Draining The Caps


Zombie Zero, DFZ
Aug 1, 1985
I do it. Can't remember why. Read that it was a good thing to do. Can't remember where.

Do you drain your caps too? Am I correct that going into standby mode (1st) is the wrong way to do it? That you hit the power button whilst rockin' the free world? Then flip it into stanbye when the amp goes quiet?

Not sure to be honest Hans, I remember reading on the unofficial Laney Forum about all of this when I got my amp. Lots of differing opinion but more than a few said they go into standby first and leave the amp for a short while.

I've done it that way for the past two years. Standby for a few secs and then full power. After playing I knock it into standby and again leave it for a bit whilst I tidy up. Then power off completely. I think this method was supposed to help prolong valve life but whether it really does...


Not heard anything about draining caps other than when working on the amp...
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Yes. When powering up the amp in the standby mode for at least 30 seconds before going to full power and when powering down, go to standby for 30 seconds before poweriing off preserves tube life.

Draining the caps? To drain the caps so you do not kill yourself...(when working on the amp). Unplug the amp. With the standby switich engaged power it up unplugged. After ten to fifteen minutes, most if not all the voltage will drain from the caps.
After ten to fifteen minutes, most if not all the voltage will drain from the caps.
That's still no guarantee. I dissipate the load with a resistor to ground just in case. Google can show specifics and videos.
100 years ago, at the dawn of tube civilization, I was taught to turn on the amp in standby mode, then strap on the guitar, tune up, etc., while the amp warmed up. Standby warms the tubes. After I'm tuned up, I power up the amp.

I reverse the process turning off....first I hit standby, then do all the put-the-guitar-away stuff, then I turn off the amp.

I don't know for sure if this practice extends the life of tubes or amp, but my tubes do last a long time (like, several years, and I only change them out of guilt!), and I really can't remember the last time an amp I owned needed a repair.

As for draining the caps, I've never heard of that except for amp repairs. What's it supposed to do for the amp in normal use?
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Funny, that is what I was taught as well Les. Use the warm up and cool down time to get situated and/or put stuff away. Draining the caps is specifically for amp repair.
Not sure to be honest Hans, I remember reading on the unofficial Laney Forum about all of this when I got my amp. Lots of differing opinion but more than a few said they go into standby first and leave the amp for a short while.

This is what I've always done, too.
The function of a standby switch is to allow a tube amp with a solid state rectifier to warm up the tubes first to prevent damage; in particular, cathode stripping. In other amps like fenders 40W and below, it is more convenience switch/mute since they have tube rectifiers. Anyone have a VOX AC15? No standby switch.

As a matter of habit, I always switched off my amp in reverse order- standby off then power off. However, it is not necessary.

The only reason for draining caps is if you are in the amp and need to work on it or perform repairs. I would go around individually to drain & check caps and never rely on just the standby. Best left to a pro!
The standby switch on a tube-type guitar amp is completely unnecessary. It is a holdover from radio frequency (RF) transmitter design. The goal of the standby switch is to prevent a phenomenon known as "cathode stripping" from occurring. Cathode stripping occurs when a high voltage potential is applied to the plate of a tube before it has reached operating temperature. Cathode stripping is a problem with tube-type RF amplifiers because plate voltages are measured in thousands of volts. However, the plate voltages found on most tube-type guitar amps are too low to cause much in the way of cathode stripping.

Here's what an RF power triode looks like when it is operating:

The power tubes used in most guitar amps are classified as receiving tubes. They are completely different animals than the 833A power tubes shown in the photo above. The 833A has a plate dissipation rating of 350 Watts and a maximum plate voltage rating of 3.3K volts DC. Transmitting tubes are designed to operate with their plates glowing. Operating a receiving power tube with its plate glowing will result in premature tube death.

Contrary to what has been said earlier in this thread, power supply filter caps can hold a charge for a long time. Some power supplies have bleeder resistors that are used to increase the working voltage of the capacitors used for the B+ node, but many do not have these resistors. A Fender Super Reverb is a good example of a vintage design that uses bleeder resistors with series connected filter capacitors to increase the working voltage rating of the capacitors used for the B+ node.

If one looks in the lower right-hand corner of schematic, one sees two squiggly lines (the schematic symbol for a resistor) with the label 220K/1W. These resistors are bleeder resistors. With the standby switch in the open position, they only drain the B+ capacitors. The standby switch must be closed to in order to get the bleeder resistors to drain all of the power supply capacitors on this amp.

The power supply in the Fender 5F6A Tweed Bassman does not have bleeder resistors. The 5F6A Tweed Bassman is the basis for most modern 50W+ rock tube-type guitar amplifiers. For example, a Marshall Plexi is little more than a modified 5F6A Tweed Bassman. The power supply on a 5F6A Tweed Bassman has to be bled manually after the chassis has been removed from the cabinet. A common way to manually bleed the power supply on amps without bleeder resistors is to run a jumper from one of the plate pins (pin 1 or 6) on the first preamp tube to ground. I use a jumper wire that has insulated alligator clips on both ends. The jumper wires that I use are usually sold as test leads.

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Draining the power supply filter capacitors after using a tube amp is not necessary, nor recommended.
Em7, why then does every single tube amplifier manufacturer making an amp with a standby switch recommend using it and letting the amp warm up for 30-120 seconds before use?

I'm not arguing, just curious. PRS, for example, recommends a 120 second warmup.

Outmoded tradition? Or a tube saver?
Not sure about other manufacturers but this is what Laney say...

How do i operate the standby and power switches for best performance?

Here is the correct procedure for switching the amp on and off : Switch on : Ensure the standby and power switch are off – both down, in the same position. Plug in the amp and switch on at the main power outlet. Switch the power switch on the amp ON (up, towards the power LED) Wait between 30 and 60 seconds. Switch the standby switch to run (up, same position as the power switch) Switch off : Switch the standby switch to standby (opposite to the above!) Switch the power switch on the amp off (down, away from the power LED) Unplug the amp from the main power outlet.

This method seems so prevalent that there must be something to it you'd have thought...?


Marshall recommend turning on for two minutes with the amp in standby. They recommend turning both off at the same time when powering down but letting the amp cool for 10 minutes before moving.
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That is the way I have done it since my first tube amp, a used Fender Bandmaster head and cab I got back around '72 or so.
I do not unplug the amp from the Furman-strip though when I am done though.

PS, I meant not the Fender back then, but my present amps I never unplug them.
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It's an outmoded tradition that was started by Leo Fender and copied by many other manufacturers. You and I are both old enough to remember tube-type consumer electronics. How many tube-type HI-FI amps or TVs in the fifties and sixties came with standby switches? How about "All American Five" tube-type table radios? Those things used to be as plentiful as dirt up through the seventies (they can still be found at swap meets/flea markets). I still know people who bang on the side of a TV when it is not operating correctly. That practice came about in the day of tube-type consumer electronics. It is a practice that I do not recommend with solid-state TVs.

With that said, there is a one good safety-oriented use for the standby switch; namely, it gives one time to ensure that one's head is plugged into a speaker cabinet before applying the B+ voltage to the power tube plates. Other than that, the standby switch is little more than a mute switch.