TB3/750 FRINEAR-750

Universal HF Amplifier for Triodes or Tetrodes

(A REBUILT YAESU FL-1000 LINEAR in RSGB's RadCom Dec 1998)

FRINEAR 750

REBUILT YAESU FL-1000 LINEAR

I purchased an old Yaesu FL-1000 (4 × 6KD6) that had changed bands and countries several times, in the course of wich it had undergone various modifications. However the HV transformer, tuning and loading capacitors and the main pi-network tank coil were in good condition. It was purchased with a view to rebuilding with quality components collected during almost 25 years. After rebuilding it produces the same output on 1.8 and 29 MHz. I exchange the PA with a Ten Tec Centaur and then build my FRINEAR 750 with the same design.

<3-500Z, TB3/750>Fig 1 FRINEAR 750

VARIOUS TYPES OF VALVES

The design of this amplifier has proved most universal, reliable and permits various types and makes of valves (Fig 1) to be used. It was tested with the following valve types: RS686, TB-2.5/300, TB-3/750, TB-4/1250, QB-3/200, QB-3/300, QB-3.5/750, QB-4/1100, 5D22, 3-500Z, 3-500ZG, 4-125A, 4-250A, 4-400A, 6155, 8802, GI-7B and GS-35B. See tests in Table 1 , Table 2 , Table 3 , Table 4 .

With tetrodes the maximum output is lower, due to the lower maximum HT. It is possible to achieve the UK legal limit with an on-load HT of 1.7 kV and 50 W of drive to a TB-3/750. The key-down current with a 3-500Z was 420 mA and standing current of 70 mA, resulting in an RF output of 430 W. The maximum output obtained with the TB-3/750 was about 560 W, but this exceeds the maximum rated cathode current.

The QB-3.5/750, QB-4/1100 and RS676 require about 85 W of drive for 400 W output. I am not enthusiastic about the 4-125A, 4-250A and 4-400A, with the gain on 10 – 20 metres falling below the European tetrodes. In general I do not recommend using American tetrodes.

Click to Enlarge Fig 2

TB-3/750 TRIODE

Full size TB3/750

A circuit diagram of the rebuilt amplifier using a single TB-3/750 triode is shown in Fig. 2. Some features may be of interest and encourage other experimenters tackling a high-power linear:

  1. Nine amateur bands, one valve, 400W HF power output (more with HV > 1.7 kV).

     

    Fig 3
  2. Universal, all-bands, pi-input network (Fig 3). A 2-pole, 5-way switch, five small coils and two miniature air-space variable capacitors will match various types of valves needing 10W, 50 W, or 100 W drive.
  3. A simple tuning indicator. This does not give scaled SWR readings but is an aid to tune-up of the input circuit for zero meter deflection. The sensitivity potentiometer can be omitted. The only critical component is the 47 Ω (2 x 100 Ω/0.25 W in parallel), non inductive resistor.
  4. A low-cost 1 mA panel meter switched by a 1-pole, 4-way switch. Yes, the positive terminal of the meter is connected to chassis.

    Fig 4

  5. Sequential chang-over system (fig 4). This ensures that the valve is "on" only when the amplifier is connected to the antenne.

    Fig 5

  6. Grid current earthing and pi-network choke decoupling. My experience is that parasitic oscillation and instability is usually the result of poor or inadequate earthing (Fig 5) and decoupling.

    Fig 6

  7. The 270 pF (Fig 6) tuning capacitor is connected to a tap on the 28 MHz tank coil to cope with the large minimum capacitance.
  8. The filament choke RFC1 (Fig 3) has a bifilar winding 1.8 mm diameter enamelled wire on a 10 cm long, 9.5 mm diameter ferrite rod from a broadcast AM radio. While the wire size might appear to be too thin for 14 A filament current, it serves also to provide a resistance to limit switch-on inrush current.