US2646472A - Amplifier control system - Google Patents

Description

J ly 1953 R. J; ROCKWELL AMPLIFIER CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Sept. 6. 1950 INVENTOR.

RONALD J. ROCKWELL ATTORNEY.

July 21, 1953 R, J. ROCKWELL AMPLIFIER CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Se t. e1 1950 llPJO INVENTOR.

RONALD J. ROCKWELL zz gw ATTORNE).

Patented July 21, 1953 AMPLIFIER CONTROL SYSTEM Ronald J. Rockwell, Cincinnati, Ohio, assignor to Crosley Broadcasting Corporation, Cincinnati, Ohio, a corporation of Ohio Application September 6, 1950, S erial'No. 183,439

, 3,Claims.

1 i The present invention relates generally to ramplifier circuits. More specifically, the present .invention relates to control circuits for parallel connected amplifier tubes wherein both a variable D. C. bias voltage and a signal potential are applied to each amplifier grid element.

Tube manufacturers usually keep up with the industrial need for tubes having a large current rating. However since consumer need always precedes ultimate production, industry has found 1 it necessary to electronically control blocks of current which are too heavy for any single avail,- able tube. When this problem arises, the only design out is to use a parallel bank, so that the large load current can be divided. The use of a 1 plurality of tubes to divide the total load current,

though solving the problem of keeping plate current flow in each tube below maximum safe operating limits, involves a new problem of controlling the platecurrent excursions in the various tubes.

It is with this latter problem that we are concerned here.

Slight variations in amplifying characteristics among tubes having identical manufacturers ratings obviously cause variations in the plate current excursion rates. Though a slight diiference in the rate of plate current change is not too important in most installations, any sizeable differf ence as wellas any diiierence in the maximum plate current drawn by different tubes in the parallel bank is objectionable.

The problem has been met by prior art teachings through use of an individualbias control for each parallel connected tube in addition to an individual input signal amplitude control for each :tube. By this means, the operating point on the characteristic curvev for each tube is separately selected and also the amplitude of the input signal, or the signal swing around the chosen operating point, is also separately selected. Though the prior art circuits have merit in that a plurality of tubes may be operated in parallel, a simplified control system would be desirable for use Where a parallel bank must include three or more tubes. 7

Where tubes are operated in parallel in a conventional circuit, the operator is forced to dynamically balance the load current by applying a signal or tone frequency as well as apply the correct bias voltage to balance the idle current flowor the plate current flowin in the absence of a grid signal. Usually the input signal transformer secondary has a large number of taps one of which must be selected to effect a dynamic balance. Especially is this true when three or more tubes.

' 2 tubes are involved, making the problem of load division rather complex and making the maintenance problem very difiicult.

It therefore becomes ageneral object of my in- V vention to simplify the amplifier control network in a parallel-bank type of circuit.

A second factor, that is involved in the need for the present invention, arises primarily because of the high cost of kilowatt rated amplifier production cost. Also, tubes of this size are usually rejected if there is any appreciable difference between their amplifying characteristic and the amplifying characteristic of the selected norm and of course the cost of rejects must be. added to the cost of the tubes having acceptable characteristics. Thus it would be desirableto devise a circuit wherein these rejects can be operated in parallel with tubes having correct normal 7 characteristics. I Therefore, a primary object of my invention is to devise a simplified control means capable of controlling a parallel connected amplifier bank in which tubes having slightly different plate current characteristics can operate side by side, each carryingits proportionate share of load current.

A still further basic object of the present invention is to provide a unitary means for controlling the plate current excursion of eachtube in a parallel connected amplifier bank.

In the illustrated embodiment of; my invention,

I choose to show a push-pull type of amplifier using three or more tubes inparallel on each side. The secondary of the input transformer has a bias source connected thereto along with a single potentiometer for each tube in the parallel bank. By connecting the potentiometers in parallel so that the same bias voltage and the same signal is impressed across each potentiometer I make it possible to control the bias and signal voltage amplitude for each tube with a single unit. After adjusting the variable arms on the potentiometer which are connected to the grid of each tube, so that the .idle current, that is the current drawnb'y each-tube in the absence of a signal is the same, I find that there is always an equal division of plate current flow among thetubes when a signal is impressed upon the circuit. In other words, I have found that there is a linear relationship between the signal amplitude and' bias voltage required to maintain similar plate current excursions in the various parallel con- Lack of demand. causes tubes of this size 7 to be predominantly handmade or at least prO- duced with a minimum of mass production tool- :ing up anda maximum of labor; Thus a high nected tubes. For a better understandin of the present invention, together with other and further objects, advantages and capabilities thereof reference is made to the following disclosure and appended claims in connection with the accompanying drawings, in which:

