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Output Transformers - Part I and II (Audio 1960/02/03)

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Category: Audio USA

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Output transformers - Part I and II
An easy-to-understand discussion of the factors that affect the performance of all output transformers.
Author: JAMES MOIR (Technical Director, Goodmans Industries, Ltd., Wembley, Middx., England).
AUDIO, FEBRUARY-MARCH, 1960, VOL. 44, No. 2-3 (Successor to RADIO, Est. 1917).

   The output tubes in common use in audio amplifiers require anode loads of between about 1000 ohms and 10000 ohms if the maximum undistorted power output is to be obtained. There are practical difficulties in winding a loudspeaker voice coil with the large number of turns of fine wire required to achieve such high load impedances directly and thus it is common practice to insert an output transformer between a speaker of low impedance and the output valves as in Fig. 1 in order to "match" the speaker to the valves. The ensuing discussion is intended to be a simple explanation of this matching process and of all the factors that control the frequency response and the distortion introduced by an output transformer.

Fig. 1. Output transformer used to obtain the correct load impedance and isolate the voice -coil current from the plate supply.

   Loudspeaker voice coils can be wound with sufficient turns to give an impedance of 3 ohms to 4000 ohms and, in fact, these were common in the very early days of radio.

Read more: Output Transformers - Part I and II (Audio 1960/02/03)

Wzmacniacz sieciowy (Mains powered amplifier) "5L"

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Category: Radioamator

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Mains powered amplifier "5L"
Radioamator, Year III, February 53, No 2
MSc. Czeslaw Klimczewski

   In the previous issue of our magazine, a description of the installation of the loudspeaker pickup working in the "push-pull" system was given. The so-called "phase reversal" was obtained in this pickup by means of a vacuum tube, not a transformer. Thanks to not using a transformer and a low-frequency choke, the cost of the amplifying attachment is relatively small and the difficulties in its assembly are also not very high..

   Now, developing the scheme of the amplification box, a full 25 watt amplifier will be described, which has a wide range of applications.

   As we can see from the schematic diagram shown in Fig. 1 - this amplifier is adapted to be powered with alternating current from the mains with different voltages (from 220 V to 110 V). The first two tubes from the "input" side to the amplifier, i.e. the 6SC7 and 6AC7 tubes - play the role of the so-called "preamplifier", in which the voltages received from microphones, adapter or receiver obtain the appropriate height in order to properly drive the final stage, consisting of a 6SN7 tube reversing the "phase|" and amplification, and two 6L6 tubes operating in push-pull arrangement, providing the necessary power to power the loudspeakers.

Read more: Wzmacniacz sieciowy (Mains powered amplifier) "5L"

The Vacuum Tube as an Amplifier (Radio 7/1922)

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Category: Radio USA

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The Vacuum Tube as an Amplifier
By B.F. McNamee
Radio for July, 1922

   The type of vacuum tube used as an amplifier is the "hard" or high vacuum tube. The vacuum is so nearly perfect that there is no noticeable effect of gas molecules; in fact for practical purposes it may be considered a perfect vacuum. The "soft' tube, that is one  containing a certain small pressure of gas and intended for use as a detector, can be used to a certain extent for amplification purposes. However this almost always results in distortion which would prohibit its use as an amplifier in radio telephone work, and its use is also restriced because it will not work with high plate voltage, thus limiting the strength of signals that can be obtained.

Fig. 1 and Fig. 2

   The hard tube therefore is the only one that will be considered in this article.

Read more: The Vacuum Tube as an Amplifier (Radio 7/1922)

Device for testing electron tubes - Przyrząd do badania lamp (Radio dla Techników i Amatorów 1949/10)

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Category: Radio dla Techników i Amatorów

Hits: 1976

Device for testing electron tubes
(Radio dla techników i Amatorów, Październik 1949, Rok IV, Nr 10)

   In our monthly magazine, we have not yet described an important and basic instrument, which is the practice of a radio amateur and radio technician - a device for testing electric lamps. Twice, however, such a device was described by our brotherly weekly "Radio i Świat", namely in 1945 No. 15 entitled "Instrument for testing the emission of electron tubes" and in 1947 No. 36/37 entitled "Instrument for testing electron tubes". Both of these apparatuses made use of the same principle, shown in Fig.1. The mains transformer has a secondary winding of the filament of the vacuum tube and some additional winding giving an effective voltage of up to 20 volts. The end of this winding is connected, through a limiting resistance of 500 ohms, protecting against the effects of possible short circuits or overloads, and a DC milliammeter - to the anode and other high-voltage electrodes of the tested electron tube. Other electrodes, such as the control grid, are shorted to the cathode, which in turn has a common point with one glow pole. When a vacuum tube is inserted into a suitable socket, a one-way current will flow through it after heating up and cause the milliammeter to deflect. The above mentioned descriptions are accompanied by tables of "normal" deflections of more electron tubes.

