Screen grid muting

[Mike Sulzer]

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."

Exactly! I prefer no standby sw myself, but IMO, the question falls way short of religion or politics in terms of controversy.

(But I would be reluctant to change dc operating points if you can just interrupt the signal path.)

[Enzo]

And like always, context matters. Need one or not, WHY are you putting a standby switch in the amp? You want it to stop making sound? OK, I have always found screen killers killed the sound perfectly well, but then I never tried to force some huge signal through it to see if I got a tiny little output. Usually in standby, we are not playing at all, let alone really large signals. And if we were strumming, would it actually matter if some small sound escaped? Chances are the sound from the strings themselves in the air would be louder than anything escaping the speaker.

If this is some sort of engineering philosophy exercise to see if we can hit absolute silence, OK, but then we are leaving the land of the practical.

[Tom Phillips]

...If this is some sort of engineering philosophy exercise to see if we can hit absolute silence, OK, but then we are leaving the land of the practical.

I need to create total silence. I want to record the silence and then play it back at full volume while I sleep so it will cancel out the sound of the neighbor's barking dog. My question is do I need to make a stereo recording or will mono due? Sometimes there are two dogs.

[The Dude]

You will need two amps- one for each dog. The silence signal does not need to be stereo, but signal exiting each amp must be in phase or the silence will cancel itself out rendering the entire setup useless against the dogs.
You could also mic the dogs and play it back through the amps out of phase thereby phase cancelling the barking dog noise.

[Enzo]

Assuming the dog is not out of phase already. That sometimes happens on Lab Series dogs.

[Tom Phillips]

I don't know about the dog but when it keeps me awake I get seriously out of phase.

[Enzo]

In those cases, it is mutting rather than muting.

[eschertron]

Sometimes there are two dogs.

If there's two dogs does make it a ca-eighteen?

A grounded (or relatively low impedance to ground) screen acts an electrostatic 'screen' or shield

Which is the exact context of the question; can the screens 'float' or do they need to be grounded? How close to ground? Is a couple hundred kilOhms small enough to get screen current to trickle to earth?

If this is some sort of engineering philosophy exercise to see if we can hit absolute silence, OK, but then we are leaving the land of the practical.

In may be in part hobby experiment, but the desire is to get practical experience - and results. My thought is: If I want to use control circuitry instead of mechanical switches, for the specific amp I'm building (B+ about 300VDC), I could probably use an SSR to do the switching, even on the screens. Higher-B+ voltages would require a mosfet (or two).

Yes, why remote control? I can't answer that. Probably will go with a SPDT switch for this application, if simply severing the screen (resistors) from B+ doesn't actually mute the signal. -20dB may not be enough, -40dB would be more than close enough to 'off'.

The whole point of the question is to fill up this empty box thingy called a 'brain'

[Enzo]

Go find a PV 5150/6505 or a XXX or an Ultra, flip it into standby and see if you can coax any sound out of it. That standby works as well as any, certainly a lot more than 20db. 20db is a mic pad on a mixer, substantial but hardly a mute.

[g1]

And that 6505 Enzo mentioned does not ground the screens, just floats them in standby. There is just the 220K bleeder across that node's filter cap (along with whatever parallel resistance from the rest of the circuit). So your "couple hundred kilOhms" seems fine.

[Enzo]

Those 220k across each cap are there to equalize voltage across the series-ed caps.

[g1]

Those 220k across each cap are there to equalize voltage across the series-ed caps.

I don't see any totem pole series caps in the 6505?

[Enzo]

Page 2 C35, C39 the screen node caps.

http://bmamps.com/Schematics/Peavey/...Schematics.pdf

6505 and 5150 are the same identical circuit. The 6505 combo does not stack caps, but that is not a 6505.

But regardless, either way there is a 220k or 440k to ground from the screen node if power is removed.

[g1]

Ah, it was the 6505 combo I was looking at.
As you say, there is at least 220K floating the node when in standby so there doesn't seem to be a need for grounding the node to kill the sound.

[eschertron]

So allowing the power tube plates to remain energized and putting every downstream node on standby sounds like an attempt to prevent cathode poisoning. But if the screens are at ground doesn't that negate the theory? :semi-off topic:
Is there something more to peavey's design? Maybe not interrupting the biggest current draw with the switch contacts?