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

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.

"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

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