Fig. 1 is a circuit diagram illustrating the teaching of the prior art;

Fig. 2 is a circuit diagram showing the invention used in a push-pull amplifier stage; and

Figs. 3 and 4 are curves illustrating the operation of the control systems shown'in Figs. 1 and 2.

In Fig. l, I have shown a typical prior art class B operated push-pull type of amplifier which uses three tubes in parallel on each side of the circuit to modulate a 200 kw. transmitter. An A. C. signal can be assumed to be taken from the secondary of transformer H), which has a center tap to ground, dividing the secondary in- .to two parts. Also, I have shown a plurality of.

The variable arm on each switching device isconnected via a blocking condenser coupling to the grid of an amplifier tube. For example, switch arm I4, of device I l, is connected through condenser IE to grid N5 of tube 17. Also, switch arm l8, of switch device i2, is connected through condenser 19 to grid Zilof tube 2!. It can'be seen that tubes 22, 23, 24 and 25 are similarly connected to the secondary of transformer I93. By adjusting the switch arm in the appropriate switching device, the operator can select the desired amplitude of input signal impressed across the/grid of each tube.

In order to keep the signal voltage from being dissipated in the low impedance bias source, a

resistor, such as resistance 36, 3! and 32, is connectedbetween the grid of each tube and the variable arm on the appropriate potentiometer. These resistors having a very high value, keep the bias source from loading the signal source.

In operating the circuit before the signal can be applied, the operator must first adjust the characteristic of tube 2! in Fig. 1 can be shown by a curve similar to curve 10, and that curve Hjillustrates the eg/ip characteristic of tube 22, it can be shown that different bias potentials must be applied to the grid circuits of these two tubes in order to cause identical idle current flow.

If both tubes have a control circuit bias potential impressed which is equal in magnitude to, point 0 on curve ill, a vertical projection of point 0 which intersects curve H indicates the idle current which would flow in the tube of curve 1!. in the tube of curve it is low and equal to ipi, while the idle current in the tube of curve H is high and equal to i o. It now becomes obvious that two tubes having different amplifying characteristics can not be operated in parallel so as to have equal idle current flow, unless a diiierent bias potential is applied to each grid circuit. In actual practice, the correct grid bias is chosen by placing a meter in each cathode circuit and adjusting the bias voltage until all meters show that the selected and the same idle current is flowing in each tube. In our case this would be i i as shown in Fig. 3.

After the operator balances the idle current flow in the various parallel connected tubes, then the circuit must by dynamically balanced, or balanced for operation under signal condi tions." A dynamic balance is reached by using a cut-and-try process to select the correct 2% taps while a signal or tone source is driving the grids through the input transformer.

used.

InEig. 2 I have illustrated an embodiment of my, invention using a push-pull type amplifier,

preferably operated in class B. Again I have used three tubes in parallel on each side of the push-pull circuit. As the explanation unwinds it will become obvious that this simplified-control system is equally applicable to circuits using more or less than three tubes in parallel as well as to circuits other than those of the push-pull type.

A signal is developed across the, secondary of the transformer which has a center tap connected to a negative source of bias potential. The remaining two terminals of the transformer secondary are each connected to one side ofidifferentbanks of parallel connected potentiometers. Onebank, or the top illustrated bank, is composed of potentiometers ll, 42 and 53 all connected in parallel. The other bank, or the lower illustrated bank of potentiometers, comprises potentiorneters 44, t5 and 46. The other terminal of the. upper illustrated bank is connected .to ground through resistance 41 while the other terminal of the lower bank of potentiometers is connected to ground. through resistanceAB. It can beseen that the source of negative biaspotential which is connected, between the secondary of transiormer wand ground will cause a currentilow throughresistance. 4 1, the upper illustratedpotentiometer bank including potentiometers .5, 42 and 43,, and one, half of the secondary of. transformer. 40. This .bias source will also cause current flow from ground through resistance 68, the lower illustrated potentiometer. bank. and theremaining half .of the secondary oi transformer. All. Resistors 4'1 and 18 have equal resistan'cevalues, and the resistance of each potentiometenbank is also identical.

