Overview

The Heathkit SB-301 receiver and SB-401 transmitter were sold as separate kits, but they were designed to work as one station. With three short coaxial cables between their rear panels, the pair behaves like a transceiver. The transmitter then takes its frequency from the receiver’s tuning dial and borrows the receiver’s crystal oscillators. The SB-401 can run on the SB-301’s crystals because in this mode it has no crystals of its own apart from a single CW carrier crystal. That is why it cost forty dollars less than the SB-400 it replaced. Turn one switch on the transmitter’s front panel and the two units work independently again, each with its own tuning, which was a real advantage for DX operators working stations that listened on one frequency and transmitted on another.

In 1966 the pair cost $545 as kits. Heath’s catalogue copy set that against the Collins S-Line, which the SB series was styled after, and it earned the SB-300/SB-400 generation the nickname “the poor man’s Collins” (Eckweiler, Heathkit of the Month #30). The receiver was sold until the end of 1970, when the solid-state SB-303 replaced it. The transmitter lasted until 1975 and worked with either receiver.

Figure 1 — An SB-301 receiver in the two-tone SB-series finish: grey wrap-around cabinet, green wrinkle panel, dark-green knobs with bright inserts. The slide-rule scale above the round dial counts hundreds o… — Figure 1 — An SB-301 receiver in the two-tone SB-series finish: grey wrap-around cabinet, green wrinkle panel, dark-green knobs with bright inserts. The slide-rule scale above the round dial counts hundreds of kilohertz; the dial below it reads single kilohertz. The band switch at lower right carries the 15.0 MHz "RCV ONLY" position for WWV that the SB-300 lacked, and the mode switch has an RTTY position. — Source: photograph via oldtuberadio.com, https://www.oldtuberadio.com/wp-content/uploads/2012/09/Vintage-Heathkit-SB-301-Receiver.jpg

1.1 What the two units are

The SB-301 is a ham-bands-only, double-conversion superheterodyne receiver. It covers 80, 40, 20 and 15 metres and all of 10 metres in 500 kHz segments, and it has an extra receive-only segment at 15.0–15.5 MHz for the WWV time signals. It receives SSB, CW, AM and RTTY. The first conversion is crystal-controlled and the tunable IF runs from 8.395 to 8.895 MHz. The second conversion uses a sealed, factory-aligned linear master oscillator (LMO) tuning from 5.5 down to 5.0 MHz, and it feeds a 2.1 kHz crystal-lattice filter at 3.395 MHz. It has ten tubes and a built-in, transformer-operated supply.

The SB-401 is the same signal chain run backwards. It is a filter-type SSB and CW exciter-transmitter with two 6146 beam tetrodes in the final, and it is rated at 180 W PEP input on SSB and 170 W on CW. Output is 100 W from 80 through 15 metres and 80 W on 10 metres. The high-voltage supply is built in, which set the Heath pair apart from the Collins 32S-series transmitters with their separate supplies (Eckweiler, HOTM #30). It has thirteen tubes and needs a separate receiver: an SB-300, SB-301 or SB-303 for transceive, or any receiver if the optional crystal pack is fitted.

Heath’s story in these years, from Benton Harbor and the Daystrom period to the arrival of the SB line, is told in the site history dive and is not retold here.

Figure 2 — Heath's own publicity photograph of the SB-301, from the specification sheet packed with the kit. The panel is an early one: the AGC switch is labelled AVC and the mode switch has no RTTY position,… — Figure 2 — Heath's own publicity photograph of the SB-301, from the specification sheet packed with the kit. The panel is an early one: the AGC switch is labelled AVC and the mode switch has no RTTY position, both of which differ from production sets. Catalogue and manual photographs were often made from pre-production units. — Source: Heath Company, "Specifications for the Heathkit SSB Receiver Model SB-301", sheet dated 3/22/68, bound with the SB-301 assembly manual, https://w5rkl.com/wp-content/uploads/2023/07/sb-301-Manual.pdf

