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by Joseph Giovanelli
Low Plate Voltage
Q. I have two old tube-type stereo amplifiers which apparently share a similar problem. At first the problem manifested itself by the plates of the output stages glowing red-indicating excessive plate current. After replacing tubes, measuring voltages, and pondering, the main problem in one was traced to a shorted capacitor and in the other to a dirty bias adjusting pot. Still the voltages are out of spec and beyond the range of available adjustments.
With all the audio tubes removed and the rectifier tube left in, I have lots of volts, but insertion of the output tubes brings the voltage to below specified levels. This would indicate that I still have a plate current problem. Why?
-Fred Portnoy, Owings Mills, Md.
A. With old equipment as you are presently restoring, it is never a good practice to operate it with the rectifier plugged in but all the other tubes removed. The filter capacitors will charge up to peak voltage which, with many amplifiers, will be close to the capacitors' breakdown point, even when they are new. You are flirting with disaster by operating the amplifier as you described.
Vacuum tube amplifiers are designed so that if the operating voltages are 10 percent low, they are still considered within specifications. If the voltage is substantially lower, then something is wrong. It may be that the output stage is drawing too much current, perhaps not enough to make the plates glow red, but still enough to cause the power supply to collapse under the load. It may also be that the capacitance of the filter capacitors is low. There may be sufficient capacitance to keep hum at a relatively low level, but not enough to keep the voltage up to specs.
It may also be that the rectifier tube is low in cathode emission. When called upon to supply the full current, the internal resistance of the rectifier may be high enough to produce an excessive voltage drop within the tube itself which would account for the low operating voltages.
Some amplifiers, including yours, employ fixed bias and the rectifiers increase in internal resistance with age, which would result in low bias applied to the output stages. This results in an increase in plate current and could be the cause of your problem. Bias is often developed via a voltage divider network. Check the value of the resistors to see if they have changed in value, for if they have, the bias adjustment controls may not be enough to compensate for this problem.
Q. My problem is with a new tuner.
I am receiving stations where there are none. There are three stations which I get all at the same time. At 95.7 mHz, I am getting 94.3, 92.1 and 97.9 mHz. What type of problem is this? Can I do anything about it? Is the tuner at fault? Is this what happens when one is close to strong signals?
-R.J. Patterson II, Lake Park, Fla.
A. Solid-state technology has brought us marvelous equipment, but we have also gotten some disadvantages. One of them is that sensitive solid-state r.f. amplifiers will overload much more quickly than their tube counterparts. The r.f. stages of the solid-state tuners are the most likely sources of the overload you described, which means that you hear more than one station at a time and on a frequency not occupied by any of the received stations. Sometimes these signals are so strong that by the time they are attenuated sufficiently to do any good, other signals have all but disappeared. It might be worthwhile writing to the equipment manufacturer to see if he has any modifications to reduce the magnitude of this problem.
If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.
(Source: Audio magazine; Apr. 1977)
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