Oh snap! my bad!

[Randall]

Wrestled a bear today with a Marshall 6100LM Anniversary 100 watt that came in for blowing fuses. (guy owns 6 of them). I found an arcing ceramic socket and a HT transformer connection that was never soldered. To pull this board you have to unsolder all of the sockets as well as fight with the very agressive glue they glopped on all the connectors, I actually damaged one of the ribbon connector trying to get it off. So, I finally get the board out, futz with the socket, fighting with this heavy and very awkward chassis, (both transformers are on the same end). I eventually get it back together to my satisfaction, install the tubes to see how they are doing, switch it on and SNAP with a little puff of smoke, and then no sound. Damn it. I put one of the tubes in wrong, and shit. The keyways on these Russian 6n3C-E are maybe just small enought that you can do that, and I didn't check, which I normally do. Sigh.

So now I find the 470R 5W screen resistor on the socket is open. OK, fair enough. But it also looks like the 100R 4W is open as well, because I have HT on one side, but not the other. Now my question is, how is it they both are bad? If the screen resistor opens, then the short is gone and the screen supply resistor should stop being stressed. And if the screen supply resistor opened, the 470Rs would see no voltage, which is currently the case.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[The Dude]

Hard to speculate without knowing how the tube was oriented in the socket. That aside, maybe they just both hit "maximum stress" at the same time. Often times component failures make perfect sense and can be predicted knowing what else failed in the circuit. Other times it's just "because". Why does a certain .01uF bypass capacitor short in a device that has hundreds of them on the same rail? Why does a barely conducting preamp transistor fail in an amp where the output transistors are just fine? It doesn't always make sense.

[Randall]

Does it make sense to have absolutely no output or current flow with the screens dead? I would have thought it would act more as a triode in this case. It does in fact make some sound in triode mode, but there is a hum of some sort there as well. I have plate voltage, bias voltage, and filament voltage, but no current flow.

Boy, I can't wait to pull this thing apart again!

[The Dude]

It does make sense. I can't recollect brands/models off the top of my head, but there were some older amps that removed screen supply as a method of standby. If you banged the strings real hard, you could still here a bit of something, but for the most part the amp was silent.

[Enzo]

About every tube amp Peavey ever made turns off the screens for standby.

yes, remove screen voltage and the tube stops conducting. Disconnect it and you do not have a triode. Triode happens when you tie the screen to the plate. The job of the screen is to accelerate electrons towards the plate. if it has no positive voltage on it, it can;t do that, so no electrons go to the plate. No curent.

[Leo_Gnardo]

While we're on it, life's a lot easier for the standby switch when using the screen grid standby method. In amps with rectifier tubes, I expect they're a good deal less stressed as well.

[Jazz P Bass]

"Does it make sense to have absolutely no output or current flow with the screens dead?"

If you stop & think about it, the Screen grid acts more like the Anode than the Plate does.

[Helmholtz]

It is called Screen for good reason. The Screen actually screens/shields the plate, meaning that the electrons mainly feel the Screen potential/field and not the plate's potential. The screen may also be viewed as another grid controlling/limiting plate current