Figure 3 — The SB-401 transmitter. Top row: FINAL tuning with concentric LOAD, the MIC/CW LEVEL control with the FREQ CONTROL switch behind it (LOCKED (RCVR) and UNLOCKED), and the multimeter. Middle: DRIVER … — Figure 3 — The SB-401 transmitter. Top row: FINAL tuning with concentric LOAD, the MIC/CW LEVEL control with the FREQ CONTROL switch behind it (LOCKED (RCVR) and UNLOCKED), and the multimeter. Middle: DRIVER tuning, the main dial, and the METER switch (GRID, PLATE, ALC, HV, REL PWR). Bottom: FUNCTION (OFF, STBY, TRCV, TRAN, SPOT), BAND, MODE (LSB, USB, CW, TUNE) and the microphone socket. — Source: photograph via QSO Today, https://www.qsotoday.com/uploads/2/4/5/0/24502639/heathkit-sb-401-transmitter_orig.png

1.2 The headline figures

Table 1 — The headline figures

	SB-301 receiver	SB-401 transmitter
Coverage (MHz)	3.5–4.0, 7.0–7.5, 14.0–14.5, 15.0–15.5 (WWV), 21.0–21.5, 28.0–30.0 in four segments	3.5–4.0, 7.0–7.5, 14.0–14.5, 21.0–21.5, 28.0–30.0 in four segments
Modes	LSB, USB, CW, AM, RTTY	LSB, USB, CW (no AM)
Frequency control	Crystal heterodyne oscillator plus 5.0–5.5 MHz LMO	Its own LMO, or the receiver’s (switch-selected)
Stability	Under 100 Hz per hour after 20 minutes	Same
Dial	1 kHz divisions, 100 kHz per turn; within 200 Hz visually and 400 Hz electrically after calibration	Same
Sensitivity / power	Under 0.25 µV for 10 dB S+N/N on SSB (1968 sheet; 0.3 µV in the 1967 catalogue)	180 W PEP input SSB, 170 W CW; 100 W out (80 on 10 m)
Selectivity / suppression	2.1 kHz at 6 dB, 5.0 kHz at 60 dB; AM 3.75 kHz and CW 400 Hz filters optional	Carrier and unwanted sideband 55 dB down; third-order products 30 dB down
Tubes	10	13, including two 6146 and an OA2
Size, weight	14⅞ × 6⅝ × 13⅜ in; 17 lb	14⅞ × 6⅝ × 13⅜ in; 26½ lb
Kit price, 1967	$260.00	$285.00

The figures come from the SB-301 specification sheet and the SB-401 manual’s specifications page. Both are reproduced and discussed in the Reference volume.

1.3 Where the pair came from

Heath’s first single-sideband station had been built from parts that were never meant to be one: the RX-1 Mohawk receiver, the Apache TX-1 transmitter, which was an AM and CW set, and the SB-10 phasing adapter, which added SSB to the Apache (Eckweiler, HOTM #30). The SB series was a clean start. It was built from the beginning around filter-type sideband generation and around one stable tuning element shared by everything. The site history dive explains why the SB pair is not a successor to the DX-100, despite what some video retellings say. The DX-100’s successor as Heath’s top transmitter was the Apache.

The SB-300 receiver and SB-400 transmitter came first. Sources disagree about exactly when. Eckweiler’s overview puts them in late 1963 and early 1964, while his SB-301 article says the SB-300 arrived in mid-1964. A contemporary Heath-list reply gives May 1964 for the SB-400 (Kechkaylo, heath mailing list, 25 March 2002). The Fall-Winter 1963 catalogue 81-30 does not list either unit, which was checked for this dive. By the time of the March 1965 catalogue both were established: the SB-300 at $265 and the SB-400 at $325. CQ reviewed the SB-400 in December 1964, and 73 Magazine ran an article on the SB-300 in August 1964 (Leedham, WA2TDH, via Nostalgic Kits Central). A mid-1964 introduction fits all of that.

Figure 4 — The SB-300 in the March 1965 catalogue at $265, with the optional AM and CW filters at $19.95 each and the 6- and 2-metre plug-in converters that mounted on its rear apron. The specification block … — Figure 4 — The SB-300 in the March 1965 catalogue at $265, with the optional AM and CW filters at $19.95 each and the 6- and 2-metre plug-in converters that mounted on its rear apron. The specification block gives 1 µV for 15 dB signal-plus-noise to noise, the figure the SB-301 would later improve on. — Source: Heathkit catalogue 800/53, March 1965, p. 39, https://www.worldradiohistory.com/Archive-Catalogs/Heathkit-Catalogs/Heathkit-1965-March-800-53.pdf

