To the Restoration Page
Main Front Page
Some Background Info.
Making It Work
Other Folks in Other Places
The battery portables collection
The Boatanchors Collection
Vintage Test Gear
Last updated 13th March 2003
Troubleshooting The Audio Output Stage
The output or sometimes called the power-amplifier stage follows the detector, and is the last stage in the signal path of the receiver. The signal level from the preceding detector stage would be about sufficient to operate a pair of headphones, so this signal must be amplified to a sufficient level to drive a loud speaker. The audio output stage accomplishes this. To get an idea of the signal levels handled by the output stage, a 6V6-G tube will give an output of 4.25 watts maximum with an input signal of 12.5 volts peak on the grid. A smaller input signal gives a smaller output; 12.5 volts is the maximum the tube will handle without producing undesirable distortion. Other output tubes will have different ratings.
Theory of Operation:
Grid Load Resistor - The input signal if fed from the plate circuit of the preceding detector stage through coupling capacitor C-32 to the grid of the 6V6 tube. The signal is impressed across grid load resistor R-12. This is usually a high value resistor in the range of 500,000 ohms or so. A lower value will result in lower gain of the stage, but better frequency response, while a higher value gives slightly higher gain at the sacrifice of tone quality.
Self-bias - The tube must
have the correct bias voltage on the grid in order to
give the best amplification and sound quality. To
maintain the proper bias, the grid must be made negative
with respect to the cathode. Resistor R-13 in the
cathode circuit accomplishes this. To see how this is
accomplished, assume no signal input and examine the
schematic diagram below. Tracing the current flow in the
tube, the current drawn by both the plate and screen
circuits must flow through cathode resistor R-13.
A voltage drop is developed across this resistor, and
because of the direction of the current flow, the ground
end is negative and the cathode end is positive. Since
the grid is returned to ground through grid load resistor
R-12, the grid will be negative with respective to
the cathode by the amount of voltage that is developed
Self-bias By-pass Capacitor C-13 - When a signal is applied to the grid of the tube, this cause the current through the tube to vary in step with the signal. This changing current will also cause the voltage developed across the cathode resistor R-13 to vary, causing the bias voltage on the grid to vary also. Since we want this bias voltage to remain constant, some way must be provided to by-pass the signal around R-13. The by-pass capacitor accomplishes this. To be effective the value of the capacitor should be such that is has a very low impedance to the audio frequencies. This capacitor is usually a low-voltage electrolytic type in the range of 5 to 35 mfd. This by-passing or filtering action then maintains a constant bias voltage on the grid.
Fixed Bias - In a stage that uses fixed bias, the cathode is grounded and the negative bias voltage is applied directly to the grid through the grid load resistor. This bias voltage is normally develop in the receive power supply.
Output Capacitor C-12 - Pentode and beam power tubes introduce a considerable amount of harmonics, which are most notable in the high AF range. Capacitor C-12 across the output by-passes some of these higher frequencies to ground preventing them from reaching the output transformer. The effect is great at the high frequencies as the impedance of the capacitor decreases as the frequency increases. Therefore the harmonic content will be reduced by the action of this capacitor.
Output Transformer T6 - The function of the output transformer is to couple the output circuit of the tube to the speaker. The high output impedance of the tube must be matched to the low impedance of the speaker to get an efficient transfer of the audio signal. The average impedance of the 6V6 tube is around 5,000 ohms, while the average speaker has an impedance of 4 to 8 ohms. This impedance match is accomplished by the turns ratio of primary to secondary of the output transformer.
Hum-bucking Coil - Because the field coil is used as the filter choke in the power supply, it is subject to the AC ripple of the power supply. This can cause hum in the loud speaker. The hum-bucking coil is wound on the speaker pole piece next to the field coil, and is connected in series with the speaker voice coil. The hum-bucking coil is connected so as to oppose the AC ripple in the field coil and cancel the hum component. If the leads to the hum-bucking coil should accidentally be reversed, the hum level will be notably increased.
Output Tube 6V6 - The 6V6 used shown in the circuit here is a beam power output tube. A beam power tube is a tetrode or pentode in which directed electron beams are used to increase the power handling capability of the tube. These tubes employ beam-confining electrodes which are metal plates located either side of the grid elements. These beam plates are tied to the cathode and prevent stray electrons from returning to the screen grid by guiding these stray electrons on to the plate.
There are several
troubles that are associated with the audio output stage,
the most common being; no or low output, poor audio
quality, and distortion.
ęBill Harris 1997