EL84 screen R´s - 1k or 100R?

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…I've decided to go ahead and replace my 1W 100R screen resistors with 5W 1K0 flameproofs…

So what’s the mitigation when a screen grid shorts? It might be counterproductive if a transformer winding becomes the weakest link in the chain.

[Roe]

In an ac30, I have replaced the choke with a resistor, using a CLC filter and a ss rectifier (32+32uf on the mains and 33uf on the screens). Anyway, I experiemented with the shared screen resistor that replaces the 20h choke.

With 100R individual screen resistors, values lower than ca 430R for the shared resitor (replacing the choke) sounded bad to me. I prefer either 470R or 560R. For higher voltage amps you need at least 560R, perhaps higher.

[brewdude]

I presume the choke and resistor that replaced it are part of the dropping string… I was under the impression that a “shared” screen grid resistor would be from the screen’s filter cap direct to the screen grids, leaving the dropping string with a choke (or dropping resist) in place.

[Roe]

I presume the choke and resistor that replaced it are part of the dropping string… I was under the impression that a “shared” screen grid resistor would be from the screen’s filter cap direct to the screen grids, leaving the dropping string with a choke (or dropping resist) in place.

No, in this case the shared resistor replaces the ac30 choke.

[rp]

I haven't been on this forum in yonks, hung up my soldering iron and closed the book on this stuff to avoid a costly habit in a low income country, but I have been dabbling again. So, hello all. As I had posted on this post it showed up in my email and this is a good place to step back in. I recalled this old info about G2 resistor topologies. I had it saved, good stuff, I think Ian aka GingerTube posts on TAG too, or used to, so with respect I'm pasting here his orig post from MEF. BTW you will see a shared G2 limiting resistor along with stoppers in IIRC the Mullard books and seen on the earliest JTM45. It was an economical way to linearize an amp. And probably why Marshall got rid of it and Hiwatt added it. So, how you set up your G2 resistors does matter. To the OP, I'd measure the resistance of your choke, if it's above 500R and your EL84 are below 370V, I'd leave the 100Rs and call it day. Tons of Voxen out there doing fine, including hard running Matchlesses, though Voxs themselves have been known to catch fire Oh, and those early Fender tweeds w/o G2 resistors? If you check they were mostly the ones without chokes, they had 5k-10k dropping resistors.

https://music-electronics-forum.com/sho ... hp?t=49190

The Screen Grid (g2) exhibits gain when a signal is applied to it with respect to the cathode - just like the Control Grid (g1).

The 1st function of the screen resistor is therefore as a grid stop to prevent parasitic oscillation. An absolute minimum value of around 47 Ohms should always be used.
This is why when triode strapping a pentode you will see a small value resistor between screen and anode rather than just a direct connection (usually 100 Ohms to 1KOhms).

The second function leads from the fact that tube current is FAR more dependent on screen to cathode voltage than on anode to cathode voltage.

The HiFi guys will tell you (correctly) that for minimum distortion you should regulate the screen grid supply, well except for those who like Ultralinear connection where feedback voltage is applied to the screen grid.

As anode current peaks then so does screen current.

You can see why this might be so. As anode current goes up the anode voltage dips as more voltage is dropped across the load. The screen grid becomes more positive with respect to cathode than the anode so more of teh electron stream arrives at the screen rather than the anode which is the same as saying screen grid current increases.

A series screen resistor drops screen to cathode voltage on screen current peaks and so imparts some compression. As this is limiting peak screen current it acts to protect the screen from melt down (over dissipating).
KOC also claims that this compression adds "Touch Responsivity" to the amp.

In a push pull pair then individual screen resitors will compress both sides of the waveform.

You will see one other "trick".
In push pull as signal goes positive, one side delivers more current and the other side delivers less, and then these swap for the negative signal swing. The screen grid currents follow this. The TOTAL screen grid current, however, is fairly constant. So some early HiFi designers used a common (470 Ohms for EL34) screen grid resistor which held the screen grids voltage at near REGULATED supply performance.
Dave Reeve copied this in the HIWATT. You don't get any compression from this common screen grid resistor.

I've copied it in a Trainwreck clone with cathode biased EL34 output and I liked it - but at B+ of +380V for 32 Watts out I did'nt need screen protection for the EL34s.

Only ever use as high as 2K2 individual screen grid resistors on one amp. Push pull 6GW8/ECL86 but used individual 1K on many INCLUDING Ultralinear connected EL34 quad output. In this last case (with Plitron super dooper output trannies) the amp oscillated with 150 Ohms screen grid resistors and I had to go to 1K to ensure no parasitic oscillation. Counter intuitively, the better the output transformer (In Ultralinear anyway) the more you needed the grid stop function of screen resistors.

Every Fender which comes across my bench has the 470 Ohm 2W carbon screen resistors replaced with 1K 5W wirewound.

Because at least part of the screen grid resistor's function is as a grid stop then it is important to fit these resistors with the resistor body close as possible to the screen grid pin on the tube socket. Just as you do for Grid 1 gridstops.

So my "pot shots at the barn door". Hope there is something useful in there.

Cheers, Ian

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As anode current peaks then so does screen current.
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I think it's worth noting that the above behaviour isn't an inherent valve characteristic, eg it won't be shown in the valve info, rather it's a side effect of how things pan out in common cathode pentode operation, as per the description that Ian provides.
I don't think it would occur in eg the common grid arrangement of some Musicman designs.

[brewdude]

I presume the choke and resistor that replaced it are part of the dropping string… I was under the impression that a “shared” screen grid resistor would be from the screen’s filter cap direct to the screen grids, leaving the dropping string with a choke (or dropping resist) in place.

No, in this case the shared resistor replaces the ac30 choke.

I have not seen a choke as a screen grid resistor before… always learn something new following these forums.

[Roe]

I presume the choke and resistor that replaced it are part of the dropping string… I was under the impression that a “shared” screen grid resistor would be from the screen’s filter cap direct to the screen grids, leaving the dropping string with a choke (or dropping resist) in place.

No, in this case the shared resistor replaces the ac30 choke.

I have not seen a choke as a screen grid resistor before… always learn something new following these forums.

Sorry, but the choke doesn't replace a screen resistor. I use the standard 100R individual screen resistors and a 470R shared screen resistor