6146B Amplifier, 100W

Front Panel View, 110W on 160M

The 6146B Amplifier consists of a pair of 6146B beam power tubes connected in parallel to provide ~100W output power in the HF amateur bands (160m – 10m). Inspiration for the amplifier came from classic Kenwood hybrids and a pair of 6146Bs I inherited from my father (VK3DCC, SK).

20 x 4 LCD Character display provides Amplifier status, Band, Plate and Screen Grid voltage and current.

Below the LCD are the following controls.

  • Main power switch
  • Power on LED
  • Standby / Enable switch. Amplifier is bypassed in Standby and Finals biased to cutoff.
  • Tune / Operate switch lowers Screen Grid voltage during tuning.
  • On Air LED

Load and Plate tuning controls with vernier reduction drives and knobs from Ameritron.

Rear Panel View

PC cooling fan used to cool Finals when Amplifier is in Enabled mode.

BNC Connectors for RF IN and RF OUT. Amplifier bypassed in Standby mode.

CONTROL is a DB-9 Connector providing serial connection to the Transmitter and Receiver.

USB provides connection to the Arduino Mega for debugging and software upgrades.

AC Power and fuse are in the bottom right of the rear panel.

Chassis Top Side View

Starting from the power transformer in the top left corner and moving clockwise.

  • Power transformer
  • 1 of 2 T/R Relay Boards (mounted vertically on divider panel)
  • 2 x 6146B Finals providing ~110W on 160M – 17M, lower on higher frequency bands.
  • Plate choke from a Kenwood hybrid. Sourced from https://kd7dny.com
  • Homebrew plug-in Output Tank Coils
  • Load and Plate tuning capacitors
  • Arduino Mega (mounted vertically on divider panel)
  • Isolated I2C bus board with ADC for Plate and Screen voltage and current monitoring.
  • 20 x 4 LCD Display with I2C bus backpack.
Power Transformer, Arduino Mega with breakout board, and Isolated I2C Bus Board with ADC.
Chassis Bottom Side View

The far right hand section of the chassis contains all the AC mains power wiring and a small transformer with a 125VAC secondary winding for the Grid No. 1 power supply, and a 12VAC secondary winding for the 12 VDC power supply.

The next section contains the High Voltage Power Supply Board, Screen Grid and Grid No.1 Power Supply Board, and Low Voltage Power Supply Board.

The High Voltage Power Supply Board is the board with the 4 large capacitors and HV cable connection, and provides plate voltage of 700V (under load) to the 6146Bs and 350V for the Screen Grid power supply on the Screen Grid and Grid No. 1 Power Supply Board.

Immediately below the High Voltage Power Supply Board is the Screen Grid and Grid No.1 Power Supply Board. This board contains an adjustable, shunt regulated power supply for Screen Grid No. 2 adjustable 150 – 200VDC, and a zener diode regulated Grid No. 1 power supply providing -75VDC. An external potentiometer allows adjustment of the Grid No. 1 bias to set idling current of the finals. There are two relays on this board: one relay is used to remove Screen Grid voltage and ground the Screen Grids of the finals, the second relay drops the Screen Grid voltage during tuning of the Amplifier.

Opposite the Screen Grid and Grid No. 1 Power Supply Board is the Low Voltage Power Supply Board. This board takes 5VAC from the Power Transformer and 12AC from the Grid No.1 power supply transformer and produces regulated 5VDC and 12VDC supplies, respectively, for the low voltage circuitry in the Amplifier.

The next section contains the following boards:

  • Final Board for 2 x 6146Bs
  • Grid No. 1 Input Board with relay switched tuning for 9 HF bands.
  • Two I2C Relay Control Boards controlling all band switching functions, T/R switching, and other relay driven functions.
  • 2 of 2 T/R Relay Boards mounted on the vertical section divider.
  • Relays for cooling fan operation and for cutting off Grid No. 1.

7 Replies to “6146B Amplifier, 100W”

    1. Hi Paul,

      Originally I had relay switched taps on the tank coil, and relay switched additional capacitance on the plate and load capacitors for the lower bands. I hadn’t seen this before but thought it might be feasible with RF power and voltage levels in this amp. I found relays that seemed promising and went ahead.

      After several weeks of operation the relays switching the tank coil taps started to fail, apparently due to RF heating from being in close proximity to the tank coil. Several weeks after that, the relays switching the additional plate capacitance started failing, I assume due to high RF voltages on the plate end of the tank coil.

      I had the relays for the tank coil mounted on the top side of a PCB facing the tank coil, moving the relays to the bottom side and leaving a ground plane on the top side may provide sufficient shielding to prevent the RF heating. I will need to find relays with higher voltage rating for switching additional plate and load capacitance.

      So for now I have reverted to plug-in coils! I will generally select a band and settle in for a while, so not a big inconvenience for me.

      Regards, Rod

  1. Excellent construction can you please give me the details of Grid tuning and switching details.

    Adi Ramesh

    1. Hi Adi,

      Thanks for the feedback!

      I have added details for the Grid Input Board. Please let me know if you have any questions.

      Regards, Rod

  2. Any plan to share schematics? I am no engineer but I really do love the old Kenwood hybrids and I have had the thought about how much I would love to build pretty much exactly what you have done. I’ve been googling for a head start was surprised and very impressed to see your work. I have some locals who would most likely help me sort things out when I get stuck. They are the 14 MegaWatt kinda guys with no interest in a 100 watt amp. I on other hand have been studying the work of the QRP crowd the last 15 years. I have an ICOM ic703 and Elecraft KX3 and they both are excellent rigs. It would be fantastic to have a 100+ amp but I’m not spending $1,300 I would rather invest in a learning experience and get to use a couple 6146 in process! I bought a donor transformer for a TS-830S thinking that it could be basis for project but what you have made here is simply beautiful work! Fantastic craftsmanship! 73 de Seth – W8FG

  3. My apologies apparently I totally missed the details page! I was up very late last night when I first looked at this and this evening it was still open on my iPad and I came back to look at everything again. Guess at least half my last comment is answered by simply paying more attention. I’m open to any advice on approaching a project like this! 73 de Seth – W8FG

    1. Hi Seth,

      Thanks for the feedback!

      If you’re interested I can post all the schematics and PCBs for the amplifier on GitHub. As far as advice, build and test in stages regardless of whether you go the PCB or point-to-point wiring route. You could simplify if you go the point-to-point wiring route by manually switching the input/output tuning and using an analog meter. Happy to provide whatever help/advice I can.



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