K6YP: Voltage and cathode keying
I much enjoyed the article on the L'il Buddy Acorn Tube QRP Transmitter in the November issue of WorldRadio. It made me explore my junk box to see if a surplus 955 was lurking there. No luck. I'll guess a 6C4 will have to do.
I am not sure that the 90-150 volt plate supply discussed in the articles likely to kill anyone. On the other hand, it will certainly bite a person enough to mess up his sending with a straight key. You mention that the W6ESL cathode keying modification removes the high voltage from the key's terminals. IT DOES NOT. The tube may be looked at as a constant current device in series with the plate power supply and key.
Placing the key in the B+ circuit leaves a dangerous situation if one touches both sides of the key. Placing the key in the cathode circuit still leaves essentially the same dangerous voltage across the terminals of the key. In both cases the the 955 triode behaves as a constant current mechanism limiting the available current. Placing the key in the plate circuit IS more dangerous because if one touches the B+ side of the key and ground there is no current limiting mechanism in the circuit.
Thus the full current available from the plate voltage supply is available to wake up the unwary operator who touches the key and ground at the same time.
The solution is to cathode key the 955 with (gasp) a high voltage NPN transistor or Hex FET switch. If this is done, the base or gate of the keying device turns the 955 on and off. The voltage in the keying circuit will be less than 5 volts and one can concentrate on the contact without worrying about being bit by the presense of
dangerous voltages on or across the key.
Again, I emphasize that the danger associated with this low power circuit is not great. However, cathode keying a 6146 with 700 volts on the plate really IS VERY DANGEROUS. I believe it is appropriate to alert your readers that cathode keying without the use of a transistor switch or keying relay is a dangerous practice.
Best regards,
John Eisenberg, K6YP
Los Altos, CA
I am not sure that the 90-150 volt plate supply discussed in the articles likely to kill anyone. On the other hand, it will certainly bite a person enough to mess up his sending with a straight key. You mention that the W6ESL cathode keying modification removes the high voltage from the key's terminals. IT DOES NOT. The tube may be looked at as a constant current device in series with the plate power supply and key.
Placing the key in the B+ circuit leaves a dangerous situation if one touches both sides of the key. Placing the key in the cathode circuit still leaves essentially the same dangerous voltage across the terminals of the key. In both cases the the 955 triode behaves as a constant current mechanism limiting the available current. Placing the key in the plate circuit IS more dangerous because if one touches the B+ side of the key and ground there is no current limiting mechanism in the circuit.
Thus the full current available from the plate voltage supply is available to wake up the unwary operator who touches the key and ground at the same time.
The solution is to cathode key the 955 with (gasp) a high voltage NPN transistor or Hex FET switch. If this is done, the base or gate of the keying device turns the 955 on and off. The voltage in the keying circuit will be less than 5 volts and one can concentrate on the contact without worrying about being bit by the presense of
dangerous voltages on or across the key.
Again, I emphasize that the danger associated with this low power circuit is not great. However, cathode keying a 6146 with 700 volts on the plate really IS VERY DANGEROUS. I believe it is appropriate to alert your readers that cathode keying without the use of a transistor switch or keying relay is a dangerous practice.
Best regards,
John Eisenberg, K6YP
Los Altos, CA
posted by Richard Fisher, KI6SN at 6:15 PM
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