Gareth Magennis wrote:



** 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.


Thanks, Phil, that makes a little more sense now.
This switch is connected to the main filter caps, as in the Bassman
schematic, though I don't think it has a choke, it's a pretty sparse 50W
EL34 design (Roost, made in UK).
One Mullard EL34 is toast, the other is OK.


** The bad EL34 may be responsible for the burnt switch.

The owner may have tried to use the standby switch when the valve was passing a large DC current - like a couple of amps. At that level, a continuous arc would form and quickly destroy the contacts.

I see there is no DC choke in the schem but the fusing would allow that sort of current to flow.

http://www.andy-duke.co.uk/roost/Non...2006_05_14.png

Maybe when you replace the switch, rewire it in series with the HT fuse - that should allow arcs to extinguish at switch off.


..... Phil