Where to ground shielded cable (5C5 content)

4 Strings · September 24th, 2008, 01:44 PM

In an attempt to get rid of some the hum in my Junkmaster (basically a 5C5 though built using nine pin pre amp tubes, an un-canned junker PT and a Magic Parts Vibrolux Reverb replacement OT) rewiring the ground, now using a ground buss, helped some. However, there is still a little too much hum for comfort.
The next step was to swith to shielded cable on all of the pre amp tube grids. I have just about finished this, but one little intricate problem - which I should of course have thought of before firing up the soldering iron - still remains. On the "upper" side of the fiber board I have grounded the shields to the ground buss. But where on earth (sorry...) do I ground the shields on the wires that connect to the pre amp sockets? There are, as far as I can see, no obvious good points to ground the shields on "the tube socket side" of the chassis.
Is it okay to ground them on the cathodes of each triode? How about the cathodes on v3, which are grounded via a 25k/25uF RC?

Any help would be appreciated!

A.

sjhusting · September 24th, 2008, 01:57 PM

Hi

No problem - you should only ground one side of the shielded cable, otherwise you risk ground loops. So the side connected to the ground buss is good. On he other side, trim back the exposed shield, and slip some heat shrink over it so that stray shield wires won't accidentally touch anything else.

Where do you have your heaters grounded? Using a CT or 100-ohm resistors? I've found that in a cathode biased amp, referencing the heaters to the high side of one of the power tube cathodes works well.

I also suggest you look at the Hoffman grounding scheme, and think hard about it, until you understand why he does it that way. In my experience, his grounding scheme works near perfect every time. Every time I try to 'improve' it, it gets worse. Once I finally understood the why, I stopped trying to improve it and my results were much better.

steven

4 Strings · September 24th, 2008, 02:27 PM

Thanks Steven!
All the shielded wire on the "upper" side of the board is grounded to the ground rail, with the opposite end of each wire insulated with heat shrink. That is, the wires from the input jacks and the volume pot(s) soldered to a 0.1uF cap each.
But there is also a bunch of wires going from the other end of those .1uF caps (and 5Meg resistors) to the tube sockets, and these are the ones whose shields I don't really know where to ground.
With the usual itch to test the results of a good days DIY:ing I am almost about to go the solder-the-shields-to-the-cathode-lugs-of-the-tube-sockets route, but this time for once I thought I'd be a good boy and wait for the get go from the rest of you pros...

As for the heaters they are grounded with the usual 100-ohm resistors. The next step could well be to move them to one of the 6L6 cathodes as you mention!

Thanks!

A.

sjhusting · September 24th, 2008, 05:11 PM

As a quick rule, ground the 'upstream' side of shielde wires, from the point of view of the signal.

steven

4 Strings · September 25th, 2008, 03:52 AM

Thank you Steven!

A.

September 24th, 2008, 01:44 PM	#1 (permalink)
4 Strings Tele-Meister Join Date: Jun 2007 Location: Gossip County Posts: 276	Where to ground shielded cable (5C5 content) In an attempt to get rid of some the hum in my Junkmaster (basically a 5C5 though built using nine pin pre amp tubes, an un-canned junker PT and a Magic Parts Vibrolux Reverb replacement OT) rewiring the ground, now using a ground buss, helped some. However, there is still a little too much hum for comfort. The next step was to swith to shielded cable on all of the pre amp tube grids. I have just about finished this, but one little intricate problem - which I should of course have thought of before firing up the soldering iron - still remains. On the "upper" side of the fiber board I have grounded the shields to the ground buss. But where on earth (sorry...) do I ground the shields on the wires that connect to the pre amp sockets? There are, as far as I can see, no obvious good points to ground the shields on "the tube socket side" of the chassis. Is it okay to ground them on the cathodes of each triode? How about the cathodes on v3, which are grounded via a 25k/25uF RC? Any help would be appreciated! A. __________________ We shall not cease from modifying our amps. And the end of all our modifying will be to arrive where we started and know the circuit for the first time.

September 24th, 2008, 01:57 PM	#2 (permalink)
sjhusting Tele-Holic Join Date: Jul 2003 Location: Germany Posts: 659 15 Photos	Hi No problem - you should only ground one side of the shielded cable, otherwise you risk ground loops. So the side connected to the ground buss is good. On he other side, trim back the exposed shield, and slip some heat shrink over it so that stray shield wires won't accidentally touch anything else. Where do you have your heaters grounded? Using a CT or 100-ohm resistors? I've found that in a cathode biased amp, referencing the heaters to the high side of one of the power tube cathodes works well. I also suggest you look at the Hoffman grounding scheme, and think hard about it, until you understand why he does it that way. In my experience, his grounding scheme works near perfect every time. Every time I try to 'improve' it, it gets worse. Once I finally understood the why, I stopped trying to improve it and my results were much better. steven __________________ "When you have to kill a man it costs nothing to be polite" - WSC

September 24th, 2008, 02:27 PM	#3 (permalink)
4 Strings Tele-Meister Join Date: Jun 2007 Location: Gossip County Posts: 276	Thanks Steven! All the shielded wire on the "upper" side of the board is grounded to the ground rail, with the opposite end of each wire insulated with heat shrink. That is, the wires from the input jacks and the volume pot(s) soldered to a 0.1uF cap each. But there is also a bunch of wires going from the other end of those .1uF caps (and 5Meg resistors) to the tube sockets, and these are the ones whose shields I don't really know where to ground. With the usual itch to test the results of a good days DIY:ing I am almost about to go the solder-the-shields-to-the-cathode-lugs-of-the-tube-sockets route, but this time for once I thought I'd be a good boy and wait for the get go from the rest of you pros... As for the heaters they are grounded with the usual 100-ohm resistors. The next step could well be to move them to one of the 6L6 cathodes as you mention! Thanks! A. __________________ We shall not cease from modifying our amps. And the end of all our modifying will be to arrive where we started and know the circuit for the first time.

September 24th, 2008, 05:11 PM	#4 (permalink)
sjhusting Tele-Holic Join Date: Jul 2003 Location: Germany Posts: 659 15 Photos	As a quick rule, ground the 'upstream' side of shielde wires, from the point of view of the signal. steven __________________ "When you have to kill a man it costs nothing to be polite" - WSC

September 25th, 2008, 03:52 AM	#5 (permalink)
4 Strings Tele-Meister Join Date: Jun 2007 Location: Gossip County Posts: 276	Thank you Steven! A. __________________ We shall not cease from modifying our amps. And the end of all our modifying will be to arrive where we started and know the circuit for the first time.