Truth or Myth?

[JammyDodger]

To All,


Simply....

An amp with a rectifier tube doesn't need a standby switch. Truth or Myth?

Cheers, Mike

[M Fowler]

When using the 5AR4/GZ34 that is said, but I use a standby switch most of the time.

[mlp-mx6]

According to many, the standby switch is not needed for ANY guitar amp. The voltages are not high enough to drive that need. (see http://www.freewebs.com/valvewizard/)

[Bierschinken]

Hello,

exactly what mlp said, stand-by switches are only needed if the power supplies voltage would exceed the cold cathode voltage when not loaded.

Otherwise there is no need for a standby but the risk of having cathode poisoning on the power tubes if they are heated but no current is flowing thru the tube.

Cheers,
Swen

[JD0x0]

a standby switch is almost never NEEDED. But it is very nice to have and can save your tubes. Having a tube rectifier means the rectifier(s) will "warm up" slowly easing the voltages nice and slow for about 10 seconds until the amp is ready to play. Essentially this "warm up" time is similar to what a standby switch does, but theres no real way to control it with tube rectifier. Also i generally like my amps to warm up for a minute or two and also i like to let them cool down for a minte when i turn the amp off, so i prefer stadby switches

[jaysg]

An alternative way of looking at it, is that a Standby switch is very useful for working on an amp, or repairing one. The attached dwg shows a couple notions of how to protect the diodes and the switch.

[Bierschinken]

Jay, the cap across the switch does not only protect switch/diodes, primary it protects the speakers from gettin the full power of the switch-pop.

But it is very nice to have and can save your tubes.

Sorry, but thats a myth.
There is no problem for a tube seeing U+ with a cold cathode as long as that voltage is below the cold cathode voltage that can be found in datasheets.

If you´re building a powerfull amp, lets say 1kW you often go for a relative high U+.
The problem with that is, if you turn the amp on, the cathodes are cold meaning that there is no current flowing thru the tube, which means that the power supply sees virtually no load.
Due to that the voltag raises up to Uac*√2 which then sometimes is too high, it would exceed the datasheets "cold cathode voltage".

You can handle that by installing a stand-by switch, so that the tubes don´t see anode voltage until their cathodes are hot enough to provide a load.

But you won´t face that problem in standard guitar-amps.

So again, there is no technical benefit from using a standby-switch.

Cheers,
Swen

[John C]

1)

...a Standby switch is very useful for working on an amp, or repairing one...

Yes!



2)

...Sorry, but thats a myth. ...So again, there is no technical benefit from using a standby-switch.

You're assuming a low potential on the grid. A stand-by switch can be beneficial - or even necessary - on a tube that is direct-coupled, for example.

[VacuumVoodoo]

A stand-by switch can be beneficial - or even necessary - on a tube that is direct-coupled, for example.

There are other ways, and cheaper than a switch, to prevent grid-cathode arcing in a tube direct coupled to preceding stage's anode. All it takes is a resistor and a diode.
Take a look at old tube based Tektronix or HP oscilloscopes, no stand-by switches on them and no arcing in DC coupled stages.

Stdby switches are not mandatory when you employ tube rectification but they give the user a sense of security, sort of like fitting your pants with both belt and suspenders. And it allows me to have both red and green inidicator lamps.

[JD0x0]

And it allows me to have both red and green inidicator lamps.

and we all know that gives better tone

[VacuumVoodoo]

And it allows me to have both red and green indicator lamps.

and we all know that gives better tone

I had no idea this secret was out

[John C]

There are other ways, and cheaper than a switch, to prevent grid-cathode arcing in a tube direct coupled...

So you're saying you need a diode or you need a switch...

[VacuumVoodoo]

Merlin Blencowe (Valvewizard)has a good explanation of grid-cathode arc prevention on his website:
http://www.freewebs.com/valvewizard1/dccf.htm

[Structo]

Is that only good on a cathode follower or can one use that on DC coupled stages?

[VacuumVoodoo]

Directly coupled cathode follower is a gain stage with gain less then 1 i.e. it is a special case of directly coupled gain stage.
You can use this resistor-diode scheme wherever needed. Does good job with split load PI too.