An impression. This is a very cool transmitter & receiver. This unit is built so well, they always work unlike Ham radio vintage gear. The transmitter is rated down to 2 mHz but actually tunes as low as 1840 KHz so covers 160 and 75 meters. As simple as the receiver is it worked quite well. The receiver does not have an RF amp tube and on lower HF frequencies the signal directly into a convertor tube can be better performance wise. I remember having a dozen NOS 1N5GT tubes and one way outperformed the others for the receiver. The tubes are cheap, buy a lot for testing. Findatube in Ohio sells NOS for $3.50 each. I also use a West Mountain DNR speaker that makes a huge difference on receive- IMO mandatory on this or any 5 tube receiver. I built a power supply and the transmitter output is about 8 to 10 watts modulating to about 35 watts using new transmit tubes. I used a Shure 104C carbon mic that worked, but replaced the element with a telephone carbon element and it had much better high frequency. I used a SS 200 watt amp to boost the power.
The power supply is built using an Antek AS-2T200 transformer ($52). TBX-8 uses 500V, 90V, -6V, -1.5V and -12 volts DC with the -12 using mod below (vs normal +12 volts). A VR tube supplies a regulated 90 volts and LM type IC regulators for other lower voltages except the transmit tube filament. I use SS bridge for 500 volts and HV winding center tap for 90 volt regulator tube with a series resistor.
MODIFICATION- the TBX-8 needs minus and plus low voltages for tube bias, receive filaments and positive for the mic and transmit tube voltages. This would require two separate 12 volt transformer windings. A better alternate is to use one winding and change the transmitter tubes and mic voltages to minus. On the right side of the transceiver change the two 25uF from negative to ground to positive ground. (Best to replace with 220uF caps). Now you can use all negative low voltages. An Antek AS-1T200 with the two 6.3 volt windings in series works well. Place filament windings in series using four 1N5322 set up as a bridge rectifier and use a low voltage drop out LM2990 IC for 12 volts Mic only voltage and use the Antek low voltage windings center tap into a 4700uF cap for the LM-337 IC for receiver filament voltage (-1.5 volts). The lower center tap voltage reduces IC heating. Use 22000uF on the 12.6 volt winding providing 14 volts DC output and then use a 1 ohm in series resistor after the 22000uF capacitor for the transmit tube filament. The transmit tube does not have a cathode, needs well filtered filament DC. I also use a LM7905 for -5 volts (normally -6 volts) bias that connects to the 14 volts DC after the bridge rectifier. Only the LM337 needs a heat sink.
More info here:
https://www.antiqueradios.com/forums/viewtopic.php?t=381718And more here:
'The TBX-8 is a NAVY WWII transceiver that receives from 2.0khz to 8.1khz and transmits 2.0 to 6.0khz. Actually performs perhaps 160 hz beyond band design and is simple to extend to cover the full 160 meter band. The power output is 30 watts CW and 7 watts beam plate modulated AM. Beam plate modulation is the prefered design regarding low level modulation that has good quality audio. The receiver is a superhet design using five tubes for single conversion and not having an RF amp. The use of the signal directly to the converter tube is usually a benefit for such simple designs. Two issues makes this a very usable receiver is the build quality is 'out of this world' and I used a fully regulated power supply. These two items adds to a stability that is just fine for listening to SSB. Fortunately the BFO is rock solid and produces a clean signal of wide frequency adjustment. The 1.5 volt DC filament tubes produce very little temperature rise which adds to the stability (LM-350T regulator produces -80db noise output). I replaced the 1N5GT with a 1P5GT in the BFO section and replaced the 1A5GT with a 1Q5GT for the audio output. The stock audio is about 100mw and the beam tube 1Q5GT produces 350mw. I also changed the audio transformer from 20k to 2500 ohm to 8k to 8 ohm (see AES in Tempe, AZ). The 350mw audio is quite loud into a Radio Shack #21-549B remote speaker. In fact, this speaker compliments the radio as the audio is better than my SB-600. This is not true on my other HF radio. I also installed a .0005uF ceramic capacitor at the 1Q5GT grid to ground to remove some high frequency responce which is not needed or wanted. The transmitter uses a rugged 2E22 with a directly heated filament. I suspect a 6CA7/EL34 could replace this tube if stock dries up in the future. Loading and tuning is simple using the RF current meter. The transmitter has two crystal control positions or a VFO. The receiver is VFO only. The radio has a 'spot' frequency control for tuning the transmitter to the receiver. The radio has a built-in antenna relay and operates full break-in. As for the power supply I converted a Drake AC-4 to provide 500v, 90v, -4.5v, 1.5v, 12v and another 12v RF isolated from the first 12v. All except the 12vdc is regulated. If you want AC-4 conversion details, please email me for a schematic. As for AM power I use a converted tube type CB linear amp. The 7 watts carrier is perfect for this application and a simple bandswitch and new tank coil completes the conversion. I would not suggest solid state as without a tank circuit in the output you may find TVI and other issues present. I find this radio to be very friendly to operate and more of a 'normal' looking transceiver not so far removed from amateur equipment of the same era. For better selectivity, a DSP filter in the CW mode is a perfect addition. The receiver does not require an aftermarket RF preamp as the sensitivity is unusally high for the design. My radio is serial number 383 which suggests less than 400 where manufactured. I was told this radio was one of a few used by the famous WWII Indian 'code talkers'. The code talkers were never successfully decoded during WWII. Their action saved many lives'.
Also- Bama Archive has the manual and includes pin out connection voltages. I just soldered wires to plugs and use Walmart meds covers on connectors.