Figure 5 — The facing page: the SB-400 at $325, "The Perfect 'Mate' to the SB-300", with a quotation from W2AEF's December 1964 CQ review. Note "All crystals supplied for complete 80-10 meter coverage", which… — Figure 5 — The facing page: the SB-400 at $325, "The Perfect 'Mate' to the SB-300", with a quotation from W2AEF's December 1964 CQ review. Note "All crystals supplied for complete 80-10 meter coverage", which the SB-401 would drop to save forty dollars. — Source: Heathkit catalogue 800/53, March 1965, p. 38, https://www.worldradiohistory.com/Archive-Catalogs/Heathkit-Catalogs/Heathkit-1965-March-800-53.pdf

The resemblance to Collins is deliberate and has been widely noted. The styling is close, the dial works the same way (a round kilohertz scale under a slide-rule hundreds scale), and the transmitter’s mixing scheme mirrors the receiver’s as it does in the S-Line. The IF frequencies differ, and so do the filters: Heath used crystal-lattice filters where Collins used mechanical ones (W2PA, “Collins S/Line Influence on Heathkit’s SB-Series”). Chas Gilmore’s account adds that several engineers on Heath’s SB radios came from Collins after layoffs there (EEJournal, part 2). That is a recollection, not a documented staff list.

Figure 6 — A surviving SB-300. Compared with the SB-301 above: the FUNCTION switch and A.F. GAIN sit lower on the panel, the mode switch has no RTTY position, the band switch has no 15 MHz position, and there… — Figure 6 — A surviving SB-300. Compared with the SB-301 above: the FUNCTION switch and A.F. GAIN sit lower on the panel, the mode switch has no RTTY position, the band switch has no 15 MHz position, and there is no front-panel converter switch. The SB-300's converter selector was on top of the chassis, under the cabinet lid. — Source: Janis, AB2RA, "SB400 & SB300 Heathkit Twins", https://wirelessgirl.com/Projects/Heathkit-Twins/

1.4 The SB-301 and SB-401 arrive

The SB-300 and SB-400 still appear in the July 1966 mail-order catalogue, 800/64. The replacements first appear in the 1967 annual catalogue, 810/67A, which carries a 1966 copyright and was printed that autumn. Both are marked “New!”. The SB-301 was priced at $260 ready to ship. The SB-401 was $285, with an “Expected Shipping Date Late September” (catalogue 810/67A, pp. 82–83). The SB-301 manual’s cover also carries a 1966 copyright, and a Radio-Electronics advertisement of December 1966 lists the receiver at $260 (Heathkit Interests blog). The earliest SB-401 manual Eckweiler found dates its block diagram 24 March 1967 (HOTM #43). Taken together, the evidence points to an announcement in late 1966 and deliveries in late 1966 and early 1967, with the transmitter slightly behind the receiver.

Figure 8 — The SB-301's debut in the 1967 annual catalogue: "New! ... Now With These Extra-Performance Features". The list is the changelog from the SB-300: more sensitivity, the RTTY mode, 15 to 15.5 MHz for… — Figure 8 — The SB-301's debut in the 1967 annual catalogue: "New! ... Now With These Extra-Performance Features". The list is the changelog from the SB-300: more sensitivity, the RTTY mode, 15 to 15.5 MHz for WWV, a switch-selected noise limiter, front-panel converter switching, and a new "sub-pack" packaging of the kit. The SB-600 speaker beside it is also new here, at $17.95. — Source: Heathkit catalogue 810/67A (copyright 1966), p. 82, https://www.worldradiohistory.com/Archive-Catalogs/Allied-Catalogs/Heathkit-1967.pdf

Figure 9 — The facing page announces the SB-401 at $285, "Expected Shipping Date Late September", and explains the new selling idea: "The SB-401 derives all the necessary crystal oscillator voltages from the … — Figure 9 — The facing page announces the SB-401 at $285, "Expected Shipping Date Late September", and explains the new selling idea: "The SB-401 derives all the necessary crystal oscillator voltages from the SB-301 or SB-300 ... eliminates redundant circuitry!" The SBA-401-1 crystal pack, $29.95, is "Required for SB-401 operation without the SB-301 or SB-300 Receiver". — Source: Heathkit catalogue 810/67A (copyright 1966), p. 83, https://www.worldradiohistory.com/Archive-Catalogs/Allied-Catalogs/Heathkit-1967.pdf

The changes from the SB-300 to the SB-301 are all receiver refinements.

