Standby, amp damage and power peaks

Discussion in 'Amps and Cabs [BG]' started by JJd2sc, Mar 5, 2006.

  1. JJd2sc


    Jul 31, 2003
    Marietta, Georgia
    I recently got my mesa 400 repaired and retubed and the guy that worked on it stressed that either turning it on with the standby on would cause the power to peak and damage the tubes or vice versa. The problem is I dont remember which he said... All i remember is that he was very adament about where standby should be while powering on and off or the tubes would peak and start hurtin. I did search for this topic, but I didnt see anything about peaking was mentioned. Thanks for the help TB.
  2. nysbob


    Sep 14, 2003
    Cincinnati OH
    If your amp has a standby switch, it should be on standby when powering up. I don't believe the off cycle makes much difference.
  3. Your tech is right about having your amp in standby mode when turning on; you should give the valves a period of time to 'warm up'* before use. the length of time depends on the previous climate of the amp. example; really cold outside going into a warm room, give it a few mnins before even turning it on.

    As far as power being at a peak thats fooey. Power cant be at a peak when the amp is turned, unless you have a wild surge. I, of course, could be way off. Once I get physics degree and break the laws of physics, I'll let you know for sure. ;)

    by 'warm up', we mean letting the current reach a steady constant value. the filaments in your vavles are very fragile so it's ideal to let them have a constant currnt before firing huge voltages across them as you do when you play.
    hope this helps.
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Standby switches are peculiar to musical instrument amps, they're seldom used in hi-fi amps. Their purpose is to allow you to turn off the amp output during a break without turning off the tube heaters, which would require warming them back up again. Turning it on after warm up can't hurt, but likely doesn't help anything either.
  5. seventhson

    seventhson Supporting Member

    Aug 12, 2005
    Seattle, WA
    But people will tell you to turn the standby back on before powering off. And to make this habit. Otherwise, the next time you use your amp and power it on with the standby off, you'll get that pop.
  6. BbbyBld


    Oct 13, 2005
    Meridian, MS
    When a tube is operating correctly, the cathode has an abundance of electrons surrounding it called the space charge. This can only happen when the cathode is heated to the right temperature. If you don't let a tube heat up all the way before playing through an amp, the electrons can be released directly from the cathode instead of from the space charge. This is called cathode stripping, and it's bad for tubes.

    As for the turn off, a well designed amp should have bleeder resistors on the supply, so I wouldn't worry about turning the standby back on before turning the amp off...if you have a well designed amp.
  7. shanmag


    Jul 27, 2005
    ok noob question here :ninja: , never owned a tube amp but just thought id ask anyway

    whats the right way to turn a tube amp on then? is it turn the standby on for a few minutes or so to let it warm up and then turn power on -> stand by off -> play?

    then when ur turning it off it doesnt matter if u turn the stand by back on, just switch it straight off?
  8. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    And what about standalone preamps or compressors containing a 12AX7... are those tubes harmed by playing through without warmup time?
  9. Preamp tubes have such relatively small emission that "cold starts" don't really hurt them. They simply don't work until they get hot. I'm telling my age, but for any of you who had TV's that had to warm up, that was exactly what was going on. Power tubes are what need standby.
  10. nysbob


    Sep 14, 2003
    Cincinnati OH
    I always check to see I'm on standby before powering up. It's a habit. ;)
  11. jokerjkny


    Jan 19, 2002
    NY / NJ / PA
    not to be a semantics nazi, but you're kinda getting your terms mixed up...

    there's an "on/off" switch, and a "standby/operate" switch.

    the amp when off, the switches should be in the "off" & "standby" positions.

    when you're turning the amp on, you first flick the "on" switch, let the amp warm up for a few minutes, then you flip the other switch from "standby" to "operate". which is essentially what you just said. ;)

    but again, you're not necessarily turning on the "standby". its more a state of being for the amp, if that makes any sense.

    bob has the idea:

  12. seamonkey


    Aug 6, 2004
    After a 100 years, don't all good amps have a delayed or thermistor controlled start? I could be wired prior to the standby to still let the switch work fine. Just a few dollars of parts.
  13. seamonkey


    Aug 6, 2004
  14. BbbyBld


    Oct 13, 2005
    Meridian, MS
    Only a 40 second delay, though. That's not really long enough.