. Because of this circuit symmetry there are two DJC. currentsfioiving in the secondary of transformer 48 in oppositedirections, each in a separatehali of thewinding thereby inducing an equaland opposite fiux in the transformer core. Since the flux set up by one current bucks-the It will be seen that the idle current flux set up by the other current, as far as the transformer secondary is concerned, it' is the same as if no current flowed through it from the bias source. Inother words, this type of symmetrical connection avoids D. C. saturation.

The variable arm on each'potentiometer is connected to the grid of a separate tube. For example. the upper potentiometer bank, which is used to control tubes t9, and 5i has three variable potentiometer arms. Potentiometer arm 54 of potentiometer i2 is connected to grid 55 of tube 5t. Also potentiometer arm 58 of potentiometer ii is connected ,to grid 5'! of tube 5!. between the lower potentiometer bank and the grids of tubes 58, 59 and 66; r

In order to understand how my novel circuit operates reference should again be made to Fig. 3, where I have shown two signals having equal amplitudes, each one swinging about the operating point on a diiierent tube characteristic. Signal curve 72 is shown as swinging around the operating point on curve H, which in this case represents the characteristics of tube 59. Dotted curve 13, having the same magnitude as wave '52 is shown swinging around theo'perating point on curve "it, which represents the characteristics of tube d9. Projecting the peaks of curve 12 and 73 upon the appropriate characteristic curve it can be seen that these two signals cause different peak plate current flow in the two tubes. A signal having the amplitude of curve 73 would cause tube is to draw a peak plate current equal to 2' 49, a value which is much higher than i sn, the current which a signal of equal amplitude draws in tube 58. Since we must have equal plate current flow in the two tubes, it becomes necessary to reduce the amplitude of the signal swing impressed across the grid circuit of tube it. Curve "i i represents the reduced amplitude of input signal swing. Vertically projecting the peak of curve is so that it intersects curve It! shows that a reduced signal amplitude can be selected which makes the plate current flow in tube 49 equal to ipfio the current flowing in tube Though this principle, of load division by reducing the signalswing in some of the parallel connected tubes, was wellknown in the prior art, no one seemed to realize that there was a relationship between the percentage change in the signal amplitude and the percentage change in the necessary bias voltage magnitude. I have found that there is such a relationship between these voltages of which advantage can be taken to eliminate the old cut-and-try adjustment process along with many circuit elements formerly deemed necessary.

' To state the relationship, if tube 50 is considered as the norm? and the bias voltage applied to its grid circuit is considered to be the becomes necessary to have a 10% lower grid bias on the grid of tube 49 than that impressed on the grid of tube 56, in order to equalize idle currents, it is also necessary to have a signal im- Similar symmetrical connections are made pressed upon the grid of tube 49 which is 10% lower in peak magnitude than the signal impressed'upon the grid of tube 50. Thus, if it be assumed that the difference (11), as shown in Fig. 3, between the bias voltages supplied to tubes 49 and 50 is equal to 10% of the bias voltage supplied to tube 50, then the difference (1) has the same magnitude of input. signal impressed across it as is impressed across the re-; mainder of the potentiometers in the potentiometer bank. Thus, when any potentiometer arm and the amplitude of" the input signal fed to grid 57 of tube 5] are reduced by the same per-- centage. This means that, if the contact is dropped or moved toward ground to a point which reduces the grid bias voltage say 10% for example, the amplitude of the input signal fed to grid 5? is also reduced 10%.

Another way of looking at this is shown by the curves'illustrated in Fig. 4. Again the characteristic curves of two tubes are shown; however, it must be realized that the principle involved applies equally as well to the relationship between the input signal and operating bias applied to all of the tubes. The higher mu tube which could be tube 49 can be assumed to have amplitudes which should be applied to each tube r I have included a dotted version of curve 80 shifted over so that the operating point of thiscurve intersects the operating point on curve 8!. Now, assuming that the input signal of both of these tubes swings around this single operating point, the correct amplitude of input signal for each tube can be plotted. If; the input signal for tube 58 has an amplitude similar to the amplitude of curve 82 then plate current will be drawn by this tube equal to inso- In order to draw the same plate current in tube 49 at the peak of the input signal, a signal having a peak amplitude similar to curve 83 must be impressedupon the grid circuit of tube 49. The difference in amplitude between these two signals is again indicated bythe letter The amplitude of signal .82 is again indicated by the letter g. The

necessary bias for tube 50 is again shown as e and the difference in bias required for tube 50 and tube 39 is again shown by the letter d. I have found that the relationship j/g=a'./e holds true for the majority of tubes which are usually rejected.

