The 400 W power amplifier (PA) uses many of the components from an old KW1000 HF linear amplifier (including the original pair of 572B valves).  The amplifier was purchased for under £200 and has been modified from a switch-tuned grounded-grid configuration to an aperiodic (untuned), push-pull amplifier, rather like the QRO Class B Modulators of the 1960s.

Toroidal transformers are used in both the grid and anode circuits using cores of Philips 3C85 material.  This material was originally proposed for use in 136 kHz transformers by David Bowman G0MRF [2].

The grid transformer (L1) was wound on a 25 mm OD 3C85 ring core, with 8 turns on the primary, and 38 turns on the secondary (centre-tapped).

With 2 kV on the anodes, 10 W of drive easily delivers 400 W (at an anode current of 350 mA).  Recently, to reduce the average load current on the PS, I have provided some negative bias so that the two 572Bs now draw about 40 mA quiescent on transmit (otherwise, they would draw about 100 mA under 'zero bias' conditions).   For CW service, this is enough bias to prevent key clicks and reduces the 'key-up' load on the HT supply from 200 W to 80 W.

Although I don't have a wide-range oscillator with which to test the bandwidth of the amplifier, my LF transverter will generate 125 - 150 kHz, and I can obtain 400 W output from the PA over this range.  I would be very surprised if the bandwidth of the PA does not extend below 100 kHz and above 160 kHz.

With such high DC and RF voltages involved, this probably would not be an ideal project for those unfamiliar with valve technology.   In any event, particular care needs to be taken with the construction of L2, the output transformer.

Circuit Diagram

Initial tests
Initial tests with the amplifier were very exciting!  Self-oscillation of the PA produced some colourful displays due to high voltage flash-over: both internal and external to the valve envelope!  Purple sparks of 8 mm length were observed within the valve and across the output transformer terminals.  Eventually, sufficient padding of the grid circuit brought the PA to a stable condition (including 5 x ferrite beads on each grid lead, plus a 1000pF capacitor from each grid to earth).  An alternative to padding the grid circuit would have been neutralisation.

KW1000 Transformer Connections
I get occasional requests for mains transformer connection details for KW1000 amplifier.  So here they are.

TAG STRIP
It is on the tag strip that the two transformer primary windings; mains lead; fuse; switch; and fan wiring is terminated.  If you are tempted to unsolder all the wires terminating on the tag strip and hence start from scratch, please keep in mind that the tag strip may break if you put any force on the paxolin.


     115  105   0    5    power lead    115  105   0    5
     RED  GRN  YEL  BLK  BLU       BRN  RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
GND                       |   GND   |                       GND
                          |         |
                          |         |
                       switch     switch


FAN WIRING
The fan has a 115v motor, so it just needs to go across one of the primary windings (0v  and 115v).


     RED  GRN  YEL  BLK                 RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
GND                           GND        |         |        GND
                                         |         |
                                         |         |
                                        fan       fan


240v STRAPPING
When operating off 240 volts all the turns of both windings are connected in series.

     RED  GRN  YEL  BLK                 RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
      |              |                   |              |
      |              |                   |              |
      |               -------------------               |
      |                      link                       |
     switch                                           switch
 (via 5A SB fuse)



210v STRAPPING
When operating off 210v, the two primary windings for 0v and 105v are connected in series.

     RED  GRN  YEL  BLK                 RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
           |    |                             |    |
           |    |                             |    |
           |     -----------------------------     |
           |                 link                  |
        switch                                   switch
    (via 5A SB fuse)



120v STRAPPING
When operating off 120v, all the turns of both primary windings are connected in parallel.

     RED  GRN  YEL  BLK                 RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
      ||             |                   |              ||
      ||             |                   |              ||
      ||              ---------------------------------- |
      ||                                 |     link      |
      | ---------------------------------                |
      |              link                                |
      |                                                  |
    switch                                             switch
(via 10A SB fuse)



115v STRAPPING
When operating off 115v, the two primary windings for 0v and 115v are connected in parallel.

     RED  GRN  YEL  BLK                 RED  GRN  YEL  BLK
---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---  ---
      ||        |                        |         ||
      ||        |            link        |         ||
      ||         ---------------------------------- |
      ||                                 |          |
      | ----------------------------------          |
      |              link                           |
      |                                             |
    switch                                        switch
(via 10A SB fuse)

 

CAUTION
If printing this page, make sure that all the verticals in the above text are in alignment and check that you fully understand the connection diagram before making a start on re-terminating the KW1000 transformer windings.  KW used poorly insulated wire around the fan and switch wiring (thin wire with thin insulation) which should be inspected for soundness along its entire length.  Replace if necessary.