Sensitivity. Heath’s figure went from 1 µV for 15 dB to 0.3 µV for 10 dB in the catalogue, and to 0.25 µV on the 1968 specification sheet.
RTTY mode. A third BFO crystal, at 3392.110 kHz, places 2125 and 2975 Hz teletype tones in the middle of the SSB filter.
WWV position. A ninth band-switch position at 15 MHz, so the 100 kHz calibrator can be checked against WWV.
Noise limiter. A full-wave, self-biased diode limiter, switched by pulling the AF GAIN knob.
Front-panel converter switch. It is concentric with the RF GAIN control. On the SB-300 the switch was on top of the chassis.
Smaller crystal filters. The new filters have tighter skirts: the CW filter was now 2.0 kHz wide at 60 dB, down from 2.5 kHz.
Assembly. The kit came in “sub-packs”, one for each assembly phase.
LMO tube. The LMO now used a 6BZ6 in place of the 6AU6.

Mort Waters, W2JDL, reviewing the new receiver for 73 Magazine, set most of these side by side with the SB-300. He found the LMO’s drift “completely unnoticeable from a cold start” (73, June 1967, pp. 76–78).

The SB-401’s changes are partly engineering and partly marketing.

The LOCKED/UNLOCKED switch. The front-panel FREQ CONTROL switch replaced the SB-400’s internal coaxial jumper. W2JDL noted that so many SB-400 owners had home-brewed the conversion that articles on it had already appeared in the magazines (73, July 1967).
An extra driver coil for the top of 10 metres. It is peaked at 29 MHz. The SB-400’s single 28 MHz coil had left drive short in the upper part of the band.
A reworked LMO mixer stage with a 21 MHz trap.
A 680 Ω resistor in the finals’ screen supply. Heath said it cut distortion products by another 3 to 4 dB.
A sidetone level control under the lid.
Crystals optional. The kit shipped with only the CW carrier crystal. Every other crystal went into the separate SBA-401-1 pack, because Heath had noticed that most SB-400s were being used with SB-300s, which already contained those oscillators (Eckweiler, HOTM #43).

How It Works covers the circuit, and Transceive covers exactly what the transmitter borrows and how.

Figure 10 — The separates and the transceivers that ran beside them. The SB-401 outlived both of the tube receivers it was designed around and worked with the solid-state SB-303 that replaced them. The SB-100,… — Figure 10 — The separates and the transceivers that ran beside them. The SB-401 outlived both of the tube receivers it was designed around and worked with the solid-state SB-303 that replaced them. The SB-100, SB-101 and SB-102 transceivers used the same LMO and dial and the same mixing scheme in one cabinet. — Source: drawn for this dive from the Heath catalogues cited in this volume and in vol07, and Eckweiler, HOTM #30

1.5 Who it was for

The copy was aimed at the serious home-station operator. It sold the pair as a full-featured station: “Single panel switch selects transceive or independent operation … perfect for DXing!”. It also sold them on value, claiming “Professional styling & features at 60% savings” as early as the SB-300 page of 1965. The station was modular, and Heath sold every part of it: the SB-600 speaker, the SB-200 kilowatt linear styled to match, the HO-10 and later SB-610 monitor scope, the SB-620 panadapter, the SB-630 station console, and the 6- and 2-metre plug-in converters. The receiver’s octal accessory socket powered the converters, and its front-panel switch selected them. That turned the SB-301 into an 80-through-2-metre receiver without a second set of knobs.

The separates cost more than the SB-100 series transceivers, and the extra money bought flexibility. A transceiver ties receive and transmit to one oscillator. The separates can listen and transmit on different frequencies in the same band, which the transceivers could only do with an extra remote VFO, the SB-640. They can also be upgraded one box at a time. Eckweiler bought his receiver in September 1968 and ran it with an old Apache until he could afford the SB-401 that December. The SB-300 series had been designed for exactly that kind of staged purchase (HOTM #36; HOTM #43).

