While the National HRO is easily recognized by a common style, the models are also easily confused. There are many cosmetic and design differences between the various models of the HRO. The National Company was continually refining the design. Besides the common "street" versions, many variations also exist only to fill specific commercial or military requirements. This will be a summary of the more common versions of the HRO.
| Sr | Sr/Jr/M | RAS | HRO-5 | HRO-5A | HRO-7 | HRO-50 | HRO-50-1 | HRO-60 | |
| 2.5V | 6.3V | ||||||||
| 1st RF | 58 | 6D6 | 6D6 | 6K7 | 6K7 | 6K7 | 6BA6 | 6BA6 | 6BA6 |
| 2nd RF | 58 | 6D6 | 6D6 | 6K7 | 6K7 | 6K7 | 6BA6 | 6BA6 | 6BA6 |
| Mixer | 57 | 6C6 | 6C6 | 6J7 | 6J7 | 6J7 | 6BE6 | 6BE6 | 6BE6 |
| HF Osc | 57 | 6C6 | 6C6 | 6J7 | 6J7 | 6C4 | 6C4 | 6C4 | 6C4 |
| 2nd HF Conv | 6BE6 |
1st IF 58 6D6 6D6 6K7 6K7 6K7 6K7 6K7 6SG7 2nd IF 58 6D6 6D6 6K7 6K7 6K7 6K7 6SG7 6SG7 3rd IF 6SG7 6SG7 Det,AVC,(AF) 2B7 6B7 6F8G 6SQ7 6H6 6H6 6H6 6H6 6H6 BFO 57 6C6 6C6 6J7 6J7 6J7 6J7 6J7 6SJ7 AF Output 2A5 42 6V6G 6V6G 6V6GT 6V6GT 6V6GT(2) 6V6GT(2) 6V6GT(2) AF 6J5 6SJ7 6SJ7 6SJ7 6SJ7 Noise Limiter 6H6 6H6 6H6 6H6 6H6 AF Inv/S-Mtr 6SN7GT 6SN7GT 6SN7GT Volt Reg 0A2 0B2 0B2 0B2 Rectifier 5V4G 5V4G 5V4G Current Reg 4H-4C
HRO Sr The HRO Senior series (sometimes known as the HRO-series) embodies an S-Meter, crystal filter circuit with phasing and selectivity controls, and coils sets A, B, C and D which have a bandspread feature. It was availablewith either 2.5 or 6.3 volt tubes.
HRO-C HRO Sr combined with SPC unit (power supply, coil container and loud speaker) in a MRR table rack (24.5" high).
HRO Jr The HRO-Junior series is similar to the HRO-series but does not have the S-Meter or the crystal filter and associated controls. It was shipped with only one general coverage coil set. The Jr was available with either 2.5 or 6.3 volt tubes. (This model must not be confused with the Navy/WWII RAS receiver below.)
RAS The RAS was designed/built for the Navy beginning in 1939 and through WWII. It is basically a HRO Jr with changes required by the Navy. The most significant difference is the 175KC IF that allowed the receiver to be used on 500KC. All of the RAS coil sets are general coverage only.
HRO-M Basically the same as the HRO Sr series except the standard coils were general coverage only. It used glass tubes and a plug-in crystal.
HRO-MX The same as the HRO-M except the crystal is inside the crystal unit housing.
HRO-M-RR The HRO-M supplied for mounting in a relay rack.
HRO-M-TM The HRO-M equipped with a cabinet for tabletop use.
HRO-5 The HRO-5 series is similar to the MX series accept it used metal tubes.
HRO-5T This is the same as the HRO-5 and is equipped with a cabinet for tabletop use.
HRO-5R This is the same as the HRO-5 but is supplied for rack mounting.
R-140/FSM-1 This is the same as the HRO-5R or HRO-MX (rack mount) except for the rewiring of the breakin switch and speaker connections.
HRO-5A1 The HRO-5A1 series is similar to the HRO-5 series accept an extra stage of audio and a noise limiter were added.
HRO-5TA1 The HRO-5A1 supplied with a cabinet for tabletop use.
HRO-7 This is an HRO-5A1 with improved HF osc and regulation.
HRO-7T This is the same as the HRO-7 and is equipped with a cabinet for tabletop use.
HRO-7R This is the same as the HRO-7 but is supplied for rack mounting.
HRO-50 Added push-pull audio output, internal P/S and direct freq readout.
HRO-50-1 Added 3rd IF stage
HRO-60 Added dual conversion for bands A and B (20 mtrs and above)
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