After this rather exhaustive explanation, it

can be seen that the unitary control device w th which I control both'the bias voltage'amplitude and the signal voltage amplitude-for each, tube in the parallel bani: is capable of' producing, in fact necessarily produces, theabove mentioned ratios. Therefore, with this simple control device I am able to divide a load current equally between the various tubes without the complexity of the cut-and-try process necessary in the prior art circuit shown in Fig. 1. All the operator has to do, in my novel circuit, is to adjust the potentiometer for each tube so that the same idle current is drawn by allot the. tubes and then apply asignal to the circuit. There is no cut-and-try process involved, because once the idle current is set, the circuit is automatically adjusted to dynamically divide theload current under signal conditions.

While I do not desire to be limited to .any specific circuit parameters, such parameters varying in accordance with individual designs, the following circuit values have been found entirely satisfactory in one successful embodiment of the invention, in accordance with Fig. 2:

Potentiometers ill-t--- 300 ohms Resistance 53,48 8875 ohms Tubes 19; 5t 5, 56, 59,-Federal 9628 or 125A Bias source (6) 3500 volts B+ 12,000 volts While there has been shown and described what is at present considered the preferred embodiment of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made re Without departing from the invention as defined by the appended claims.

Having thus described my invention, I claim:

1..111'2tIl-3l1l131lfi61' the combination comprising arplurality of tubes having e /z' characteristics such that substantially the same ratio exists in each tube between the bias potential and the peak signal potential necessary to cause a given-platecurrent to flow, each tube having at least an -anodaa cathode and a control grid, means connecting the anodes of said tubes directly in parallel, means connecting the cathodes of said'tubes to a common equipotentialplane, an output circuit, coupled between said. parallel connected anodes and said equipotential plane,

pled across said first and second terminals,.and a source of bias voltage having a positive terminal D.-C. coupledto said second terminal and a negative terminal D.C. coupled to said first terminal, whereby the amplifiers are adjusted for dynamic load current division by adjusting the potentiometers so as to balance the idle current among the tubes,

2. In an amplifier of the balanced parallel tube; type, the combination comprising a first terminal and a second terminal, a plurality of potentiometers each having a resistance element potential necessary to cause a given'plate current to flow and each tube having at least acontrol grid, an anode, and a cathode, each of said tubes being associated with a separate potentiometer, said anodes being connected to a first common point and said cathodes being connected to a second common point, a source of anode potential and a load means series coupled between said first and said second common points, means for D.-C. coupling each one of said plurality of control grids to the associated potentiometervariable contact and means for coupling said second terminal to said common cathode point, whereby the amplifier tubes are adjusted for dynamic load current division by adjusting the potentiometers so as to balance the idle current among the tubes.

3. In an amplifier of the balanced parallel tube type the combination comprising a first terminal and a second terminal, potentiometer means connected between said first and second terminals having a plurality of variable contact arms, a source of signal voltage coupled across said first and second terminals, a source of bias voltage having a negative terminal D.-C. coupled to said firstterminal and apositive terminal D.-C. coupled to said second terminal, a plurality of amplifier tubes each having e /i characteristics such that substantially the same ratio exists in each tube between the bias. potential and the peak signal potential necessary to cause a given plate current to flow and each havin at least a control grid, an anode, and a cathode, each of said control grids being connected to a separate variable contact arm on said potentiometer means, circuit means connecting said anodes and cathodes in parallel tube relationship, and means D.-C. coupling said second terminal to said cathodes, whereby the amplifiers are adjusted for dynamic load current division by adjusting the potentiometer contacts so as to balance the no-signal idle current among the tubes.

RONALD J. ROCKWELL.

References Cited in the file of this patent UNITED STATES PATENTS McCarty July 3, 1951

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