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The DX-35 80...10 meter AM and CW Transmitter came on the market in 1956 and was produced until 1957. It superseded the CW only AT-1 Amateur Transmitter that came on the market in 1951. The DX-series intoduced a new styling that started in 1955 with the introduction of the advanced DX-100 160...10 meter AM and CW transmitter with built in VFO. The DX-100 was produced until 1957 and was replaced in 1958 by the DX-100-B that in turn was produced until 1960.
The DX-35 was crystal controlled, had half the power of the DX-100, lacked the 160 meter band and had no built-in VFO. An external VFO could easily be plugged in and a socket for the VF-1 (the external VFO for the previous AT-1 transmitter) was placed on the rear panel.
In 1957 the CW only DX-20 transmitter was launched. It had 20% less power than the DX-35 but almost twice the power of the AT-1. The DX-20 was basically a modernized AT-1. The DX-20 was produced until 1960 and in 1961 it was replaced by the HX-11.
In 1958 the DX-35 was replaced by the DX-40 and was produced until 1960. The DX-40 was basically a 15% higher power version of the DX-35. All these DX-series transmitters had a buit-in pi-network for antenna impedance matching.
The Brittish Heathkit factory in Gloucester produced several kits with the letter U added to the designation. Among them was the DX-100U, DX-40U and VF-1U. The U meant The United Kingdom. The DX-40U differed from the DX-40 in terms of a modernized style but slightly also in the choice of tubes and a few other parts. The DX-40U had a similar styling as the HX-11. The Brittish made versions DX-100U, DX-40U and VF-1U was still in the 1966 catalogue and the DX-100U even in the 1968/1969 catalogue so they were marketed a few years longer than the American counterparts.