Correct power on / off sequence for valve amp with standby?

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  • ICBMICBM Frets: 71960
    I've just had an idea. I don't know why it never occurred to me before, but… if you have a valve-rectified amp with the standby switch in the wrong place, and it's difficult to rewire it to the right place due to the filter caps being PCB-mounted (like the AC15 I'm about to replace the PT on), or some other consideration, how about…

    Simply using the standby switch to turn off the rectifier valve *filament* supply. This is always a separate winding on the PT so it should always be possible to access. It's low-voltage across the contacts (even though not relative to ground) so the switch rating isn't a consideration. It will always provide the correct slow-start since it's exactly the same as what happens when an amp with no standby switch is turned on, and it will also avoid causing a flyback voltage in the PT since the rectifier will cool slowly - and it shouldn't cause a pop either, I don't think.

    Has this ever been done before? I'm not aware of it, but I can't quite believe no-one else has thought of it.

    @ecc83 @jpfamps @martinw @DJH83004 - comments appreciated :).



    "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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  • normula1normula1 Frets: 639
    That sounds like neat solution. 
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  • ecc83ecc83 Frets: 1591

    No one has ever thought about it IC COS IT'S WEIRD! But like you I cannot see any problem with the idea.

    Dave.

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  • DJH83004DJH83004 Frets: 196
    Like Dave, I can't see a reason why it wouldn't work, certainly worth experimenting with. But to be honest the only real problem I have experienced with bad standby switching was on the AC30 CC2 and variants which switch the B+ centre tap, they do appear to be rectifier killers!
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  • normula1normula1 Frets: 639
    Many years ago I build myself an amp that switched the centre tap, but I used a varistor in series with it to limit the inrush current.
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  • ICBMICBM Frets: 71960
    DJH83004 said:
    Like Dave, I can't see a reason why it wouldn't work, certainly worth experimenting with. But to be honest the only real problem I have experienced with bad standby switching was on the AC30 CC2 and variants which switch the B+ centre tap, they do appear to be rectifier killers!
    That's exactly one of the candidates I was thinking about - it's difficult to rewire the standby into the B+ after the first filter cap because it would involve modifying the PCB. It's not just the AC30 - Orange AD30s and Marshall JTM45/Bluesbreakers (these have it in the B+, but upstream of the first cap) do it too - and I'm sure there are more. The AC15 is slightly different in that it switches both HT connections from the PT - arguably the worst of all possible worlds because it doubles the chance of switch failure too! They really didn't have a clue.

    The only disadvantage I can think of with the filament switching is that switching from standby to on will mean a delay as the rectifier warms up - so the initial impression will be that the amp isn't working.

    "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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  • DJH83004DJH83004 Frets: 196
    Good point IC, in fact one of the amps I look after (a 700watt PA amp - 14xKT88), has an EZ80 in circuit whose sole purpose is to warm up from mains switch on and pull in a DC relay to switch the HT onto the valves. From cold it takes about 10 seconds, but obviously if you switch it on and off it is almost instantaneous. So yes it may be a bit unnerving for someone when the amp is not instantly 'ready to rock'    
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  • jpfampsjpfamps Frets: 2723

    There also maybe issues with approvals re switching the rectifier filament, however I suspect that is not insurmountable.

    I've also thought about this, but not tried it.
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  • Danny1969Danny1969 Frets: 10357
    How about using the standby switch to short the input jack signal to ground if all your using the switch for is a mute ?
    www.2020studios.co.uk 
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  • ICBMICBM Frets: 71960
    Danny1969 said:
    How about using the standby switch to short the input jack signal to ground if all your using the switch for is a mute ?
    No, because a proper standby is far more than a mute switch - the much more useful thing is that it removes the HT from the circuitry so you can change cabinets, swap valves or otherwise troubleshoot without powering off the amp.

    If you want to mute the amp at the input you just pull the plug out halfway.

    "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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  • Danny1969Danny1969 Frets: 10357
    ICBM said:
    Danny1969 said:
    How about using the standby switch to short the input jack signal to ground if all your using the switch for is a mute ?
    No, because a proper standby is far more than a mute switch - the much more useful thing is that it removes the HT from the circuitry so you can change cabinets, swap valves or otherwise troubleshoot without powering off the amp.

    If you want to mute the amp at the input you just pull the plug out halfway.
    I know,  I was just joking :)


    www.2020studios.co.uk 
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