Figure 11 — The complete station in the Winter 1970 catalogue: SB-301 at $270, SB-401 at $295, SB-200 linear at $220. The HDP-21A microphone stands between them. The SB-110A six-metre transceiver on the same p… — Figure 11 — The complete station in the Winter 1970 catalogue: SB-301 at $270, SB-401 at $295, SB-200 linear at $220. The HDP-21A microphone stands between them. The SB-110A six-metre transceiver on the same page shows the family resemblance. — Source: Heathkit catalogue, Winter 1970, p. 45, https://www.worldradiohistory.com/Archive-Catalogs/Heathkit-Catalogs/Heathkit-Winter-1970.pdf

1.6 Prices and the catalogue years

Table 2 — Prices and the catalogue years

Catalogue	SB-300	SB-400	SB-301	SB-401	SB-303
800/53, March 1965	$265.00	$325.00	—	—	—
1966 annual	$250.00	$325.00	—	—	—
800/64, July 1966	listed	listed	—	—	—
810/67A, 1967 annual (© 1966)	—	—	$260.00 (new)	$285.00 (new)	—
June 1967	—	—	$260.00	$285.00	—
1968 and 1969 annuals	—	—	$260.00	$285.00	—
Winter 1970	—	—	$270.00	$295.00	—
1971 annual	—	—	—	$295.00	$319.95 (new, “Available November”)
810/72	—	—	—	$299.95	$319.95
Christmas 1975	—	—	—	$389.95	$389.95
Spring 1976	—	—	—	not listed	$389.95
Christmas 1976	—	—	—	—	not listed

The optional crystal filters followed the same pattern: $19.95 each for the SB-300’s, $20.95 for the SB-301’s in 1967, and by 1975 $24.95 for the AM filter and $29.95 for the CW filter. The SBA-401-1 crystal pack cost $29.95 from the first listing to the last.

Figure 12 — The receiver's replacement: "New Heath SB-303 Solid-State Amateur Receiver", "Available November", $319.95. The catalogue counts 27 silicon transistors including four dual-gate MOSFETs, one integra… — Figure 12 — The receiver's replacement: "New Heath SB-303 Solid-State Amateur Receiver", "Available November", $319.95. The catalogue counts 27 silicon transistors including four dual-gate MOSFETs, one integrated circuit and nine plug-in boards, and adds a 25 kHz calibrator step, an RF attenuator and a smaller cabinet. It also promises full SSB and CW transceive with "the famous '400' or '401'". — Source: Heathkit 1971 catalogue, p. 76, https://www.worldradiohistory.com/Archive-Catalogs/Allied-Catalogs/Heathkit-1971.pdf

The receiver’s end is clearly dated. The SB-303 was advertised in the 1971 annual catalogue, printed in autumn 1970, as “Available November”. Heath’s first SB-303 service bulletins are dated 25 November 1970 (Lutz, SB-3XX service bulletin index). This matches Eckweiler’s statement that the SB-301 was replaced in late 1970. The transmitter’s end is less precise. The SB-401 is in the Christmas 1975 catalogue at $389.95 and is gone from Spring 1976, which fits Eckweiler’s “into 1976”. The SB-303 lasted one catalogue longer and was absent by Christmas 1976. By then Heath’s HF line was the solid-state SB-104 transceiver. The stand-alone receiver and transmitter as a product category ended there, and no production figures for either model have been found.

Figure 13 — "SB separates" in catalogue 810/72: the SB-401 at $299.95 beside the SB-200 linear, in one of Heath's more atmospheric colour photographs. The copy now says the transmitter works with a "300-series… — Figure 13 — "SB separates" in catalogue 810/72: the SB-401 at $299.95 beside the SB-200 linear, in one of Heath's more atmospheric colour photographs. The copy now says the transmitter works with a "300-series receiver", which covers the SB-300, SB-301 and SB-303. — Source: Heathkit catalogue 810/72, p. 69, https://www.worldradiohistory.com/Archive-Catalogs/Heathkit-Catalogs/Heathkit-1972-810-72.pdf

1.7 The SB-303

The SB-303 is covered in this dive because the SB-401 was built to work with it. It is not a tube receiver with transistors added. It is a new design with the same architecture: crystal first conversion, 8.395–8.895 MHz tunable IF, a now solid-state LMO, and the 3.395 MHz crystal filters that are interchangeable with the SB-301’s. It also has the same transceive jacks. Its assembly manual (595-1124, © 1970) describes 28 silicon transistors, four of them MOSFETs, and an integrated-circuit calibrator on plug-in “Switch-Boards”. The receiver needs one small change to the SB-401 before the two will transceive. That change, and the reasons for it, are covered in Transceive. There was also a shortwave-listener sibling, the SB-310 (and later the SB-313). It had the SB-301’s cabinet and circuit layout but covered the broadcast bands, and it came with a wider 5 kHz AM filter (Eckweiler, HOTM #30).

