Why should I run screen resistors?

[leadfootdriver]

If B+2 is already dropped in the power string, why do I need another resistor at the tube?

[d95err]

The screen B+ node has a reservoir capacitor, so for short bursts of current (AC) it's the same as no resistance at all. So the screen resistor is necessary If you want to limit scrren current.

[Enzo]

What he said.^^^^

The resistors in the B+ string are involved in dropping the voltage. The tube screen resistors are there to limit screen current. Helps prevent your screens from melting.

[leadfootdriver]

What's a good value for the JJ KT77???

[loudthud]

Screen current can go sky high when an amp is overdriven or operated into an open load. Screen current can even exceed plate current when there is no load connected. Screen resistors suppress any tendancy for oscillation and act as fuses when certain tube failures occur.

[imaradiostar]

What's a good value for the JJ KT77???

In what kind of circuit at what operating voltages?

1k is a good starting point but in an abusive circuit I'd go higher. I suppose you could find reasons to go lower as well.

jamie

[EFK]

So you guys - who I realize know a lot more than me - are saying that even in some of the older style amps w/ no choke that use something like a 2K-3K resistor (or even larger, but I like to keep it around 2-3K) between the plate and screen nodes should STILL have more resistance added at the tube socket itself? I'm not talking EL34 here, mostly 6L6 and 6V6.

[Diablo]

So you guys - who I realize know a lot more than me - are saying that even in some of the older style amps w/ no choke that use something like a 2K-3K resistor (or even larger, but I like to keep it around 2-3K) between the plate and screen nodes should STILL have more resistance added at the tube socket itself? I'm not talking EL34 here, mostly 6L6 and 6V6.

I was convinced by the knowledgable folks here to add 1K-3W screen resistors to 6L6 amps that I rework or build. Part of the "be kind to your screens program".

[Wombaticus]

So you guys - who I realize know a lot more than me - are saying that even in some of the older style amps w/ no choke that use something like a 2K-3K resistor (or even larger, but I like to keep it around 2-3K) between the plate and screen nodes should STILL have more resistance added at the tube socket itself? I'm not talking EL34 here, mostly 6L6 and 6V6.

The resistance between the plate and screen nodes is irrelevant to the question at hand. The screen node has its own capacitor, which means that a large reservoir of charge is built up and available at that node... And until that reservoir is pretty much drained and needs to be recharged by sourcing current through the big resistor you mention, nearly all of that charge is available to drive current should it be called upon to do so. And caps have a very low intrinsic source impedance. This is another way of saying that it can provide a lot of current, and do so very quickly. So yes, you need screen resistors! Its really for the same reason you wouldn't grab the positive terminal of the screen node cap with your hand, and put the other to ground..it would just as happily source a large amount of current into *you*.

[pdf64]

My take is that it's best practice to fit screen grid resistors, as (already mentioned) they are beneficial in suppressing oscillation, limiting current and acting as a crude fuse (given a type and suitable power rating); the issue what value to use.
In the case of a 'no choke' B+, where there's already significant resistance in the screen supply, eg >2k, 470 should be fine and as the node resistance increases could probably be reduced to a minimal value, eg 100 ohms.
As an example, for the 5E3 RI, Fender have fitted 470 ohm screen grid resistors, which weren't in the original design.
I'm a bit sceptical about how much surge current a (regular sized) cap, fed from a saggy supply, could put into the screen grid, and how much harm it would really do; rather my feeling (based on not much!) is that it's a sustained heavy overdrive condition that's more of a killer for the screen grid, and that a saggy supply is better mitigation for it than large value screen grid resistors, in regard of detrimental impact on tone.
Pete.

[Chuck H]

So, to fan the flames, what would the tonal effects be of using a single resistor from the screen node to feed both screens? Would the anti phase AC coupling of not having individual resistors make any tonal difference? What about AC bypassing that resistor or even individual screen resistors? What would that do? (he wondered aloud)

[Steve Conner]

You guys ask all the hard questions!

The way I see it: For large signals, when one tube is completely cut off, it's the same as having one screen resistor per tube.

For small signals in the Class-A region, it's, erm, like no screen resistor at all? After all, the variations in screen current of the two tubes ought to be equal and opposite, so they should cancel at the junction and the voltage across the resistor shouldn't vary.

So, that suggests to me that a shared screen resistor ought to boost the gain for small signals relative to large ones. Hence, give more compression than two independent resistors.

Also, the odd distortion harmonics of the screen current won't cancel. They'll add and you'll get more odd-order distortion than you would if the screen voltage were perfectly constant (or if the node were bypassed by a bigass capacitor, which amounts to roughly the same)

The original EL34 datasheet specified one screen resistor shared between the push-pull pair, and that's how I built my first amp. The Selmer T'n'B 50s were like that too.

[EFK]

Interesting question Chuck. I've often wondered about something along these lines, as I've 'payed with' a lot of Selmer TnB50 amps and Selmer always used a single big 1.5K screen resistor to feed both EL34 screens. I remember someone telling me that this was an old hi-fi thing, and it somehow involved some type of phase or nfb issue.

Ha! Steve you got that in there while I was typing. Gotta love Selmers.

[pdf64]

That single shared screen grid resistor arrangement is reputed to cause big stability problems with the 100W / 4 x EL34 Selmers, at least with modern tubes, see c1968 Selmer P.A.100/Reverb Amplifier and c1970 Selmer P.A.100/4 SV Amplifier
It does seem ok with 50W / 2 x EL34 amps though; I can't think why the difference should be so.
Talking of strange screen grid supply arangements, mention should be made of the JTM45, 3 resistors, shared then split, 1k to 2 x 470 ohms.
It does seem to affect the large signal ouput waveform (NFB disengaged); I tried switching in / out a bypass cap at the 1k / 2 x 470 node and stock, without that bypass cap, the peak of the sine gets compressed, it seems to be more accurate with a bypass cap.
So that correlates with Steve's analysis.
Pete.

[Steve Conner]

Any time you connect a bunch of tubes in parallel, each one needs its own individual stopper resistors right at the socket pins. The control grid stopper is an absolute minimum even for a 1-tube output stage, but the more tubes you gang up, the worse the stability problems get. The Ampeg SVT, with its sextet of oscillation-happy 6146s, even had plate stopper resistors.

I'd say the 100W Selmer should have had four screen stopper resistors, but then they might as well just have used four individual screen resistors.

When I turned my old Selmer into the Ninja Deluxe, instead of the 1.5k shared screen resistor, I used a 1k shared screen resistor plus two individual 470 ohm ones. So, the same as the JTM45 I guess.

The 1k can be shorted out by a toggle switch if you want to run 6L6GCs. (I also rewired the bias circuit as a voltage doubler to allow for this)