid resistor value at your bias point
you can measure the difference between the plate and screen at the socket pins

[Buddha's Guitar Tech]

its a good opportunity to set the screen grid resistor value at your bias point
you can measure the difference between the plate and screen at the socket pins

Do you mean change the value of the resistor, so at idle, the screens are a particular voltage below the plates?

In this amp i've noticed at idle, there's little difference between the two voltages, but i was under the impression that that all changes once you begin to play - screens pull more, sends their voltage down, etc.

[pdf64]

The 1 watt resistor only blows if there's a tube problem - it's telling you something.
If you want a tube short to carry on conducting until the line fuse blows, then >=5 watt resistor should be used.
A 2 watt resistor might get super hot and damaged but still hang in there, walking wounded, maybe with an altered value, waiting to fail.
Unless more fusing is added, I prefer to stick with 1 watt in this application.
A 5 watt resistor allows the power transformer to carry on supplying fault currents for much longer, putting it at risk from the likely, foreseeable hazard of modern crummy output tube shorts.
Suitable fusing in the B+ is highly beneficial if you go that route.
Pete.

[husky]

The 1 watt resistor only blows if there's a tube problem - it's telling you something.
If you want a tube short to carry on conducting until the line fuse blows, then >=5 watt resistor should be used.
A 2 watt resistor might get super hot and damaged but still hang in there, walking wounded, maybe with an altered value, waiting to fail.
Unless more fusing is added, I prefer to stick with 1 watt in this application.
A 5 watt resistor allows the power transformer to carry on supplying fault currents for much longer, putting it at risk from the likely, foreseeable hazard of modern crummy output tube shorts.
Suitable fusing in the B+ is highly beneficial if you go that route.
Pete.

I agree with this big time especially when there is no HT fuse.
Stick with a good 1W it acts like a fuse (not the best design use of course)

[Buddha's Guitar Tech]

i like what you're saying.

[greekie]

A resistor is not a fuse, it is a resistor. It does the job it is told to do, just like a fuse is doing its job when it pops from carrying too much current.

Install a fuse instead.

[Buddha's Guitar Tech]

It has been said that in an SVT, the 10R plate resistors on the 6550s are kinda like fuses. Go figure.

[husky]

A resistor is not a fuse, it is a resistor. It does the job it is told to do, just like a fuse is doing its job when it pops from carrying too much current.

Install a fuse instead.

Well yes of course that is preferred but isn't original.
The point is don't make them bigger unless you add a fuse and even still I see no reason to up the wattage.
I have also seen my fair share of Marshall screen resistors failing with Power tube failure even with an HT fuse which may have blown as well or not.
Another one where fuses might not be great is in the heater circuits you will see on Canadian JCM800's. From memory they fuse the heaters on both legs, the added resistance especially on a Slo Blo winds up with too hot to touch. The fuse holders eventually unsolder themselves from the PCB and sometimes the fuse themselves come apart.

[pdf64]

Re Marshalls with a HT fuse, it's there to protect the power transformer from power tube failure, not prevent the power tubes failing.

Re if a fuse is needed then install a fuse, in this instance the resistor acts both as a fuse and a current limiter. Very crudely and far from perfect in both aspects - better fuses and current limiters are available - but nonetheless it fulfils those functions adequately.
Just because a paperclip comes out of a box that says 'paperclips' on it shouldn't blind you to its potential to fulfil other functions.
Including a fuse (joke!).
Installing a specific fuse for the screen grids would be beneficial in some aspects but detrimental in others.
The design has stood the test of time. A change that may initially be perceive as an improvement (eg uprating the power spec of the resistor that's blown) may leave system sub-optimal.