Shielded cable ground?

[punkykatt]

Hey Guys. this Marshall Im working on (JMP Master Model 50W MK2 Lead) someone put shielded cable going to V2 pin 7 control grid and connected the shield to the plate pin 6.  I have never seen this before.  They probably did this when they cascaded the input  jacks like the JCM 800.  Is this acceptable practice? If so what does it do? I always thought the shielding went to ground on one end not high voltage.  Thanks in advance   Punky

[sluckey]

What does the other end of the cable connect to?

[kagliostro]

Sometime it is used, but I don't consider it as safe


Franco

[punkykatt]

the other end of the  cable connects to a 68k grid resistor a 1M grid to ground  and the high input jack.

[kagliostro]

the other end of the  cable connects to ................

Of the cable, I think Sluckey is asking about the shield

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I consider the above schematic the same of this, but I prefer this second option



Franco

[Fresh_Start]

Sometime it is used, but I don't consider it as safe


Franco

I agree. The plate is at AC ground, but no future repair person would expect high DC voltage on that shield.

In this case, I think the shield should be grounded at the inout jack, not the tube's control grid. But I get mixed up sometimes about which end is supposed to be grounded.

Another "bad" practice is using the shield as a ground conductor for a pot. IOW both ends of the shield are connected to save running a separate ground wire. I confess that I've done that a couple of times in amps I planned on keeping. A future repair tech won't know that the Reverb pot is grounded via the shield unless he really pays attention.

Chip

[punkykatt]

Ok , makes sense now. Instead of using a cap they hooked it up that way.  The 68k is mounted on the pcb  with shielded wire from there to the tube. Upstream of the 68k is non shielded wire to input  jack. That wiring setup is coming out.  Going to put the 68k on the tube pin and run shielded cable  from the 68k to the input jack and ground the shield at the input jack.   Thanks for the input Guys. 

[HotBluePlates]

... someone put shielded cable going to V2 pin 7 control grid and connected the shield to the plate pin 6.  ...  Is this acceptable practice? ...

Kagliostro showed what it does (introduce a little treble roll-off by adding capacitance from plate-to-grid; i.e., feedback).

Acceptable practice? Well, I doubt it would have been done much at all except that Gerald Weber wrote about this connection in one of his books. IIRC, the idea was to fight instability (unwanted feedback) by introducing a treble roll-off through intentional feedback (the cable capacitance of the shield-to-conductor). But even he noted this puts high voltage on the shield and could be dangerous.

I wouldn't do it, because there are safer ways to do the same thing (essentially what Kagliostro's drawings show). I also wouldn't call it a "widely acceptable practice" since it put high voltage where many techs wouldn't expect to see it.

[punkykatt]

Thank you for the explanation HotBluePlate.