Mowgli
Tele-Meister
This AM a video from a very sharp electronics expert, Mr. Carlson ("Mr. Carlson's Lab" fame), popped up on my YouTube feed. It was entitled something like, "1959 Electronics; Let's see if it works." He was discussing an old Tektronix square wave generator.
During his discussion of the two rectifier tubes that were employed to supply a lot of current to the downstream circuit he mentioned something that may have addressed a question I wondered about decades ago but forgot!
That question was: "How come low wattage amps don't have standby switches while higher wattage amps do?"
Mr. Carlson may have provided a reason, if not THE reason, for this.
There is a vulnerable junction in vacuum tubes where the individual elements (i.e. cathode, grids, plates) are mated with their outside pins. Think of this junction as a "weak link." Mr. Carlson explained that if "too much current" is quickly drawn across the junction of the cathode-pin in rectifier tubes (Diode tubes) it will blow the junction (burn up) and an open circuit will occur as a result of this excess current burning up the connection.
In "low" wattage amps the amount of current drawn from the low wattage power tubes does not pose a threat to the "weak link" junctions of the rectifier or power tubes.
But in "high" wattage amps, the higher current demands could pose such a threat to these "weak link" junctions if a cold tube was suddenly exposed to a high current.
[NOTE: Incandescent light bulbs, when they fail, almost always fail when you "flip on" the switch. Cold light filaments pose very little resistance. So when we turn on the switch, a rush of high current flows through the cold filament. As the filament becomes increasingly hot, its resistance increases (old oscillator circuits employed light bulbs as current limiters; they could "automatically" limit current gains as a function of the current flow!). But cold filaments having low initial resistance, if they are weak or thinned from use, can blow and the bulb fail! This is why one should strategically use dimmer circuits on expensive specialty and rare incandescent bulbs; start out at a low setting, wait about 20 seconds or so for the filament to heat up at the lower voltage, then slowly increase the brightness; less chance of sudden bulb failure!]
So to avoid this risk of damaging the tube(s) and tube failure, the standby switch and observing a "warm up" period were employed.
Not only does a standby switch and observing the recommended "warm up periods" allow for the filaments to warm up and help extend the life of the cathode coating, but it also allows the "weak link" junctions to warm up through heat conduction, thus increasing its intrinsic resistance and decreasing its vulnerability to sudden high current flows.
I thought this was interesting and worth sharing. I think it addresses the question of why standby switches are not used in low wattage amps but are employed in high wattage amps. It appears to be an issue of sudden high current draws during start up.
Additional comments/insights encouraged and welcomed.
During his discussion of the two rectifier tubes that were employed to supply a lot of current to the downstream circuit he mentioned something that may have addressed a question I wondered about decades ago but forgot!
That question was: "How come low wattage amps don't have standby switches while higher wattage amps do?"
Mr. Carlson may have provided a reason, if not THE reason, for this.
There is a vulnerable junction in vacuum tubes where the individual elements (i.e. cathode, grids, plates) are mated with their outside pins. Think of this junction as a "weak link." Mr. Carlson explained that if "too much current" is quickly drawn across the junction of the cathode-pin in rectifier tubes (Diode tubes) it will blow the junction (burn up) and an open circuit will occur as a result of this excess current burning up the connection.
In "low" wattage amps the amount of current drawn from the low wattage power tubes does not pose a threat to the "weak link" junctions of the rectifier or power tubes.
But in "high" wattage amps, the higher current demands could pose such a threat to these "weak link" junctions if a cold tube was suddenly exposed to a high current.
[NOTE: Incandescent light bulbs, when they fail, almost always fail when you "flip on" the switch. Cold light filaments pose very little resistance. So when we turn on the switch, a rush of high current flows through the cold filament. As the filament becomes increasingly hot, its resistance increases (old oscillator circuits employed light bulbs as current limiters; they could "automatically" limit current gains as a function of the current flow!). But cold filaments having low initial resistance, if they are weak or thinned from use, can blow and the bulb fail! This is why one should strategically use dimmer circuits on expensive specialty and rare incandescent bulbs; start out at a low setting, wait about 20 seconds or so for the filament to heat up at the lower voltage, then slowly increase the brightness; less chance of sudden bulb failure!]
So to avoid this risk of damaging the tube(s) and tube failure, the standby switch and observing a "warm up" period were employed.
Not only does a standby switch and observing the recommended "warm up periods" allow for the filaments to warm up and help extend the life of the cathode coating, but it also allows the "weak link" junctions to warm up through heat conduction, thus increasing its intrinsic resistance and decreasing its vulnerability to sudden high current flows.
I thought this was interesting and worth sharing. I think it addresses the question of why standby switches are not used in low wattage amps but are employed in high wattage amps. It appears to be an issue of sudden high current draws during start up.
Additional comments/insights encouraged and welcomed.
