Why does Marshall use 1K 5W Screen resistors

[Jerry2013]

And Fender just use 470K 1W screen resistors like on the Blackface Bassman?

[tictac]

The screen grid counld be likened to a govenor in a gas motor and the screen gris resistor sets the max idle...

if you put 1 Meg ohm screen grid resistors in your amp it will product no power....no sound (I know, I accedentally did this once!)

The smaller the sreen grid resistor the more current will flow thru the power tube....

So with an amp like a Fender that's made to play clean there's less stress on the power tubes so less protection is needed. A 470 ohm screen resistor sounds good an provides some protection, not much but they do occasionally burn out like a fuse.

With a Marshall amp; players like to crank them up so a 1K 5W screen resistor offers a bit more protection against tube failure. Kevin OConner of London Power Amps reccomends 2K screen grid resistors for EL84 amps...

I'm sure someone with more smarts than me will chime in if I've errored in my explaination somehow... but that's how I see it currently.....

TT

[rock_mumbles]

There's a difference in screen dissipation between beam tetrodes (6L6) and pentodes (EL34) ... the screen in a pentode has a higher dissipation so it needs a larger resistor to keep the dissipation in check ...

[Roe]

And Fender just use 470K 1W screen resistors like on the Blackface Bassman?

actually quite a few Marshall 50w amps used 470R (not 470K) like fender did. but most techs recommend 1k+ in a marshall

[Structo]

Yep the actual value is 470R or 470 ohms.

Big difference between that and 470K.

The other difference is that Fender's predominately use 6L6GC power tubes while Marshall's use EL34 power tubes.

Different tubes and different specs.

The reason that Marshall uses 1K screen resistors is because the screen grid on EL34's are a bit more delicate and need the voltage to be dropped more.

[RCGPNY1]

Actually, I dont believe old Marshalls had those screen resistors(?) As I understand they were not needed in the days of Mullard EL34's

[PaisleyTube]

EL34s draw more G2-current so 1k is better
but....
lots of 50 Watts Marshalls didn't have any G2-stoppers at all.... subtily slightly more dynamic behaviour but harder on the tubes

[Mark]

The EL-34 has a screen grid dissipation of 8 watts (according to svetlana data sheet), the 6L6 has a screen grid dissipation of 6 watts. All things being equal you'd expect the Marshall to have the smaller value resistors.

The metal face Marshall 100 watt amps do run voltages up to 100 volts higher than Fenders (depending on models of course). As P=E*I, higher voltage means higher watts.

I'm inclined to think that these values became an accepted standard rather than the correct value required by the circuit. I may be mistaken but I seem to remember when Marshall used 5881's for a short while these amps used a 2.2K screen grid resistor.

I suspect most people stick with these values in fear of changing the sound of the amp.

[Roe]

Actually, I dont believe old Marshalls had those screen resistors(?) As I understand they were not needed in the days of Mullard EL34's

the 50w amps didn't but all 100w amps came with 1k screen grid resistors. even mullard el34s will have problems with 470R screen resistors at higher voltages. I've tried it

[talbany]

This has been an on going debate as to why the 34's need better screen regulation when they can handle more power (8w)Than a 6L.. The earlier 34's (Mullard,Amperex) valves were built to higher standards so you might could get away without using screen resistors and why the really old Marshall's didn't use them in the 50w (although this can be debated)..Those days are long gone and the 34's today have IMO much wimpier screen supply..You may want to consider the fact that you might be running the poor EL34's way over their max dissipation limits with 480V on the plates and a 3.4K load (for two tubes in a 50W, which is the same as 1.7K for four tubes in a 100W)..The other problem you run into in a typical high-voltage Marshall is that you are not only running at or above max ratings on that load line, you also have to contend with the duty-cycle modulation caused by the long-tail pair phase inverter design and the relative matching of the phase inverter tube sections under full clip. This causes the output square wave to not be equal on top and bottom - one side is wider and the other is narrower, so the wider side dissipates more power...This is the side that generally blows the screens and takes out the screen resistor or worse..
Put these 2 together!...A wimpy screen supply and an unbalanced phase inverter at full clip and your now prone to screen blowouts..

Tony

[Mark]

you also have to contend with the duty-cycle modulation caused by the long-tail pair phase inverter design and the relative matching of the phase inverter tube sections under full clip. This causes the output square wave to not be equal on top and bottom - one side is wider and the other is narrower, so the wider side dissipates more power

How does this happen Tony, the PI is running in class A. I usually associate this sort of thing with a classAB output stage than the PI.

[talbany]

you also have to contend with the duty-cycle modulation caused by the long-tail pair phase inverter design and the relative matching of the phase inverter tube sections under full clip. This causes the output square wave to not be equal on top and bottom - one side is wider and the other is narrower, so the wider side dissipates more power

How does this happen Tony, the PI is running in class A. I usually associate this sort of thing with a classAB output stage than the PI.

Mark
I am not sure what you mean by the PI is running class A..
Perhaps you can elaborate.. The issue I was referring to has more to do with clipping symmetry..This is due in part by the low value (10k) tail resistor used in the Marshall/Bassman design that clips more asymmetrically (harder)..

if you did design a more symmetrical PI, (up the tail 22k or 47k) it would reduce the tendency to red plate on only one side...Also something like a split-load PI driving either two equal stages.or a Williamson, or two cathode followers, would all lead to more symmetrical duty cycles.

On the flip side by running a more symmetrical PI you would loose part of that Marshall tone that does not exist in a "more" symmetrical duty cycle square wave..Like say a BF Fender or a Vox or a SSS w/ CF followers running closer to AB2..
I hope this makes some sense now..

All The Best!

Tony