K2RIW 1kW Linear Amp


Dick Knadle, K2RIW, designed what I think is probably the best 432MHz Kilowatt Linear Amplifier ever produced, utilising a pair of 4CX250B’s in a Parallel Stripline configuration, which appeared in the April and May editions of QST in 1972. The design could be difficult to reproduce, mainly due to the use of double sided PCB for the Anode Line, and missing dimensions for its rounded corners.

The late Fred Merry, W2GN refined the design, manufacturing and selling an easy to build kit version, through his company, Arcos. The dimensions for this amplifier are shown in the VHF/UHF DX Manual, (sadly now out of print). There are errors in the articles though, so check VHF/UHF DX Book errata file on Downloads page.
My amplifier is based on this design, but with the grid and anode inductances  being made from copper sheet instead of brass. It has been suggested that it will overheat, due to the use of copper, but with proper cooling this won't be a problem.
K2RIW claimed in his article that because the design utilised a Parallel Stripline design, neither matching pairs of valves, (or even types), or neutralizing was required.

Construction Notes

I bought the enclosure for my K2RIW off ebay, and it looks professionally built. The only problem is that it has been built the wrong way round, with regard to the design, so the valves sit at the right hand side of the amplifier, with plate tuning controls on the left, and grid tuning controls on the right. Please bare this in mind when looking at pictures in this article.
The housing in a 12” x 8” enclosure, with  the anode cavity being 3” deep, and the lower cavity 2 ” deep. The Grid Cavity is within a 5” x 7” box sited within the lower enclosure. The Arcos design calls for a 2 3/4” deep cavity, so we will have to wait and see what effect having a 2” deep cavity will have on the grid tuning.

The anode line was made from 1mm copper sheet, instead of double sided PCB as in K2RIW’s original article, as the PCB variant rarely tunes up, first time, so I sized it as per the Arcos design. The finger stock was soft soldered in position with a blowlamp. If you want to control where the solder goes, you can use Tipex correction fluid, even when silver soldering with a blow lamp, and the metal red, the solder will not run onto the Tipex.Bases

When building this amplifier only use SK620 or SK630 bases, as these have the correct screen bypass capacitors built into the base.
A pair of Eimac SK630 valve bases were used, which have pins 2, 4, 6, & 8 grounded. I only had to ground pin 7, which was accomplished with copper braid and a rather large 200W soldering iron.  

Tuning Flapper

The output tune and load capacitors are Beryllium Copper flappers controlled by 20lb fishing line. I had a hard time sourcing the Beryllium Copper sheet, finally obtaining some from Nogend Metals.
I turned some blind nuts from 5/16” hex brass rod and soldered them to the anode line tuning flapper, and tapped them 5BA. The tuning flapper was made 9/16” wide and is soldered to the output socket. Both flappers sit 1/2” below the slab line in their unbiased position.
I made PTFE mushroom headed bolts to attach the fishing line to the flappers, and to act as a stop to keep them away from the plate line. The fishing line was fastened to 1/4”dia. brass control rods which have a 6:1 reduction drive to give braking fine adjustment at one end, and a brass support block at the other end of the shaft.

The grid line was made from 0.8mm copper sheet. The input tuning and load capacitors are miniature 0 – 2.5pf butterfly types, which were also hard to find, mine came from Surplus Sales of Nebraska. The capacitors are mounted on “L” brackets, made from fibreglass board and 1/4” brass angle, as the rotors need to be left floating.
The control rods were made from 1/4”dia. nylon rods, which were drilled with a No. 13 drill making a good interference fit on the butterfly capacitors shafts. They were then cross drilled 1/32”, and a split pin fitted to keep them in position. This seemed a better idea than using metal couplers, from a stray RF coupling point of view.

Screen, Grid and EHT Supply

Grid CavityThe design’s power supply is crude and archaic by today’s standards, so I would suggest that would-be builders first read the VHF/UHF DX Manual,  paying special attention to the chapters on Linear Amplifiers, power supplies, and their control, and the K2RIW linear amplifier, and pay a visit to GM3SEK’s website.

Obtaining screen and bias supplies off the EHT supply with dropper resistors and thermonic regulator valves or Zener stacks, is not the way to do it, if a clean, narrow signal is required.

The maximum rated plate current for a pair of 4CX250B’s with 2000V HT is 500mA, I found a HT transformer with a 1600Volts @ 550mA secondary, which should just do, as my licence limits me to 400W at the antenna.
The HT transformer will be fed via a soft start circuit to limit current through the rectifier stack at switch-on, stopping the shack lights dimming and that audible thump at switch-on. A future plan is to look into fitting a thyrister control board to regulate the HT supply, and provide a ramp up type of soft start.

The screen and bias supplies need to be regulated, and be able to sink as well as source current. I have decided to use modified GM3SEK’s Tetrode Boards at the centre of the power supply control system, regulated bias and screen supplies, and a regulated heater supply to G4JZQ’s design. I based part of the logic, control and monitoring off the GW4FRX/G4JZQ design.
John Nelson also wrote a very good article in Shortwave Magazine in the early eighties about 4CX250B/350A linear amplifier power supplies and their control, well worth a read!.


The cooling of the amplifier is unconventional, in that the air is directed into the anode cavity first, then some of the air is forced up through the cooling fins of the valves, and is then directed out of the anode cavity by the valve chimneys, which are made from PTFE (Teflon), and are sandwiched between the lid and the top of the plate line, whilst the rest of the air flows through the bases into the Grid Cavity, cooling the valve bodies, pin base and grid elements.
When thinking blowers, the Eimac spec sheets state that at 250W plate dissipation, a single 4CX250B needs 6.3 Cubic Feet / Minute airflow with a back pressure of 0.82in water at sea level! That’s 12.6 Cubic Feet / Minute airflow, you need at least a 5” diameter centrifugal fan to deliver the goods, an axial fan is out of the question. Don’t skimp on the blower!

This is an ongoing project......arn't they all!