Ken,
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The answer to your question is yes. ?Using the external Dow Key auxiliary switch in the open position will provide sufficient isolation to all of the active devices of the receiver front end. ?The switch opens upon applying 115 VAC transmit transition to the Dow Key.
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Explanation:
With the mute line (a shielded center conductor of coaxial cable) connected to one of the Dow Key auxiliary contacts with its other connection to the shield of the coaxial cable as a return to ground at the 2-B and that aux switch closed during receive mode, hence creating an externally controlled ?ground, the 2-B in STANDY mode is not muted. ?When the aux switch is open, the 2-B is muted, completely. ?Reviewing the 2-B schematic, this external ground provides a ground to the gain bias negative DC voltage divider circuit, reducing the AC-DC rectified negative voltage to a level established by the gain and bias potentiometer resistors that have been set properly for nominal (best) receive operation. ?When that ground is removed that circuit no longer has a return current path to ground, no voltage division occurs, and AVC-2 instantly goes to maximum magnitude negative voltage biasing the receiver vacuum tubes in the front end OFF, completely. ?This is normal operation. ?You can follow this explanation in the attached schematics I found online. ?(My 2-B manual has gone missing.)
Now, let¡¯s say an EMP was produced by a rouge entity, or even a very unusual cosmic event from the Sun, ?with those type of signal levels unknown because they haven¡¯t happened or recorded happening. ?There is a probability the receiver will survive, but not sure. ?The peak voltages of the EMP traveling wave coupled to the antenna and through the feed-line (if breakdown doesn¡¯t occur there) could be above rated inter-electrode elements¡¯ ratings and electron breakdown (arc) could occur in the front end first amplifier tube.
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As a side note to your question, you may be interested in a better T/R switch. ?The 1970 edition of the ARRL Handbook has a nice electronic T/R design with instructions to build using two vacuum tubes, one is a low gain voltage amplifier and the following that is a cathode follower, to complete a nice isolation amplifier. ?The output of the transmitter is connected to the input of the voltage amplifier with a grid-leak bias resistor connected at its input. ?When a large signal (signal greater than normal receive signals) with large positive voltage peaks such as the transmit signal occurs at the control grid, electron grid leak instantly occurs through the resistor, biasing that voltage amplifier off, completely. ?Hence, the antenna and transmitter are completely isolated from the receiver. ?The control is instantaneous providing a full break in keying arrangement. ?Another bonus to this T/R switch is that it provides another stage of selectivity for each band to the 2-B or any receiver; the WARC bands are not included in this selectable tuned circuit stage and would need to designed and added if desired. ?The parts should be obtainable; searching will be required. ?Using this T/R switch, another external means of muting would be needed to keep the receiver silent during transmit. ?It may be acceptable to listen to your transmit signal, weakly leak-coupled through both chassis¡¯, from the transmitter to the receiver, as others have used as an automatic side tone oscillator for CW transmissions. ?Note that this T/R switch would protect a receiver front end from RF EMP, possibly with the expense of the grid-biased voltage amplifier tube in its first stage. ?See the attached information from the ARRL Handbook.
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I¡¯ve included the extra information because you indicate that want to protect your 2-B from damage, and I don¡¯t blame you. ?To be clear, I¡¯m not speaking of EMP such as lightening; there is way too much energy contained in a lightening strike to protect the receiver. ?Destructive currents travel EVERYWHERE in that case.
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73, Michael Smith, N4KZO
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