Just ordered some new 12AX7 valves for my Marshall amp.

[Simon Barden]

Aha! I'm afraid that the standby switch myth has recently been debunked and you don't need it at all. In fact, it can actually do harm if you leave it on too long as you can get cathode poisoning from just the heaters being on. There are now several YouTube videos about it and it's been mentioned in most of the guitar magazines. My friend Dave McCready (who was an amp designer at Blackstar until recently) confirmed it.

All the stuff about cathode stripping in valves only applies to larger valves using a 1kV+ power supply. Voltages in valve amps only get to about half that.

The first standby switches occurred in some Fender amps because as models increased in output power over time, their standard store of capacitors weren't rated for the higher voltages present on initial switch-on before the tubes started conducting current and the voltages at the capacitors dropped as a result. If they allowed the tubes to pre-heat, then they conducted straight away and so the capacitors never saw the higher no-load voltages.

Marshall basically copied a Bassman circuit and stuck with the standby switch. No '50s or '60s Vox or Gibson amp had a standby switch. They only appeared on modern re-issues where the standby switch had become a marketing tool.

You'll find that a lot of amps are now dropping the standby switch. It was only ever designed to protect the capacitors, not the valves - and only because of Leo Fender's penny-pinching designs where one extra switch cost a lot less than buying uprated capacitors! In a properly specified valve amp, they are simply not required.

[DrNomis_44]

Aha! I'm afraid that the standby switch myth has recently been debunked and you don't need it at all. In fact, it can actually do harm if you leave it on too long as you can get cathode poisoning from just the heaters being on. There are now several YouTube videos about it and it's been mentioned in most of the guitar magazines. My friend Dave McCready (who was an amp designer at Blackstar until recently) confirmed it.

All the stuff about cathode stripping in valves only applies to larger valves using a 1kV+ power supply. Voltages in valve amps only get to about half that.

The first standby switches occurred in some Fender amps because as models increased in output power over time, their standard store of capacitors weren't rated for the higher voltages present on initial switch-on before the tubes started conducting current and the voltages at the capacitors dropped as a result. If they allowed the tubes to pre-heat, then they conducted straight away and so the capacitors never saw the higher no-load voltages.

Marshall basically copied a Bassman circuit and stuck with the standby switch. No '50s or '60s Vox or Gibson amp had a standby switch. They only appeared on modern re-issues where the standby switch had become a marketing tool.

You'll find that a lot of amps are now dropping the standby switch. It was only ever designed to protect the capacitors, not the valves - and only because of Leo Fender's penny-pinching designs where one extra switch cost a lot less than buying uprated capacitors! In a properly specified valve amp, they are simply not required.

But still, I reckon it's a good habit me to get into when powering-up my Marshall amp anyway, while trying out the new preamp valves, I noticed that the Marshall amp is pretty quiet on it's own so the problem is not in the amp, but when I plug any of my guitars into it at uni I get a noticeable hum coming through, apparently Room 105 ,where I have my Marshall amp, is a pretty noisy room electrically because of the fluorescent lights, I'm seriously thinking of putting some copper shielding-tape in my Gibson LP Studio guitar, since it seems to be the most hummy out of all my guitars, which is a bit odd cause the pickups are both Humbuckers, I might check to see if the bridge hardware is properly electrically-grounded to the guitar's circuit-ground, I know that it is definitely not an instrument lead issue cause both the leads I was using at the time were a couple of known good ones that I had made myself, so I'm guessing that my LP Studio is picking up some RFI from the fluorescent lights in room 105, hopefully the copper shielding-tape will sort it out.