The phase inverter
The phase inverter used is of the type in which the out-of-phase voltage is obtained from the cathode circuit. Actually, the 25,000-ohm resistor at the ground end of the 6J5 cathode is really part of the plate circuit. Where additional amplifier gain is not needed, this form of phase inverter is preferable from the standpoint of simplicity to the double-triode type, and was used for this reason. Notice the 250,000-ohm resistors connected from the plates of the 6L6-G tubes to the plates of the...
A Decirel Nomograph
a handy nomograph is one relating decibels to voltage or power ratio. The three variables are input, output and decibels. In the figure, the left-hand scale is calibrated in values from 1 microvolt to 100 volts in two sections, A and B. The right-hand scale indicates from .5 volt to 500 volts. The center scale shows decibels in two sections, C corresponding to A and D corresponding to B. As the nomograph stands it indicates voltage gain or loss, but since current varies directly with voltage in...
Resistancecoupled amplifiers
Resistance-capacitance coupling is employed with triodes as well as pentodes. Figs. 1 and 2 are actual circuit diagrams of single stages of resistance-coupled amplification employing triode and pentode tubes respectively. It will be noticed that there is a resistor in the plate circuit of each tube. This is the plate resistor and is designated by the symbol, Ri. Also attached to the plate is a condenser which has its opposite end attached to the grid resistor and the grid of the following tube....
Rsout Toi Controls
he human ear responds differently to sounds of different frequencies and intensities, and attempts to compensate for losses in high-and low-frequency gain which are common in most types of audio amplifiers. It is necessary, for satisfactory listening, to compensate for these deficiencies of the ear and of electronic devices. The frequency-response characteristics of an amplifier may be altered to suit the most critical listener. Any method used to control the frequency response of an amplifier...
Sj
Fig. 23 above This circuit employs negative feedback direct to pickup. Fig. 22 left Negative feedback for tone control R 50,000 ohms, C 8 pf . Fig. 23 above This circuit employs negative feedback direct to pickup. Fig. 22 left Negative feedback for tone control R 50,000 ohms, C 8 pf . A procedure seldom employed but nonetheless effective is to use negative feedback direct to the pickup, arranging the circuit so that the amount of feedback is proportional to the pickup's impedance. This results...
Advantage of inverse feedback
Several types of distortion are likely to be generated within the vacuum-tube amplifier stage. One advantage of inverse feedback is that it tends to reduce the percentage of distortion present. Let us assume that D represents the percentage of distortion present in a stage having a voltage gain of A without feedback. Now if B is taken as the feedback factor, when feedback is introduced into the circuit, we find that although distortion is still present in the circuit it will have a value of D'....
TransformerCoupled L s
This amplifier was designed for the reproduction of musical programs with a high degree of realism. The amplifier had to be more than just ordinarily good, as its ultimate purpose was to please the discriminating music lover. In the tube line-up, triodes are used, with the exception of the output stage which uses 6L6-G's in push-pull with inverse feedback. High-fidelity audio transformers are used in input and output of the Fig. 45 Appearance of the finished transformer-coupled amplifier. Fig....
Info Elr
Thus the input of zero level to the microphone is lowered to 60 db in passing through the microphone and the power that the amplifier begins with is very small. The entire gain is therefore 96.4 db as the amplifier ends up with a 36.4-db level. In amplification work it is desirable to know to what level above zero the amplifier will raise the sound of the speaker's voice, and, therefore, the maximum reading on the control should be 36.4 db and not 96.4 db. A high-gain amplifier when used with a...
Info Vsq
or the output W0 equals .6 watt. After all, the decibel gain is not so important. It is the decibel level above zero that counts. Table I lists the efficiency for certain decibel gains or losses. The table's use can be demonstrated by means of the following examples. It is well to point out that the table may be used for any value of decibel gain. Suppose the efficiency at 15-db gain is to be found. Fifteen decibels equal 10 db plus 5 db, but the resulting efficiency is the product of the...