    I can tell you from Peavey's experience with the Classic 30, that even when you eliminate the standby switch people complain. People like using standby switches. Plus, that's just one more thing to go wrong.
  15. seamonkey


    Aug 6, 2004
    Seems like you can keep the standby switch, and use a thermistor to switch on the HV once the tubes are warm. You're right that it's one more thing to go wrong, but you could also have a bypass switch on the back of the amp to bypass the delayed start circuit.

    In any case it must not be that important or you'd think amp manufacturers would have added some protection circuits already.
  16. Several of the tube amp gurus that live in Guitar Land suggest that you turn the volume control full counterclockwise before powering up as well. They suggest you start with the volume control(s) all the way down and the amp in standby mode. Then turn the power on. Wait at least a minute under normal temperatures, or longer if your amp just came out of a cold trunk. Then throw the standby switch into operating mode. Then slowly turn up the volume. If you don't get full power with no unusual distortion immediately when you start turning up the volume, you are not letting the tubes warm up enough.

    I back down the same sequence when I shut down, so my amp is in the correct condition when I power up the next time.

    The guys in Vintage Guitar Magazine get deeply into the science of all this, but I don't understand it. They do seem to be referring to power tubes (as opposed to preamp tubes) when they speak of the consequences of not doing things this way. I just believe that they know more about it than most folks, so I follow their protocols. With a quartet of decent quality KT88's going for well over $100 USD, I figure it is worth my time.
  17. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    This thread belongs in a FAQ or sticky! Seriously useful information here.
  18. The standby feature is for the power tubes only. Hence you'll find it on the tube heads which have tube power amp and tube power amps.

    Standby serves two purpose:
    1) let the power tubes "warm up" (the tube filament needs to be glowing)
    2) give the power tubes some rest time

    First purpose: because the power tubes are power amplifiers (they amplify both voltage AND current) it is a good idea to have them ready before applying the signal to amplify.
    Compare it to running uphill at 6am on a freezing day... it is better to be warmed up (Army memories anyone?).

    Second purpose: without going into the details of vacuum tube operation, what standby does is preventing the very high voltage to be applied to the power tubes for them to do their job (turning them on if you will). This voltage can vary depending on the tubes from 400V to 600V.
    The preamps are usually below the 200V (still "shocking" on the fingers though :eek: ).
    So when you take a break on a gig or rehearsal, give a rest to your power tubes, put them on standby.

    To wrap things up: When in standby, the power tube operation is basically turned off (not the tubes themselves, the filament is still glowing).

    Turning on the amp:
    1) amp off, standby on
    2) amp on, standby on
    3) wait 30sec to 1mn
    4) amp on, standby off

    Turning off the amp:
    1) amp on, standby off
    2) amp on, standby on
    3) amp off, standby on
  19. Once the filaments are warmed up, there's no need to put the amp in standby mode again while it's on. There's no advantage to turing the HV off during breaks and whatnot. It's actually probably worse for the tubes to power cycle them 3 or 4 times a night as opposed to once. Having the HV on with the tubes at idle sin't going to cause much trouble. The only reason to put the amp back in standby before you shut it off is so that it's in standby when you go to power it up the next time. Most tubes are adequately heated to pass signal in less than 30s. The glass doesn't need to be hot for the tube to pass signal.

    We don't use a standby on our products (hifi) but I did design a soft start circuit that slowly ramps the HV up. We haven't noticed any effect on tube life from our early standby equipped prototypes. In fact, after several years of working with tubes, I can say with a large amount of certainty that tubes are a lot more robust than they're given credit for.
  20. BbbyBld


    Oct 13, 2005
    Meridian, MS
    That's not really the case. The standby should turn the screen voltage on and off, not the plate. There's not really any power cycling going on in the power tubes.

    That's not really true. The plate voltage for the preamp tubes is turned on and off with the screen voltage when the amp is on standby.

    They will be heated enough to work, but chances are, the cathode won't be heated all the way and you will get cathode stripping on the cold spots.