mick wrote:
On Wed, 30 Dec 2009 23:59:58 +0000, Ian Bell wrote:

Nice ASCII art John!
snip
relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND
Not quite, swap the LOAD and SUPPLY and it is correct - in other words
the HT supply goes to the common and is switched either to the load or
the discharge resistor because nearly all the charge storage is in the
HT supply not the load.


Ah. Just as I thought.
As David has pointed out though, moving the rectifier(s) and reservoir
caps etc. onto the amp chassis removes most of the problem. It has
several advantages:
You don't need a relay.
Only AC goes through the plug & socket.
The rectifier protects the socket against a back feed from the HT.
The plug & socket are "dead" as soon as the mains is switched off.
All you need to do is to make sure that the plug & socket can't be
disconnected while the mains is on, or rig up something so that a tool is
needed.


It also has some serious drawbacks.

1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.

2. AC inside a box where I am trying to keep it out by having an
external PSU

Incidentally, I used a flying lead from the PSU and a socket on the amp.
Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an electrical
interlock so that the relay couldn't pull in until the plug & socket had
made.


Funnily enough, those are precisely what I was planning to use and I
even bought a few pairs. Trouble is they do not lock and can very easily
be pulled out.

I am now planning on using an 8 way Speakon locking connector - a tad
more expensive but you won't be able to pull it out.

Cheers

Ian