Standby switches

Can we settle this once and for all please...

If I am using a GZ34 or 5AR4 rectifier, do I need to install/use a standby switch?  

If not, would fitting a thermistor at any point in the power supply benefit any part of the amp in terms of longevity?  (E.g. before the rectifier to protect it, after the rectifier to protect the caps/valves etc.) 
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Comments

  • ICBMICBM Frets: 71951
    You can't settle it once and for all because it depends where in the circuit the standby switch is fitted. In some places it will be a risk to the rectifier valve. (Quite a few modern amps are like this.)

    A correctly sited standby is useful and can extend power valve life a bit, depending on how hot the amp runs at idle - better than a thermistor.

    The standby needs to be after the first filter cap, not before it or in the centre tap connection of the PT.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • jpfampsjpfamps Frets: 2723
    Not needed in any amp really (with the caveat that the filter caps are rated for the unloaded HT voltage)

    In fact putting a standby switch into an amp with a valve rectifier can imperil the rectifier as Korg found out with the VOXAC30.
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  • CirrusCirrus Frets: 8481
    edited April 2014
    You don't need to, but if you install one put it after the first filter cap to avoid stressing the rectifier when you flick it. That's my understanding anyway.
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  • Cirrus said:
    You don't need to, but if you install one put it after the first filter cap to avoid stressing the rectifier when you flick it. That's my understanding anyway.
    I did wonder why the standby always came after that first pair of caps on the blackface Fenders.  Presumably the .05 cap on the 5F6A Bassman is there precisely for this reason.

    My chassis is pretty busy, so I'm trying to save space.  I have been using a three position switch as an on-off-standby switch, but I'm not sure about sweeping across the 'off' setting to get from 'standby' to 'on'.  Is this dodgy?  Would I be better off without a standby position at all?
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  • ecc83ecc83 Frets: 1589
    jpfamps said:
    Not needed in any amp really (with the caveat that the filter caps are rated for the unloaded HT voltage)

    In fact putting a standby switch into an amp with a valve rectifier can imperil the rectifier as Korg found out with the VOXAC30.

    Absolutely agree. Yes I know "they" fitted them to the A series but I wish they hadn't. Still, it would I think be hard to sell a "classic" handwired all valve amp without one!

    FYI the HT-5 opens the AC feed to the sstate bridge and the rest of the HT and S1 series just slam -100 volts or so on the power valve grids.

    Dave.

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  • CirrusCirrus Frets: 8481
    Weird, when I posted I couldn't see the two posts before mine, even after i clicked post and did my usual self-check for stupidity. I promise I wasn't ignoring or parroting ICBM and JPF!
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  • ICBMICBM Frets: 71951
    Regardless of whether it's strictly 'necessary' (which it isn't) a standby switch is still *useful*, and if implemented correctly isn't a risk to the rectifier valve. The fact that some modern designers don't seem to know how to do it right doesn't make standby switches a bad thing.

    I still think that turning the HT off is the best method because it can help with troubleshooting and makes it safer to work on the amp. It's not just a mute switch.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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