Post by Amanda Hambidge on Dec 22, 2008 17:09:42 GMT
Hi all,
This has been mentioned before but here's a post from the vintage amps Vox bulletin board in the states by Lyle Caldwell who is a well respected amp tech over there:
Better for the amp if you either just leave it on with the master down or turn it off until 5 minutes before you start playing. Standby offers no benefit but every time you engage the standby switch there is a large current inrush and voltage spike. This gradually damages the GZ34 and eventually the filter caps and output tubes. Nominal B+ on these amps is about 325vdc, and the filter caps are rated 400-500vdc. When you flip the standby switch, voltage spikes of over 700vdc with brief but heavy current surges are common. These put undue stress on the parts mentioned above, as well as eventually causing the switch itself to fail.
Sometimes its a lot of fun when the spike/surge causes the rectifier tube to actually arc weld itself. You get a light show and a funky smell. The light show is over fast, but that smell lingers. And sometimes it takes out other parts of the power circuit! Yay!
And the reason people tell you TO use standby is so there is no instantaneous plate voltage on the tubes until the heater filaments in the tubes are warmed up. Thing is, no B+ (plate supply) flows in these amps until the filaments in the GZ34 have warmed up themselves, by which time every other tube in the amp also has its filaments heated.
So there is no valid reason to use standby, but lots of empirical, explainable, and predictable reasons not to.
Note, for the sake of this argument, I am only discussing amps that use a GZ34 rectifier tube or the Weber replacement. So JTM45, AC30, Bassman, etc. Amps with other tube rectifier types or solid state rectifiers are not part of this discussion.
Though really, unless you're talking about an extremely high voltage amp like an SVT (700+vdc B+) cathode stripping is really not a danger, so there still is no compelling reason to use Standby on a Twin or Marshall either.
Lots of techs "know" things based on what they have been told. That really is not knowledge. That is just a good memory.
Let me put this in less technical terms.
It's rare for you to be sitting in a room reading a book and the light in the lamp just goes off.
But it's common to switch on a lamp and have the bulb die at that point.
In the first scenario, the light has a steady voltage and steady current through it. Unless physically vibrated, that filament inside that bulb will keep on going a long long time.
But that same bulb when subjected to the voltage spike and current increase due to the electricity quickly being switched on will fail. The filament that could withstand hours and hours and hours with a steady voltage and current will fail instantaneously when the voltage and current briefly exceed its threshold of failure.
Now, that same bulb on a dimmer would not fail at power up if the dimmer was all the way down when the switch was turned on, and then you slowly brought the voltage up with the dimmer.
This last scenario is what happens when you let the filaments of the GZ34 control the voltage sent to the rest of the amp, rather than using the Standby switch.
Standby = instant on.
GZ34 = slow dimmer increase.
Regards
Amanda
This has been mentioned before but here's a post from the vintage amps Vox bulletin board in the states by Lyle Caldwell who is a well respected amp tech over there:
Better for the amp if you either just leave it on with the master down or turn it off until 5 minutes before you start playing. Standby offers no benefit but every time you engage the standby switch there is a large current inrush and voltage spike. This gradually damages the GZ34 and eventually the filter caps and output tubes. Nominal B+ on these amps is about 325vdc, and the filter caps are rated 400-500vdc. When you flip the standby switch, voltage spikes of over 700vdc with brief but heavy current surges are common. These put undue stress on the parts mentioned above, as well as eventually causing the switch itself to fail.
Sometimes its a lot of fun when the spike/surge causes the rectifier tube to actually arc weld itself. You get a light show and a funky smell. The light show is over fast, but that smell lingers. And sometimes it takes out other parts of the power circuit! Yay!
And the reason people tell you TO use standby is so there is no instantaneous plate voltage on the tubes until the heater filaments in the tubes are warmed up. Thing is, no B+ (plate supply) flows in these amps until the filaments in the GZ34 have warmed up themselves, by which time every other tube in the amp also has its filaments heated.
So there is no valid reason to use standby, but lots of empirical, explainable, and predictable reasons not to.
Note, for the sake of this argument, I am only discussing amps that use a GZ34 rectifier tube or the Weber replacement. So JTM45, AC30, Bassman, etc. Amps with other tube rectifier types or solid state rectifiers are not part of this discussion.
Though really, unless you're talking about an extremely high voltage amp like an SVT (700+vdc B+) cathode stripping is really not a danger, so there still is no compelling reason to use Standby on a Twin or Marshall either.
Lots of techs "know" things based on what they have been told. That really is not knowledge. That is just a good memory.
Let me put this in less technical terms.
It's rare for you to be sitting in a room reading a book and the light in the lamp just goes off.
But it's common to switch on a lamp and have the bulb die at that point.
In the first scenario, the light has a steady voltage and steady current through it. Unless physically vibrated, that filament inside that bulb will keep on going a long long time.
But that same bulb when subjected to the voltage spike and current increase due to the electricity quickly being switched on will fail. The filament that could withstand hours and hours and hours with a steady voltage and current will fail instantaneously when the voltage and current briefly exceed its threshold of failure.
Now, that same bulb on a dimmer would not fail at power up if the dimmer was all the way down when the switch was turned on, and then you slowly brought the voltage up with the dimmer.
This last scenario is what happens when you let the filaments of the GZ34 control the voltage sent to the rest of the amp, rather than using the Standby switch.
Standby = instant on.
GZ34 = slow dimmer increase.
Regards
Amanda
