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Power / Standby switches Why

If your guitar amp has both a Power switch and a Standby switch, then it uses vacuum tubes (a.k.a. valves).  As you may know, the amp takes some time to warm up before producing sound.

Proper use of the Power and Standby switches could help you prolong the life and sound quality of your tubes.  Just follow two simple guidelines.




The Guidelines



  1. Before switching the Power to On, make sure the amp is set to Standby mode.  Let the amp warm up for several minutes before you switch the amp out of Standby mode.

  2. To quiet the amp during a break without letting it cool down, switch to Standby mode.  But don't leave it in Standby mode for more than 15 to 20 minutes.




The Warm-Up



To understand the reason for the guidelines, recall how a vacuum tube works.

Every tube has at least two elements that conduct electrons.  The cathode emits electrons and the anode (a.k.a. plate) collects them.

The tube's cathode is heated to such a high temperature that electrons in its coating, often an alkaline earth-metal oxide, gain enough energy to break away from the coating.



This thermionic emission produces a red glow.  Eventually, a cloud of electrons (negative charge) builds up in the space around the cathode.  At that point, the tube is standing by, ready to go to work.


When you exit Standby mode, you're applying a relatively positive voltage (sometimes called B+) to the tube plates.  The positive anode attracts the negative electrons that surround the cathode.

The resulting current powers your amp's circuitry.  Electrons continuously circle back to replace those that escaped the cathode's coating.




Cathode Stripping
The Reason for Guideline 1



If B+ is applied to a tube's plate before its cathode is fully heated, electrons will be ripped from the cathode's oxide coating, instead of being drawn from the surrounding electron cloud.  The resulting damage to the coating is called "cathode stripping".

The Standby switch was invented to withhold B+ from the plates until the tubes are warmed up.  You can reduce cathode stripping by letting the tubes reach their operating temperature before you put them to work.




Cathode Poisoning
The Reason for Guideline 2



Conversely, if a cathode is red hot for long periods without a B+  to collect the electrons, a destructive process called "cathode poisoning" occurs.  The old-timers called this "sleeping sickness".

Without current, a layer of high resistance builds up between the cathode's metal base and its oxide coating.  This resistance layer robs gain from the tube and increases its noise.  To fight cathode poisoning, limit the length of time the amp is in Standby mode.


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