shielding with plate voltage

[maxkracht]

What’s the deal with shielding with plate voltage? I know it works, and why it works, but seems unnecessary and somewhat dangerous compared to using ground. (Especially as input shielding.) I think I have only seen this done in Marshalls, and usually poorly implemented, making me assume superstition over reasoning. Is it a macho thing, or is there some reason to do that instead of using ground?

[TUBEDUDE]

It also provides some out of phase high frequency signal to fight oscillation. Not my preferred method. Sketchy method championed by Gerald Weber or Torres back in the day.

[R.G.]

Huh????
Never saw that mentioned before. Is this a thing? Really tying the shield on a signal line to plate voltage? B+ supply to the stage or the actual plate signal?
Enquiring minds want to know.

[WhopperPlate]

Huh????
Never saw that mentioned before. Is this a thing? Really tying the shield on a signal line to plate voltage? B+ supply to the stage or the actual plate signal?
Enquiring minds want to know.

I figured you would have heard of this before

[WhopperPlate]

It also provides some out of phase high frequency signal to fight oscillation. Not my preferred method. Sketchy method championed by Gerald Weber or Torres back in the day.

Changes the feeling in an apparent way . Immediate dissatisfaction in my attempts to utilize .

[Roe]

known as hot shield, not safe really but may sound good

[maxkracht]

I don't think I have ever noticed any audible difference, but it's usually a pretty short run from input to the board or from the board to the tube socket. I've never seen both. I routinely undo this mod and shield, with ground, to the tube socket, then attach grid stoppers. Easy enough, and no oscillation or radio.

This amp has 2" of hot shielded wire going to the board mounted grid stoppers, but 4" of unshielded wire between the high and low inputs, then another 4" of unshielded wire from the PCB to the tube socket. There's also a rectifier with mismatched diodes and someone replaced the bias supply caps at different times. I think one is from the '80's and one from the 2000's...

IMG_0874.jpg

[Stevem]

Mesa is still at it!
Due there packing of 45 lbs of 3 channel high gain amp into a 20 lb box they are still using the jump some high level signal to somewhere else crappy thing to tame oscillation as in this photo of a Mk V .

Notice the red wire placed over the preamp tube socket and glued down.

[pdf64]

As the input stage anode is fed via 100k, the voltage / current at the input if the screened cable shorted is rather limited.

[TUBEDUDE]

A short of ~150V can still be enough to ruin the day.

[TUBEDUDE]

Mesa is still at it!
Due there packing of 45 lbs of 3 channel high gain amp into a 20 lb box they are still using the jump some high level signal to somewhere else crappy thing to tame oscillation as in this photo of a Mk V .

Notice the red wire placed over the preamp tube socket and glued down.

Not to mention my favorites, the board mounted tube sockets and ribbon cable.

[R.G.]

Huh????
Never saw that mentioned before. Is this a thing? Really tying the shield on a signal line to plate voltage? B+ supply to the stage or the actual plate signal?
Enquiring minds want to know.

I figured you would have heard of this before

Apparently I neglected this part of my education.

So is this whole thing as described on the sticker on the photo? Input jack leads with shields connected to B+ somewhere? Where? At the input tube plates? Power supply plate supply??

If it's to reduce RF interference and only involves the inputs, why not just use a CRC RF filter at the jack?

Given the imperfect nature of the plate supply bypassing/dropping string, I can see this feeding back to the inputs a random-phase signal depending on where the "shield" is connected. That could lead to a bunch of stuff, including introducing peaks and valleys in the overall response of the amp.

Wow. Never heard of it.

[maxkracht]

So is this whole thing as described on the sticker on the photo? Input jack leads with shields connected to B+ somewhere? Where? At the input tube plates? Power supply plate supply??

On this Marshall, shield is connected to each input's plate, but I have seen it connected to the other end of a plate resistor as well. I think the more common method is to shield after the grid stopper, so you at least have a bit of resistance between high voltage and your guitar cord if the insulation fails.

I don't claim to have a golden ear, but I have never heard any sonic improvement with this method. I have also never received a complaint after unmodding, shielding the normal way, and dealing with any oscillations in normal ways. No idea how many I have changed, but probably half a dozen in the past year at least.

[sluckey]

It's more about local negative feedback than shielding. The shield is connected to the plate and acts just like a small cap between plate and grid.

[maxkracht]

It's more about local negative feedback than shielding. The shield is connected to the plate and acts just like a small cap between plate and grid.

The telephone game version of different implementations I have seen was leading me to believe this was more about shielding. No useful feedback loop when connected to the wrong side of the plate resistor... I was familiar with the diy cap, wrapping a wire around another wire, thing. Cheap shielded cable with high voltage is too sketchy for me. I think I will stick to lead dress/ high voltage caps instead. This specific amp already has grid to cathode caps on the tube socket anyway.