Screen grid muting

[eschertron]

Sorry if this has been covered elsewhere; I've searched some keywords here and also tried a general internet search, to no good response. I vaguely remember discussion about muting power tubes by lifting cathodes as well as severing the screen connection, but cannot find details.

I'll have some time in the next few weeks to set up a 6V6 on a test stand and see for myself if what I'm thinking is valid. But why not get some advice while on the way?

I want to see if simply severing the connection between the screen node and the screens (or screen resistors) will allow the screens to interrupt the flow from cathode to plate. My intuition says if the screens are not physically connected to ground there will be some residual signal current, since the screens in a high-Z state might simply become electrically transparent. However, if my intended final target is a PP output section, the one screen - draining charge to the screen in the paired tube - might behave differently than in the SE lab experiment.

Grounding the screens aught to work better. If there's a nominal voltage under which the screens shut the tube 'off', I can calculate what size resistor I can place from the screen resistors to ground to draw the screens down sufficiently, assuming there's a minimum current that flows from the cathode to screens regardless of cathode-screen potential. Comments and conceptual corrections are welcome!

Thanks for reading

[Enzo]

Just taking voltage off the screens should be sufficient. Grounding them unnecessary. Just about every Peavey tube amp ever made used screen kill as the standby. Plates remain powered. Well, every PV tube amp that HAD a standby.

The screens don;t just turn transparent, they no longer have the ability to accelerate electrons toward the plate.

If the screen is severed, it no longer will attract electrons to itself either. Zero volts cathode to zero volts screen - no attraction. Or cathode bias, +30v cathode to zero volts screen, no attraction. Any electrons that accumulate on the floating screen will make it more negative, not more attractive.

I will not telly you it is impossible to cram some tiny amount of signal through the tube, but I can say I have never heard anyone say their screen kill standby was leaking signal.

[Leo_Gnardo]

If I'm not mistook Ampeg SVT Classic & similar accomplish their standby by disconnecting the screen grid supply.

[pdf64]

The screen grid is a grid, and just like the control grid, the idle voltage on it is a key part of the tube's operating conditions; lower it enough and the tube will go into cut off.
A power tube with a disconnected / floating screen may, with a sufficiently big signal at the control grid, allow a tiny bit of very distorted signal through, likely just a fraction of a watt.
Similar thing with a floating (eg 47k resistor to 0V) cathode.
If the screen grids are pulled down to 0V, then the muting should be total, so a dual throw switch could be used to achieve that, eg screen grids to switch common, one side to the screen grid HT node, the other to 0V.

[eschertron]

I've looked at the peavey schems, interrupting the HV rail after the plate node. There's certainly some load that drags the rail down towards ground.
And I'm probably over-thinking this whole thing. A single or double-throw switch in the right place is going to be simpler and cheaper than anything else I can come up with.

I hadn't considered using a spdt switch to select HV or ground. Worth a try! Thanks

[Tom Phillips]

If I'm not mistook Ampeg SVT Classic & similar accomplish their standby by disconnecting the screen grid supply.

Actually the standby switch was in the primary of the HV transformer. When in standby mode that complete transformer and all the B+ in the amp was turned off. The heaters stayed on because they were supplied from a separate transformer.
I don't recall any other popular amp that operated like that.

[Leo_Gnardo]

Actually the standby switch was in the primary of the HV transformer. When in standby mode that complete transformer and all the B+ in the amp was turned off. The heaters stayed on because they were supplied from a separate transformer.
I don't recall any other popular amp that operated like that.

That was the case for "old fashioned" SVT - the original type made from late 60's to early 90's.

Again, if I'm not mistook, for modern SVT-CL a.k.a. 'Classic' late 90's on - what SGM would have called CratePeg - there's a single PT and standby switch makes/breaks the connection between screen grids & their supply. Dam' if I can find a schematic to support this though, all my searches come up with the original 60's item, drat.

[Mike Sulzer]

What is wrong with "control grid muting"? That is, disconnect the input signals to the two power tubes.

[nsubulysses]

It seems that the power tube would still be on and conducting then, and there would be a noise floor?

[Leo_Gnardo]

What is wrong with "control grid muting"? That is, disconnect the input signals to the two power tubes.

Sure, that's been done too, mostly in some 50's and early 60's amps where the "standby" switch simply grounds the signal to the power amp.

Then there's the old "why do you need a standby switch anyway" discussion, we've been round & round with that one in times past. Boils down to "whatever the customer wants, or expects."

[frus]

If the screen is severed, it no longer will attract electrons to itself either. Zero volts cathode to zero volts screen - no attraction.

How do triodes work then?

I mean, I know what you're saying is true, I've seen amps that are dead silent with B+ on plates and disconnected screen, but I would expect at least some attraction from the plate itself

[mhuss]

A grounded (or relatively low impedance to ground) screen acts an electrostatic 'screen' or shield, in a similar way that a braided shield around coax does. In this case it basically 'screens' the positive potential on the anode so that the electrons coming from the cathode can't 'feel' it and aren't attracted to it.

[Gregg]

In more recent amps screen grid muting is used in Marshall JVM410 fro example. Nothing new or fancy. The plate voltage is still on and it allows you to use the preamp only for PC recording, PA or whatever you want.
With no power tubes the B+ will go up but not much. If you want to be very accurate you can add a resistor in the preamp B+ line and switch it together with the screen to match preamp's voltage with and without power tubes.

[frus]

in a similar way that a braided shield around coax does

I guess so, although the screen doesn't look like a very effective shield to me, it's just a coarsely wound wire
It's probably supposed to be aligned with the grid, maybe that's why it's more effective?

[Mike Sulzer]

It seems that the power tube would still be on and conducting then, and there would be a noise floor?

True, but in a carefully designed and built amp you have to stick your head in the speaker to tell if it is on with the master volume down.