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Heater Concerns
Now, where you live might help you decide on how many output tubes to use. In Japan, 8 would be a good choice; but in the United States, 10 would be the better choice. Why? These numbers lend themselves to a nice trick. One of the problems with any power amplifier is having to feed the heater string. The EL509 heater draws 2.5 amps. which means given 10 tube output stage, we will need a 25 amp at 6.3 volt tap on the power transformer. They make such transformers, but they are huge and the wire coming out of them is usually something like 10 solid-core (definitely a headache to work with).
Now, if all 10 tubes heaters are placed in series, the total voltage requirement would be 63 volts and the current draw per heater string would be only 2.5 amps. If the heater string from one channel is placed in series with the other channel's string the current draw would still be 2.5 amps, but the total voltage requirement for the string would be 126 volts. 126 volts is very close to the 120 volts we have coming out of our wall sockets in the United States. But if 8 output tubes are used per channel, then the total series requirement for this heater string would be 100.8 volts, which is very close to the 100 volts wall voltage in Japan. (Understand that running the heaters directly off the wall voltage is not the same thing as running the amplifier's rail voltage directly of the wall voltage as in some of the Futterman amplifier and many cheap tabletop radios from the 1950s.)
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I would never trust the wiring to be correctly set up throughout the house's wall sockets and in all the audio gear that hooks to the amplifier to forgo the safety provided by using a power transformer. With the heaters, I am much less nervous. The heater must be insulated from the cathode to prevent the heater voltage from contaminating the audio signal. Furthermore, in spite of dealing with hundreds of tubes, I never have found a tube that went bad because its heater shorted to its cathode. Open heaters, yes; shorts, no. This brings up the subject of Christmas tree lights.
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The old style string of Christmas tree lights had a major defect: when one bulb burnt out, it took much effort to find it, as all the bulbs were in series, so all the bulbs went dark. How do we spot the bad tube with an open heater out of a group of 20? The solution is to wire a neon bulb in parallel with each heater; thus, if a heater opens, the neon bulb will see the full 120 VAC and light up. While all heaters are working, the 6 volts across the neon bulb is not nearly sufficient to ignite the bulb. An alternative to the neon bulb would to use a LED in series with a small valued disk capacitor (0.1 µF).
Output Stage Design
Power output tubes are driven hard and often the grid is driven positive relative to the cathode, which causes the grid to conduct current from its grid resistor. The voltage the develops across this resistor shifts the bias voltage for the output tubes. The usual solution is to use a fairly low valued grid resistor. However, if the resistor value is too low, the previous stage will be excessively burdened. Other solutions are to use an interstage transformer or coupling capacitor and choke or a Cathode Follower in place of the usual coupling capacitor and resistor combination.
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