Fig. 1. Principle of operation of the most primitive device for testing electron tubes. All electrodes are connected either to the anode or to the cathode. A one-way rectification system is obtained, and the device measures the rectified current, which depends to some extent on the emissivity of the cathode. The shortcomings of this instrument are discussed in the text.

   Instruments of the type shown in Fig. 1 operate on the principle of one-way rectification. Each electron tube, regardless of its proper purpose, is of course capable of rectifying, and it does so in a manner dependent to some extent on its "emission". Of course, there is no need to emphasize that the system in which we examine vacuum tubes is not even roughly similar to the conditions in which the vacuum tubes we use work in amplifiers, receivers, oscillators, etc. It even happens that we do not see a case at all any vacuum tube was ever supposed to work under such or even similar conditions.

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Circuit Sentry Protects Tubes in P.A. Amplifier (Audio September 1960)

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Category: Audio USA

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"Circuit Sentry" Protects Tubes in P.A. Amplifier
J. LEVITSKY - Chief Engineer, Fanon Electrobic Industries
Audio September 1960, Vol. 44, No. 9

Simple protector circuit added to conventional amplifier prevents damage to output tubes in case of shorts on the loudspeaker line.

In most commertial and industrial public address systems utilizing fairly high power amplifiers, break-down of the amplifier output tubes often results from a short or a severe overload of the speaker line. In many such systems,  the amplifier feeds power via the 70-volt line to numerous speakers distributed over wide areas, each speaker being provided with its own separate matching transformer. Under such conditions, due to the long runs of line and a large number of components connected across it, partial or complete shorts may occur rather frequently.

The severity of the problem can be seen by a glance at the data in Table 1. This data was taken with the Fanon 70-watt amplifier (model 3370), employing two EL-34 power output tubes, operating in class AB₁. Columns 1 and 2 show the audio power output for different levels of input under normal trouble-free conditions. Column 3 shows the corresponding power dissipations per tube under the same conditions. Column 4 shows the tube dissipations for the same levels of input signals, with the 70-volt line shorted to ground. Since the average audio power output of a P.A. amplifier may be somewhere between 25 and 30 per cent of its peak output, when signal is being applied, the data in column 4 indicate that if a short occurs in the speaker line, each tube is dissipating roughly three times its maximum rated power. Even if a high-resistance short takes place, say about 25 per cent of the rated load, the dissipation in each tube is much higher than the maximum allowable, as shown in Fig. 4.

Read more: Circuit Sentry Protects Tubes in P.A. Amplifier (Audio September 1960)

RADIO (USA)) 11/1921

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Category: Radio (USA)

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Heat her with A-P Tubes
(To receive our daily concerts from the California Theatre, and enjoy them completely, use A-P. tubes and De Forest Inter-panel C. W. equipment. At your dealer or direct from us. Write for catalog.)
ATLANTIC-PACIFIC RADIO SUPPLIES CO.

• Radiotorial Comment.
• The New Federal Arc Station at Palo Alto.

Main Power Switchboard

• The How and Why of Radio Tuning.

Variation of Radiated Wire Lenght with Frequency of Vibration

• Cutthroat Competition in General Public Business.

The Great Northern Fisheries of San Francisco were in receipt of the following telegram:
"Send twelve audion bulbs; terrific explosion occured last night. In addition, ship complete set of meters for power house, one armature, new leftside engine flywheel and sixty fell belting. Nosey Olsen hanging around when explosion occurred. Notify Swedish-American Life Insurance Co."

• Read more: RADIO (USA)) 11/1921

Universal Feedback Amplifier Circuit (Audio January 1960)

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Category: Audio USA

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Universal Feedback Amplifier Circuit
ARNOLD J. KAUDER (Principal Engineer, Bendix Aviation Corporation, North Hollywood, California)
AUDIO, January, 1960, VOL. 44, No. 1
A simple amplifier of exceptional performance which should be adequate for practically any installation is the basis for this article, but its greatest value lies in the "universal" instructions for adjusting any feedback amplifier.
(Note: The original notation of units used at the time of writing the article has been preserved.)

   The amplifier to be described has performed well with five different output transformers, which has led the writer to use the designation "universal." The amplifier has in each case been found completely stable with (a) no load, (b) 8-ohm resistive load, (c) 8-ohm loudspeaker load, and (d) a 0.1 μF capacitor load added to any of the load conditions of (a), (b), or (c) above. The feedback factor employed has been 20 db ± 1 db.

