shileld of input coax to b+

[funkmeblue]

I read a couple of times that Ken didn't ground the shield at the jack to ground but instead attached it to b+ at the tube socket. This is mentioned in the francesca files that this is only done on the liverpool. Did not find any mention of this in the liverpool files. Will this make a noticeable difference? Extra capacitance? I grounded it.....just wondering if it's worth messing with.....thanks again

[dynaman]

I do it in certain builds. I've never heard a difference except that it tamed oscillation a couple of times. Others may disagree.

[JamesHealey]

If for any reason the insulation failed you'd get a nice surprise! Wouldn't do it myself

[VacuumVoodoo]

10-15pF ceramic cap directly on the socket, between grid and plate pin is a much more sensible solution that offers computable, predictable and consistently repeatable results. In contrast to connecting input wire shield to the plate it's safe.

[guitarmike2107]

If for any reason the insulation failed you'd get a nice surprise! Wouldn't do it myself

Agreed,

If you do try it make sure your coax cable can handle the voltages, most can't!

[Firestorm]

IIRC, when Dave Funk was building with Ken's blessing, this is one area where they disagreed, so Ken must have done it or talked about it often enough for Dave to object.

[keewee]

yes, but I want more than a 10pf (shorted) between me and B+. Safety first!! IMO

[geetarpicker]

I've NEVER even heard of this one!

[WhopperPlate]

Did it on v2 of a troublesome super lead, I felt it took out a lot of life and made the amp feel way different. my 2 cents

[rp]

I tired it after reading about it but I couldn't sleep at night, so I undid it.

[dynaman]

If for any reason the insulation failed you'd get a nice surprise! Wouldn't do it myself

Agreed,

If you do try it make sure your coax cable can handle the voltages, most can't!

Not sure if you're referring to the outer insulation or actual shield?

Regardless, when performed with suitable cable that's been properly dressed, I've found active shielding to be a safe and effective trick to cure the occasional problem that simply didn't respond to passive shielding. Admittedly, I only use this method very sparingly. In my very limited experience, I've learned that proper lead dress usually doesn't require active shielding.

Rather, I'm much more fearful of the other completely exposed runs of cap leads, turrets, wire, etc. that will most definitely cause harm if mishandled.

However, I think rp did the right thing by reversing his actions. There's absolutely no fun in agonizing over the fear of getting hurt or starting a fire!!

[VacuumVoodoo]

There's nothing active in electrical sense in connecting a cable shield to supply voltage (B+), it's just a DC offset.
Shield attached to plate is in effect equivalent to connecting a small capacitor between grid and plate and may have a stabilizing effect on a nervous amp.
As I wrote earlier it is much better and safer to just use a small capacitor, you may even want to use a small 5-25pF variable trimmer capacitor and tune in the "almost unstable" condition.
Now, if you can live with losing 4db gain in the first gain stage....remove its cathode decoupling capacitor, use a plastic input jack NOT grounded to the chassie (have ground at PSU) and connect one end of input cable shield to the jack and the other to 1st gain stage cathode.....the treble clarity will surprise you. This is an old method of neutralizing influence of shield-signal wire capacitance in sensitive measurement systems. It allows use of long shielded cables without losing bandwidth.
Is it safe? yes, shield will be only about 1.5V DC above ground.

[funkmeblue]

would this capacitance change with the voltage drop due to sag in the preamp or would it remain constant?

[VacuumVoodoo]

Which capacitance are you referring to? There's more than one under discussion.

[funkmeblue]

oh, sorry man, I meant connecting the shield to the plate. If the preamp sagged, the plate voltage would change on the shield. Would this then in return effect the capacitance of the shielded cable? Just wondering if this is why ken chose this route over the capacitor. thanks