The Johnson 250-39 is the best full break-in T/R switch ever made. It has two stages to provide enough gain to overcome the insertion loss and then some. Other contemporaries often only had one RF tube section, which was noisy and caused signal loss, especially on 15 and 10 meters. The 250-39 does not provide receiver muting, since it assumes you are going to want to hear your own CW sending. It does have some correctable deficiencies. I provide you with the fixes here.

The Johnson 250-39 can be used with a TUBE receiver for full break-in CW with any transmitter. DO NOT EVER use it with a SOLID STATE RECEIVER. It does not reduce the transmit signal sufficiently to protect the radio from damage. Some bodacious pulses come through it during the initial switching. See the web site below for proof.

A comprehensive analysis of how it works as well as tests of the Johnson 250-39 has been done by W8JI. He uses a SPICE simulation employing a 6SN7, which he thinks is a close approximation. I would advise you NOT to use a 6SN7 as a substitute for the 6BL7, since the plate dissipation is too low and other characteristics are different. However, his test methods and analysis are good. (I do not rip off other's work, but I do refer you to it, if I believe it is useful. You should spend some time looking at the other areas of his web site.) You should take a look at this for more information on the Johnson 250-39:
https://www.w8ji.com/johnson_tr_switch.htm

One more thing I plan to do is to install strings of fast diodes with a good voltage rating back to back across the receiver port J4. The coupling capacitor C5 blocks any DC from the tube. I think a NTE 558 or 575 diode should do the job. I think 2 or 3 in each string, to make the forward drop higher than anything you would encounter in normal use would prevent cross modulation during receive. I will update this when I do the experimental work to prove this concept. This T/R switch was in common use back in the day, even without the diode protection, with no reports of damage to receivers. Even with the diodes, I still would not trust this scheme to protect a solid state receiver.

The MFJ-1708B-SDR is a solid state reed relay antenna switch that is intended for use with a transceiver and SDR sharing the same antenna. It costs about $100. It uses four MMBD1403 diodes in parallel back to back configuration as receiver input spike protection. The MFJ unit claims to be safe for 100 watt transceivers and solid state receivers. That diode might be a good choice to install on the receiver output port of the Johnson T/R switch. Here is the manual for the MFJ unit:
https://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1708B-SDR&filename=MFJ-1708B-SDR_SDRS%20Manual.pdf&company=mfj

Failure of C5 (short) applies full DC B+ to the receiver antenna coil. I plan to replace it with a 2 KV capacitor. Failure of the tube to cut off due to failure of any of the biasing tricks described by W8JI also could be problematic. The diodes above could be a fail safe second line of defense. I think about these things after spending a lot of time in the aerospace industry.

There is no AC switch. The manual below tells you to use a master AC switch for you whole station (an outlet strip will do). The T/R switch is generally installed behind the equipment with extremely short leads from the transmitter. To switch power on the T/R device would then require reaching in back of the station gear and doing it by feel. This is OK. However, there is no FUSE provided in the Johnson 250-39. I find this oversight bad for safety reasons. The photos show where I installed a fuse inside the case of the unit, after I removed the filter capacitor can.

Long coax leads between the transmitter and T/R switch will prevent proper operation. The manual states there may be problematic lengths of coax, and to experiment, but I have found that less than 6 inches works for me. Reasonable lengths between the receiver and switch will work fine. The manual states that a low pass filter is advised between the antenna and the transmitter or T/R relay antenna port. This is because harmonics of the otherwise clean transmitter are generated by the rectification process that creates cut off bias for the 6BL7 grid. Note that this implies the antenna will be 50 ohms with reasonable SWR, since the low pass filter will burn out with excessive SWR. Failure to observe this specification exceeds the voltage ratings of C2 and the tube. Failure of a coax cable, antenna selector switch, or antenna, as well as antenna tuner misadjustment could also cause damage.

This T/R switch in stock form gets hotter than the Very Hubs of Hell. Two things can be done to help with this. First, remove the 6X4 rectifier and replace it with solid state diodes (1N4007) and a series resistor [value TBD to get voltages to schematic specs]. This eliminates the heat from the 6X4. The 6BL7 is an early black and white TV vertical sweep tube which dissipates a lot of power. This is unavoidable. I painted the screen enclosure inside and out with flat black spray paint. This helped a surprising amount. The bottom photo shows the black paint looks good, just as if the factory did it. Be sure to allow plenty of air circulation.

The image with the test lamp is the set up I used to test the T/R switch before connecting a receiver. I applied full transmit power into a dummy load and saw no light, and observed safe voltages on a scope connected to the receiver port. Not all active T/R switches of this era do this well. Suspect all ham fest swap specials until you have verified their operation by this type of test.

Here is the original Johnson 250-39 manual. Click on the thumbnails to enlarge them.