   Few of the "Williamson Type" and other amplifiers seen by the author have been capable of meeting such a stability test. Breathing of the loudspeaker cone, due to very-low-frequency oscillations, and supersonic oscillations readily seen on an oscilloscope are all too common. Either type of oscillation can produce negative charges on the grid sides of the output-tube coupling capacitors, with distortion and limited power output resulting. Marginally stable amplifiers have also been observed which do not normally oscillate, but are highly regenerative at extreme frequencies and do oscillate when audio signals with steep leading edges on the waveforms are fed to the input terminals.

   A brief history of the development of the circuit is believed to be of interest and is as follows:

Development

   The author was a "high fidelity" fan many years ago and is still not ashamed of the performance of a class-A push-pull 2A3 triode amplifier (power output of 7 watts) still on hand. After a lapse of 10 years, a reviewed interest in high fidelity led to a study of feedback and the present day amplifiers which have achieved recognition in the literature. The writer found to his annoyonce that it was not possible to duplicate a published amplifier circuit and employ a different output transformer and a more compact layout - unless extensive redesign of the coupling and feedback circuits was carried out.

Read more: Universal Feedback Amplifier Circuit (Audio January 1960)

Amplifiers with Positive and Negative Feedback (Audio 1961/04)

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Category: Audio USA

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Amplifiers with Positive and Negative Feedback
CHARLES P. BOEGLI (Product Planning Manager, Bendix Corporation, Cincinnati, Ohio)
Audio, April 1961, Vol 45, No. 4

Contrary to a widely held belief, this author discovered that the cathode-coupled phase inverter (“long-tailed pair”) introduces a significant amount of distortion. By including this stage in the negative feedback loop he achieved an unusually low-distortion amplifier.

   Several years ago, the writer had two articles¹ published on the design and construction of audio amplifiers utilizing over-all negative feedback with internal positive feedback. A number of readers constructed these amplifiers and satisfaction was the general result.

   Those who are interested in the details of these amplifiers should refer to the original articles. Several difficulties were encountered with the circuits, primary among which were:

1. The output transformer was not designed for the manner in which it was operated.
2. The output transformer secondary was at a small d.c. potential above ground.
3. The inverter (the first stage of the amplifier) was not included in the negative feedback loop, so that the distortion introduced by this stage appeared undiminished in the output.

   Both amplifiers used ordinary output transformers with the secondaries connected in unusual fashion. The speaker lines were connected to the 0- and 16-ohm taps of the secondary and the 4-ohm tap was grounded (for a.c.), so that a balanced output was being drawn from a transformer intended for unbalanced operation. The output transformer was carefully specified, and those who were foolhardy enough to construct their amplifiers with other transformers usually paid the penalty of instability or oscillation. For some time, the reason why one transformer worked well while another did not, remained a mystery, but it was thought that unbalanced capacitances between each end of the winding and ground might be responsible.

   One hundred per cent negative feedback was obtained by connecting the ends of the secondary directly to the cathodes of the driver tubes. Internal positive feedback was brought from each driver plate to the grid of the other driver. Bias for the drivers was obtained by inserting a bypassed resistor between the center tap (that is, the 4-ohm tap) of the output transformer secondary and ground, so that the entire secondary was at a d.c. potential equal to the bias on the driver cathodes. If a speaker line became shorted to the chassis of the amplifier, the bias was disturbed, and oscillation usually occurred. Nevertheless, speaker lines are usually not grounded, and this did not prove to be a very great shortcoming.

Read more: Amplifiers with Positive and Negative Feedback (Audio 1961/04)

Push-Pull in HiFi (Audio 1959/4)

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Category: Audio USA

Hits: 3601

Push-Pull in HiFi
Author: MANNIE HOROWITZ
AUDIO, APRIL, 1959, VOL. 43, No. 4 (Successor to RADIO, Est. 1917).

The push-pull amplifier has become standardized as the optimum circuit arrangement for providing adequate power output with a minimum of distortion - so long as the tubes are used under proper conditions. The author makes the performance of this type of amplifier thoroughly understandable.

   The push-pull power output stage can be studied from many angles. A theoretical discussion on composite tube characteristics is interesting and informative. A survey of the practical applications of different push-pull or driver circuits is an important asset to any audiofan's library.

   In this article, several refinements in push-pull circuits will be discussed. These refinements are frequently designed into the amplifier on an intuitive basis rather than a scientific one. The importance of a scientific analysis rather than instinctive motivation can be well appreciated by the serious hi-fi enthusiast.

Graphical Analysis

   A typical self-biased triode push-pull output amplifier is drawn in Fig. 1. Everything discussed about this triode refers to the pentode as well – but to an even larger degree due to the greater curvature of the tube characteristics.

Read more: Push-Pull in HiFi (Audio 1959/4)