Gareth Magennis wrote:

I have a fairly vintage valve amplifier with a dead HT (standby) switch.


** Is the failure mechanical or are the contacts burned up ?


It's this kind of format, SPST rocker:
http://uk.farnell.com/arcolectric/c1...ack/dp/2068581

Now, this switches the rectified HT winding from the transformer,


** Where is the switch connected - in the AC feed ?


which is
around the usual 500v with no valves in place.

I've been looking at replacements, but have not found any such switch that
has a rating of 500v DC.


** And you won't.


Many of these seem to be only rated,or at least tested, at 24v DC (?)
Nothing I have found has been anywhere near 500v.


And, here is a Carling switch typically found in Fenders, Mesa etc, to
switch the typical 500v HT.
http://www.hotroxuk.com/carling-spst...-sw110-63.html
It has a maximum DC voltage rating of 250v, way higher than anything else I
have managed to find, but definitely not 500v rated.


I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

Unless I am missing something here.


** You have missed the fact that the breaking capacity of a switch depends on the current flowing at the time. That rocker switch will break 24V DC and 250V AC all day long as long as the current does not exceed 16 amps.

In the Standby position of a valve amp, it maybe has to break 0.4 amps at most - so the voltage rating goes way up and it can be expected to do the job fine.

Plus any 250VAC mains switch is insulated to way more than the nominal voltage to allow for spikes etc - so that is OK too.

The best idea is to have the switch in the transformer wiring, breaking the AC voltage rather than the DC after the rectifier. Marshall did this in most models, using a double pole rocker.

But Fender got it *horribly* wrong in a great many models - like the Bassman 100 - connecting the switch in the one spot it should never be.

http://www.thevintagesound.com/ffg/s..._100_schem.gif

Imagine the switch is operated when the amp is on and running at high volume - maybe because of a sudden acoustic feedback.

Momentarily, as the switch opens, OT is fed HT from the 20uF electro but now in SERIES with the filter choke as the main electros are disconnected. Audio frequency current flows in the choke and generates a very large AC voltage pulse - making the switch contacts arc and the same voltage also appears on the output valve plates.

I have measured this voltage at more than 3kV rms.

A similar thing happens when the HT fuse in a70s or 80s Marshall blows, since it is wired in the same place.

Explains a lot of arc burnt valves and octal sockets.


..... Phil