tube amp-leave it on or turn it off

Discussion in 'General Electronics Chat' started by leon23, Feb 4, 2016.

  1. leon23

    Thread Starter Member

    Oct 11, 2008
    21
    1
    I have a Marshall guitar tube amp. I'm wondering if turning it off and on several times during the day stresses the tubes more than just leaving it on all day.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    A good compromise is to put the filaments at half power in a standby mode when the amp is not used.
    That allows the amp to come on rapidly when full power is applied, but there is much less thermal shock to the filaments, which is likely what causes filaments to fail sooner.
    Depending upon the design, using a DPST (or DPDT) switch to connect a power resistor of the appropriate value in series with the filament power, and also switch off the plate voltage, would do that.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    In TV sets with heater chains; some manufacturers put a rectifier across the on/off switch, pointing the opposite way to the HT rectifier. That tactic was defeated in later models because manufacturers found they could reduce the dropper resistors by half-wave rectifying the heater supply.

    Valves take the biggest hit from switching on from cold, the heaters draw more current when cold and the turn on surge is a lot of stress. NTC thermistors were common in heater chains, but not very practical on all parallel heaters.

    The best solution is switching over to a lower voltage standby heater transformer, maybe 4.5 - 5V or so - you'd have to also kill the HT or it'll strip the cathodes.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Hi, ex bass player here.

    First I doubt that the TS wants to modify his amp. Modified amps are worth less than pristine ones.

    Doesn't your amp have a standby switch? (This switch turns off the high voltage but leaves the filaments on.)

    Regardless, tubes have a finite life, but they are easy to replace. You have to re-tube an amp every so often anyway. Leaving your amp on all day just consumes the filament life. Just use good judgment about when it is going to be used and turn it off.
     
    Sinus23 likes this.
  5. leon23

    Thread Starter Member

    Oct 11, 2008
    21
    1
    All great answers. Yes the Marshall JCM900 has a heater switch. I always turn that switch on first but I don't leave it on long enough before turning on the main power. I got no help from the Marshall forum or from the lady receptionist there who apparently they have put in charge of answering tech questions.
     
    Last edited: Feb 6, 2016
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,779
    932
    Tube amps are increasingly popular, so the price of the common tube will continue to fall as suppliers fight to win market share.
    Yes, you can extend the lifetime of a tube by using standby heater techniques, but the amount of savings is hard to value and the tube are currently cheap enough to not be a problem for an employeed individual.
     
  7. leon23

    Thread Starter Member

    Oct 11, 2008
    21
    1
    Somewhere I read that when you put in new tubes you have to tweak/adjust the circuits somehow and that this is something a professional should do. ?
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    Sometimes in RF/IF circuits, manufacturing tolerances cause slight variations in electrode capacitance that can take the tuning off trim.

    With audio power stages its not a bad idea to make sure the plate current is within spec, sometimes you have to tweak the bias.

    Its never a bad idea to check any capacitors in C/R coupled stages - especially if the previous bottles imitated electric fires.

    Even slight leakage can wipe out the negative bias on the grids and cause excessive plate current - full on HT from the previous stage can destroy the grid, then your expensive beam-tetrode is just a rectifier diode shunting the HT.
     
    leon23 likes this.
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