1.8 Variants and revisions

Table 3 — Variants and revisions

Model	Years (catalogue)	What changed
SB-300 receiver	1964–1966	First SB receiver; 6AU6 LMO (110-13); large 404-200 filters on #6-32 studs; no RTTY, WWV or noise limiter
SB-400 transmitter	1964–1966	All crystals supplied; transceive or separate chosen by an internal coax jumper; 6AU6 LMO
SB-301 receiver	late 1966–1970	Changes listed above; LMO 110-32 with 6BZ6, then 110-40 from November 1967 (service bulletin SB-301-1D)
SB-401 transmitter	late 1966–1975	Front-panel FREQ CONTROL switch; crystals in the optional SBA-401-1 pack; 404-283 filter; LMO 110-32 then 110-40; later kits with a dual-primary 120/240 V transformer (54-198 replacing 54-143, believed autumn 1968)
SB-303 receiver	November 1970–1976	Solid-state; same IF and transceive jacks; needs a small modification to the SB-401
SB-310 / SB-313	SB-310 by 1968; SB-313 later	Shortwave-listener versions of the SB-301 and SB-303

Eckweiler records one visible sign of an early SB-401: on the earliest units the FREQ CONTROL switch was labelled “LMO MODE” (HOTM #43). The two LMO part numbers matter to anyone restoring a set, and they are covered in the Restoration volume.

The same LMO, dial and mixing scheme are packed into one box in the SB-100 family of transceivers, documented in the SB-102 dive. The HW-101, the budget transceiver derived from those, is covered in the HW-101 dive. The solid-state SB-104A that ended the tube SB line has its own dive.

Sources

Heath Company. Assembly Manual, SSB Receiver Model SB-301, 595-946-01 (copyright 1966; printing of 11-29-68), with specification sheet dated 3/22/68 and change letter of 6 January 1969. W5RKL scan
Heath Company. Assembly and Operation of the SSB Receiver Model SB-300. W5RKL scan · Internet Archive, abridged
Heath Company. Assembly and Operation of the SSB Transmitter Model SB-401 (later printing covering the SB-303). Internet Archive
Heath Company. Assembly Manual, Solid-State SSB Receiver Model SB-303, 595-1124 (copyright 1970). Internet Archive
Heathkit catalogues, World Radio History: 800/53, March 1965 · 1966 · 800/64, July 1966 · 810/67A · June 1967 · 1968 · 1969 · Winter 1970 · 1971 · 810/72 · Christmas 1975 · Spring 1976 · Christmas 1976 · Fall-Winter 1963, 81-30
Eckweiler, Bob, AF6C. “Heathkit of the Month #30: The Amateur Radio SB-Line Overview” (2011). PDF; “#36: SB-301 HF Ham Band Receiver”, rev. A (2012–2023). PDF; “#43: SB-401 HF Transmitter” (2012). PDF
Waters, Mort, W2JDL. “The Heathkit SB301 Receiver.” 73 Magazine, June 1967, pp. 76–78. Internet Archive; “The Heathkit SB401 Transmitter.” 73 Magazine, July 1967. Internet Archive
Leedham, Charles, WA2TDH. “The Heath SB-300 Receiver.” 73 Magazine, August 1964. Nostalgic Kits Central
Kechkaylo, Dave, W8QIZ. “Differences between SB-400 and SB-401?” heath mailing list, 25 March 2002. Archive
Lutz, Joseph W. “Heathkit Service Bulletins – SB-3XX (Complete).” heath mailing list, 19 January 1998. Archive
W2PA. “Collins S/Line Influence on Heathkit’s SB-Series” (2008). Link
Gilmore, Chas, Chuck Penson and Terry Perdue. “‘H’ is for Heathkits and Hams: Part 2 – The 1960s.” EEJournal. Link
Heathkit Interests blog. “SB-301 Amateur Receiver” (2019). Link
Janis, AB2RA. “SB400 & SB300 Heathkit Twins.” Link