2M amplifier 4CX1000K

Hoping this will not be yet another page with a half finished building story, but I’m late in the project and have some spare time to write.

This all started when unsuccessfully I tried to modify my 6m AR313 MOSFET amplifier to work on 2m. That resulted in a blown mosfet and some burned balancing resistors. It’s very hard to combine 2 let alone 4 300W modules to get to KW. Any difference in each of the  power modules has to be dissipated by the balancing resistors. Mine where getting very hot  – so I was losing alot of power due to imbalance in the 4 AR313. This was just to critical on 2m, they are better matched on 6m.

I started looking for a solution, and my father had started a project, building a tube amplifier based on 4CX1000 tetrode. So, I started scavaging for parts, was going to “borrow” the tube and socket from him. For some dumb luck, when I called a friend of mine TF3HRY, which is a endless source of hard to get components 😉 I don’t quite remember what I was after, but when I described the project I was starting, he offered me a RF unit he had pulled out of a derelict German research container that was to be trashed. I got the RF housing with a socket and actually an 4CX1000K tube in place. So I started to plan, or not 😉

The original plan was to house the power supply and the RF deck in separate 19″ chassis. So I went about and constructed two 19″ chassis. I used a contact I have with a local metal work company that has some very cool machines. So they made the sides and flooring in the case. I’m a bit bound by the complete RF deck of the amplifier. As I made the chassis bit big, later it was decided to cram the power supply as well into the same chassis. So this is far from optimal, the chassis construction is still on my original plan just to house the RF deck.

So, here are some design facts and stats.

  • Control board: GM3SEK Tetrode boards.
  • Power supply: HV-2 by WD7S – usd $45 is nothing for this quality PCB.  (even for the hassle as he does not accept paypal)
  • HV transformer is cabable of 650mA ICAS – so not the greatest but got it from TF3T
  • RF deck – from a Linear amplifier from Richter and co. (German?)
  • LCD front panel for measurements.
  • Stepstart on Filament and HV transformer.


Done on a control board I’m designing, using Teensy2++/Atmega AT90USB1286

  • Grid / Screen / Plate / filament voltage
  • Grid / Screen / Plate current
  • input and exhaust temperature
  • Filament and transmit timer (later versions)
  • Forward and Relected power using a analog log-amp AD8307

Picure gallery1 – RF Deck:

Picture gallery2: Status 9 Apríl 2011 + soldering of the AD8307 log amp “kit” I got from Ebay – it turned out cheaper to buy a small kit with all the components than to order the AD8307 from digikey. This I believe is after a design from OZ2CPU.

New status 27 April

After working on the amp nearly every night I have something to show – it works!!!  only problem is that the bird element I had for 100-200Mhz – 1kw is faulty – so I could only meter up to 250W.

The GM3SEK control board works perfectly, I have tried and tested that it trips the amp in overload conditions. I really think I would not have come so far without Ian’s excellent work – it simply amazes the work that Ian has done with the tetrode boards. The manual for the tetrode boards is just a schoolbook on Tetrodes – and he seems to have really thought this through. I once did not read his text 100% and got a burned resistor – due to me not putting a IC in that was a must 😉 But money well spent!

Here are pictures of the 90% finished amp.

There are lot of things left to do – the digital MCU metering is not finished – and when I transmit the atmel/control board / LCD just goes blank, but I don’t have the cover over the RF unit – so that might help. I put a normal meter on the plate and had a extra digital meter on the grid current.

2 thoughts on “2M amplifier 4CX1000K”

  1. Hi Benni
    Very nice project, and good to see how it has all come together. (The Tetrode Boards work much better when you use all the parts 🙂

    73 and good DX on 2m!

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