• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

An Umbilical Question

I have (old 211) amplifier, where inside, the power supply wires are longer than this. 🙂

Try the amp first without DC-Link. Use it for few weeks, then insert this capacitor to HV.
If the experience (or measuring) confirms this modification legitimacy, use this configuration.
My motto: what isn't broken, doesn't need to be fixed. 😊

BTW which type amplifier it is?

This is for a headphone amplifier. I want to make the PSU as a separate project so that I can maybe re-use it with multiple projects in the future to get more projects going in this hobby faster. And also I like the headphone Amp to fit on my nightstand, putting the PSU on the shelf under nightstand will save space on top. I'm incorporating a 5V supply line to power a Bluetooth board and remote control boards in the Amp box, all of which I will purchase or design for 5V. My wife uses Bluetooth phones, I use 300 ohm Sennheisers, by adding in Bluetooth the headphone Amp will be able to use two sets of headphones at once without having to have the OPT mess up its reflected impedance trying to drive two wired phone sets.
 
We are talking about a 1-1.5m long cable. It's not transmission line, nor the signal (which is -in good case- DC).
If you build it correctly (individual cold -ground- and hot wires for each supply voltage, each pair gently twisted, possible screening or shielding for each pair -if necessary-, good shielding for umbilical), the disturbances on amp socket side are almost imperceptible/immeasurable.
Do you have a vendor link for a cable like that?
 
Ah, I see. Looks like you're making your own cable: putting multiple single conductor wires (in twisted pairs) wrapped in a braided sleeve inside some kind of outer wrapper. Cool! Btw: can the braided sleeve serve as the earth ground wire?
 
"can the braided sleeve serve as the earth ground wire"
Never use it such way! It's shielding, not conductor.
Use individual ground wire (separated from each "cold" wires of power supply). Using it (connecting on both end) depends of implementation.