Can someone explain this schematic symbol?

[SILVERGUN]

What does this symbol represent, and how would it be wired in this circuit? ....thanks in advance for your replies

In welding machines that would be a hall device or current sensor....in guitar amps, I just don't know

[kagliostro]

Usually that symbol means shielded (the imput wire is shielded)

but is very strange that the shield is connected to the plate of the tube  

Can you post the entire schematic ?

K

[SILVERGUN]

Thanks for the quick response K......that's kinda why it had me scratchin' my head....maybe it's a mistake?

[kagliostro]

I don't know if the shield was deliberately connected to the plate
(my knowledge does not go so far)

what I can note is that there isn't the usual input resistor (33k --- 68k) and neither the alternative ferrite bead to avoid RF interference to be captured

K

[darryl]

A short length of shielded lead from input socket to grid has a small capacitance when the shield is connected to earth. However, if the shield is connected to the plate as it appears to be here, then this small capacitance value is multiplied by the gain of the valve.

Perhaps this is being used on the input stage as a low pass filter.

An similar idea is discussed here: http://www.guitargear.net.au/discussion/index.php/topic,32882.0.html

[kagliostro]

Hi Darryl

so if that is a low pass filter that is a trick to reject RF noise

interesting

K

[Willabe]

It's a hot shield. Be VERY CAREFULL with it if your going to use it in an amp.

I bet you would have figured it out if the circle symbol had gone to ground as it does 99.9% of the time.

G. Weber wrote of this type of wiring connection in 1 of his books for getting rid of parasitic oscillations. He said he used it only as a last resort.

IIRC, it's in a chapter called tricks of the ninja.


                Brad     

 

[SILVERGUN]

Thanks Brad,...I just ran across it while doing some research, and I'm one of those people who just HAS to know,,,,but I'll be the first to admit, I'm no ninja

[Willabe]

Now K and darryl might be right too. I don't know.

           
                  Brad     

[stratele52]

I won't use this amp with a shield at few hundred volt potential .  DANGER

[SILVERGUN]

Thanks stratele, I was just comparing a bunch of schematics and this stood out to me as extremely odd.....thanks for the help

[kagliostro]

Google Sir #34 and Sir #36



K

[Willabe]

I won't use this amp with a shield at few hundred volt potential .  DANGER

I agree.

G. Weber was using it where needed, which could be another pre tube and not the input pre that would put the full dc plate voltage on the guitars input cables shield if it were to short to the input cables shield.

2'nd or 3'rd stage pre/tone recovery would be safe.


              Brad      

[kagliostro]

And also

if I've to use such a mod I'll prefer to use RF shielded cable who is rated for high voltage, than common shielded cable for audio purpose

http://www.belden.com/pdfs/Cable101/coaxrefc.pdf

EDIT: a friend of an italian forum (who is a careful reader also of this forum) indicated to me this interesting tread

http://www.el34world.com/Forum/index.php?topic=2467.msg100402#msg100402

and also this alternative way to achieve the same result



(Many Thanks Robi)

K

[Willabe]

if I've to use such a mod I'll prefer to use RF shielded cable who is rated for high voltage, than common shielded cable for audio purpose

Yes.

And the small blocking cap seems like a good solution.


              Brad       

[SILVERGUN]

Thanks a lot guys, there's nothing like getting a good explanation when you ask for one...you guys are the best

[jeff]

I wouldn't do it.
If this is done as a "quick fix", I'd fix the problem, but I wouldn't do this.
If it's an absolute must at the very least put a cap between the input wire and jack to block DC if there were a short.
But I wouldn't do it.
If there were a short this would put B+ on the tip of your guitar cord. So picture this, it shorts, now your amps not making any sound, you unplug your guitar to see if it's the guitar or the amp, touch your thumb to the tip of the cord to see if you get a "buzzzt, buzzzt". You'll get a BUZZZT alright.
Like I said I wouldn't do it.
It may work forever with no problems but if it were to go wrong it'd go really wrong. Same reason I wouldn't use a 3 to 2 plug adaptor. Yeah it'll work but what happens when something does go wrong?
I wouldn't do it.

So I guess what I'm trying to say is I wouldn't do it.

I wouldn't want to play an amp that had this done either.

[SILVERGUN]

Thanks Jeff....don't worry, I wouldn't put this in an amp either.....I'm just in a heavy learning stage and came across this while comparing schematics, and I just couldn't wrap my head around what I was looking at (just assumed it was a mistake).....thanks to all of you guys, I got my answer same day, and got on with my buisness

[Tone Junkie]

The idea is you dont connect it at both ends that way its not a hot shield if you only connect at the tube no voltage can make it out of the amp. It gets rid of oscillations but doesnt create bigger problems.
 I pursonally like using the small cap that kagliostro showed.
Thanks Bill

[jojokeo]

I've seen Three-Stooges Amplification use this wiring "trick" in all their amps w/out any problems!?

[SILVERGUN]

Thanks jojo, I knew I could count on you to lighten this up

[jojokeo]

Moe comes in, see's what Curly's wiring and says, "Hey numbskull! What are you doing that for?" Curly says, "Moe, I can't see I tell ya!" Moe, "Why not?" Curly, "I got my eyes closed nyuk nyuk"

[Willabe]

The idea is you dont connect it at both ends that way its not a hot shield if you only connect at the tube no voltage can make it out of the amp.

No.

Even with the shield only conected at the plate, it still is a hot shield because the shield has a high dcv on it.


             Brad     
 

[HotBluePlates]

The idea is you dont connect it at both ends that way its not a hot shield if you only connect at the tube no voltage can make it out of the amp.

No.

Even with the shield only conected at the plate, it still is a hot shield because the shield has a high dcv on it.

When the military hands you a handgun, they tell you to never pull it out of the holster unless you intend to kill someone with it. The gun has a safety, and you might think you won't pull the trigger. But, if you pull out a weapon capable of deadly force, you shouldn't do it as a show of force, because there's a reasonable expectation that the safety could fail and you could get caught up in the moment and pull the trigger. So, if you're that military member and pull your gun, you might be facing a court martial and a murder conviction.

With anything you do in an amp, you have to ask yourself what are the possible failure modes. If you use a 47pF cap, you'll be using a silver mica or ceramic cap with a 500-1000v rating (because those are the most commonly available parts in this value), which is gonna be higher that the insulation rating of most wire you're likely to use in a guitar amp (600v rated wire has *very* thick insulation).

It's also easier to make an installation mistake which compromises the voltage rating of shielded wire than it is with a cap. It might work forever, or it might fail. When it fails, you might create big problems for yourself (or a customer).

The value of the hot shield is in curing an oscillation by reducing the high frequency response of that following tube stage. You're using the capacitance of the wire to act as a cap between the grid and plate, which then reduces the gain at some high frequency. You could look at it as negative feedback from the plate to grid (180 degree apart) at that high frequency.

But you can do the exact same thing, if it's absolutely necessary, in a safer way with the cap. Or you could reduce high frequency gain in other ways. So the hot shield just doesn't seem to be better than any other method, and adds a reasonable expectation of added risk.

Keep the hot shield in its holster. 

[SILVERGUN]

Thanks for taking the time HBP........nice hyperbole
You don't have to worry about me putting this in an amp,,,,,I've seen what electricity can do,,,, when a 600 volt 100 amp saftey switch failed and blew off the wall, burnig/puncturing three holes (one for each phase) in the wall it was mounted to....I've seen the light , and it aint pretty

I was just doing some homework and ran across what I thought had to be a mistake......guys will try anything!?!?!