Amp power down question.

[alerich]

My Marshall style amps have power and standby switches. My PRRI has a power switch only. All of my amps are plugged into power strips that also supply power to pedal boards. When I shut off my PRRI off I turn the power switch off (or course). On my amps with power and standby switches I leave the standby on and switch the power off so that the tubes will drain off the filter caps before they cool down. This does work. Check it with a meter.

On any of these amps after I shut the power switch on the amp off if I then turn the power strip off while the tube are still hot I'll hear a little "thump" out of the amp as I switch the power strip off. Why does that happen? If I turn the power switch off am I not disconnecting the amp from the AC wall supply? Once the power tubes cool enough to longer conduct this "thump" does not occur. This has been puzzling me for some time.

[sluckey]

Maybe some device plugged into the power strip is causing a little bump when the power strip is turned off. Leave the amp switch on and just turn the strip off. Still happen?

Or unplug everything from the strip except the amp. Still happen?

Next try this... With everything plugged into the strip and powered up as usual, turn the amp off. Now unplug each of the other devices in turn. Does unplugging some particular device cause the thump?

[alerich]

Good ideas. I will try each of those. I also will only plug in the amp to the power strip and see if it happens when it is the only device on the power strip.

[eleventeen]

I think that what is happening is that the effects pedals, one or many or all of them, usually generate a glitch or a noise or whatever you want to call it when they are shut off. I have a POD and it makes a fearsome squawk when I turn it off....not a "thump" but a very high-pitched and high volume squeak that is positively annoying if I turn it off before doing anything to the amp. A real ear twister and maybe even a speaker-blower.

In your case, your amp remains "amping" a little bit even with B+ turned off (eg; standby) and draining fast...and you are getting that turn-off transient when you kill the power strip.

One solution IMO is to just wait more time in between stand-bying the amp and shutting off the strip. More like a full minute. It's of course dependent upon how fast your particular amp bleeds off B+ when switched to standby. Some: 5-10 seconds. Others: almost a minute. It has a lot to do with whether you have some form of bleeder resistor installed on the amp power supply. For example, on Twin Reverbs where you have those 2 qty 220K "balancing" resistors strapped parallel to the series-connected filter caps, those *do* function as a bleeder, but not a very "aggressive" one, as those R's only pull maybe a mil or so. Also consider: if the negative bias on the output tubes happens to fade a lot faster than the rest of the B+, the output section could probably amplify with a very low B+ of around 100 volts! Enough for one big "whump".

[alerich]

I think that what is happening is that the effects pedals, one or many or all of them, usually generate a glitch or a noise or whatever you want to call it when they are shut off. I have a POD and it makes a fearsome squawk when I turn it off....not a "thump" but a very high-pitched and high volume squeak that is positively annoying if I turn it off before doing anything to the amp. A real ear twister and maybe even a speaker-blower.

In your case, your amp remains "amping" a little bit even with B+ turned off (eg; standby) and draining fast...and you are getting that turn-off transient when you kill the power strip.

One solution IMO is to just wait more time in between stand-bying the amp and shutting off the strip. More like a full minute. It's of course dependent upon how fast your particular amp bleeds off B+ when switched to standby. Some: 5-10 seconds. Others: almost a minute. It has a lot to do with whether you have some form of bleeder resistor installed on the amp power supply. For example, on Twin Reverbs where you have those 2 qty 220K "balancing" resistors strapped parallel to the series-connected filter caps, those *do* function as a bleeder, but not a very "aggressive" one, as those R's only pull maybe a mil or so. Also consider: if the negative bias on the output tubes happens to fade a lot faster than the rest of the B+, the output section could probably amplify with a very low B+ of around 100 volts! Enough for one big "whump".

I believe you are correct. After I had replied to Steve it hit me that the pedals are still under power and plugged in and that one or more of them are creating the sound I am hearing. All of my amps have at least one pedals in front of them or in the loop. It's not really a problem, per sé. I was just puzzled at the phenomenon. Now it makes perfect sense.