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12AU7 acting as rectifier
How much current can 1/2 of a 12AX7 push out if it's used as a rectifier?
Many years ago I worked as a technician servicing industrial instruments. In those days vacuum tube amplifiers often drove servo motors to position pens on recorders or pointers in big gauges. Designs for these servo amplifiers dated from about 1939. The first ones came from the Brown Instrument Company in Philadelphia which had been bought by Minneapolis Regulator (Honeywell) about 1936. The early units used twin triode loctal tubes. When WW II came along 100 % of the production capacity was eaten up be the military but after the war civilian products came out using new miniature tubes. Outside of tube size and filament circuit changes, it was the same 1939 design. At any rate, there were usually two 12AU7's and two 12AX7's. (I can't remeber the old loctal numbers). The 12AU's were a sort of class B or C parallel push/pull that drove the servo motor directly .. no transformer. The 12AX's were preamps and phase splitters except that 1/2 of one 12AX was the power rectifier for the whole works. As I recall the grid was tied to the cathode and that was it! It always seemed to me that it was asking an awful lot of that one puny 1/2 12AX7 to power everything, not to mention the filament to cathode voltage issues it made. But the things worked and the tubes lasted in them for years and years. The things were redesigned with solid state amps about 1968 but tubes remained an option. The tube units were much better for RFI rejection, handling voltage surges and other "industrial" issues and remained in prodution till the late 80's or early 90's.. The fifty year production run says something about the quality of the original design! But I still wonder about that rectifier stage and if any tube maker ever published numbers for that kind of service? Rod |
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Rod Keys wrote: How much current can 1/2 of a 12AX7 push out if it's used as a rectifier? Many years ago I worked as a technician servicing industrial instruments. In those days vacuum tube amplifiers often drove servo motors to position pens on recorders or pointers in big gauges. Designs for these servo amplifiers dated from about 1939. The first ones came from the Brown Instrument Company in Philadelphia which had been bought by Minneapolis Regulator (Honeywell) about 1936. The early units used twin triode loctal tubes. When WW II came along 100 % of the production capacity was eaten up be the military but after the war civilian products came out using new miniature tubes. Outside of tube size and filament circuit changes, it was the same 1939 design. At any rate, there were usually two 12AU7's and two 12AX7's. (I can't remeber the old loctal numbers). The 12AU's were a sort of class B or C parallel push/pull that drove the servo motor directly .. no transformer. The 12AX's were preamps and phase splitters except that 1/2 of one 12AX was the power rectifier for the whole works. As I recall the grid was tied to the cathode and that was it! It always seemed to me that it was asking an awful lot of that one puny 1/2 12AX7 to power everything, not to mention the filament to cathode voltage issues it made. But the things worked and the tubes lasted in them for years and years. The things were redesigned with solid state amps about 1968 but tubes remained an option. The tube units were much better for RFI rejection, handling voltage surges and other "industrial" issues and remained in prodution till the late 80's or early 90's.. The fifty year production run says something about the quality of the original design! But I still wonder about that rectifier stage and if any tube maker ever published numbers for that kind of service? Rod Any triode or triode strapped pentode can be used as a rectifier, and the rules for peak cathode current and power dissipation apply just as they do for diode tubes. The numbers must be crunched for the circuit in mind, and since you have not told us what schematic and voltages you want, we can't advise to have something last 50 years. Power tubes were also used as power rectifiers, with grid bias voltage controlling the output voltage from a power supply. afaik, the bias voltage control chopped portions off the sine wave and the rectifier wave input was varied for regulation. It could mean noisy operation. Patrick Turner. |
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"Patrick Turner" wrote in message ... Rod Keys wrote: How much current can 1/2 of a 12AX7 push out if it's used as a rectifier? Many years ago I worked as a technician servicing industrial instruments. In those days vacuum tube amplifiers often drove servo motors to position pens on recorders or pointers in big gauges. Designs for these servo amplifiers dated from about 1939. The first ones came from the Brown Instrument Company in Philadelphia which had been bought by Minneapolis Regulator (Honeywell) about 1936. The early units used twin triode loctal tubes. When WW II came along 100 % of the production capacity was eaten up be the military but after the war civilian products came out using new miniature tubes. Outside of tube size and filament circuit changes, it was the same 1939 design. At any rate, there were usually two 12AU7's and two 12AX7's. (I can't remeber the old loctal numbers). The 12AU's were a sort of class B or C parallel push/pull that drove the servo motor directly .. no transformer. The 12AX's were preamps and phase splitters except that 1/2 of one 12AX was the power rectifier for the whole works. As I recall the grid was tied to the cathode and that was it! It always seemed to me that it was asking an awful lot of that one puny 1/2 12AX7 to power everything, not to mention the filament to cathode voltage issues it made. But the things worked and the tubes lasted in them for years and years. The things were redesigned with solid state amps about 1968 but tubes remained an option. The tube units were much better for RFI rejection, handling voltage surges and other "industrial" issues and remained in prodution till the late 80's or early 90's.. The fifty year production run says something about the quality of the original design! But I still wonder about that rectifier stage and if any tube maker ever published numbers for that kind of service? Rod Any triode or triode strapped pentode can be used as a rectifier, and the rules for peak cathode current and power dissipation apply just as they do for diode tubes. The numbers must be crunched for the circuit in mind, and since you have not told us what schematic and voltages you want, we can't advise to have something last 50 years. Power tubes were also used as power rectifiers, with grid bias voltage controlling the output voltage from a power supply. afaik, the bias voltage control chopped portions off the sine wave and the rectifier wave input was varied for regulation. It could mean noisy operation. Patrick Turner. Hi Patrick, I guess I was not hoping to build a new one nor even analyse it to death .. I was just making an observation. It's been thirty five years since I had a meter in one of those beasts but as I recall they had pretty hot voltages, like over 300 volt B+. Sign wave was never an issue .. the input came from a DC Wheatstone bridge converted to AC by a mechanical "chopper" in the front end of the amp. The output of the amp drove a winding in a motor. (Another winding was powered from straight 120volts AC). Phase of the output reflected polarity of the input and so drove the motor (which re-balanced the wheatstone bridge) one way or the other,. Interstingly, gain was adjustable by a simple pot in a voltage amp stage. It was used to "damp" the motor action .. I thought adjustable gain was kind of odd in view of the near class C output. Class C outputs usually have an all or nothing character but in this case it worked .... I still marvel that the poor 1/2 12AX7 didn't melt its little guts out trying to power all this. Rod |
#4
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Website @ http://webpages.charter.net/dawill/tmoranwms
"Rod Keys" wrote in message ... How much current can 1/2 of a 12AX7 push out if it's used as a rectifier? I would guess a good bit if you tie grid to plate. I know 12AU7 and 5687 have characteristics such that grid and plate currents are equal when voltages are equal (coincidentially). Gets a pretty steep curve, I believe 5687 goes up to an ampere total at 50V while 12AU7 does 200mA between the two at 30V. I'd have to see 12AX7 curves, but based on what I've seen of transmitter tubes, hi mu vs. low mu, it's probably quite stiff. Certainly moreso than the plate curves at Vg=0V. Tim |
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