"Loki"
(Norse, God of fire)
By Matt Erickson KK5DR
This is a new project, and number six in the series of home-brew amplifiers I have built since I became a ham. The picture below shows what the unit looked like originally. It started its life long ago as a Henry 2K-3, but last year it was killed by a lightning strike. I purchased the unit as salvage for its new life as a home-brew Tetrode HF amplifier.
The Henry 2K-3, as it originally appeared.
My plan is to install four EIMAC 4CX250B tubes. A new power supply, new control circuits, and band coverage from 160-15 mtrs.
The project is scheduled for 2-3 year design and build period. So, this page will be added to periodically during the process. This will be a slow step by step operation, so don't get impatient.
With each of my project amplifiers, I have challenged myself to do better on the next. Some of the first amps I built were pretty crude, and didn't work that well. But, each unit became more and more refined as I learned more. Now, the basics are pretty easy for me, but the innovative refinements have become much more difficult. I look for new ways to build old tube type amps. Long ago I gave up on glass tubes, as they are quickly heading for extinction. Now I only use metal/ceramic tubes, and only EIMAC brand.
It is my hope that this page will help other hams to understand the home-brewing process, and perhaps gain some ideas for their own projects.
Step #1, Strip it down!
Strip the chassis down to bare metal. Removed all the bad parts or ones that will not be used in this project. There are some good parts that I plan to modify and reuse, such as the RF deck, output tank circuit, band-switch. Some of the nice things in the old 2K-3 hulk are the band-switch, which is a multi-bank Radio-Switch® Co. model 86. These are band switches that can handle 7kV @ 30A., very robust. The 2K-3 used a Pi-L output network, so this is another feature I shall reuse. A Pi-L output network is the preferred type, since this network is superior to a simple Pi network, because of its greater level of harmonic suppression. I will also be retaining the plate tuning capacitor or "C-1" as well as the loading capacitor or "C-2" configuration, with only minor modifications. The 2K-3 covered 80/75 to 10 mtrs, but I will shift all bands down one band, so the new coverage will be 160-15mtrs, with a dedicated band position for 17 mtrs. The tuning dials on the front panel are nice as well, they are an elaborate gear driven arrangement. This feature will also be retained in the new amp.
Front tuning dials and stripped down RF deck.
Step #2, buy parts;
Starting from the bottom of the chassis working up, I must build the HV power supply first, then work upwards. A Peter W. Dahl Co. HyperSil® plate transformer rated at 1600Vac RMS @ 1A. CCS will be the heart of the PS. Plate impedance works out to about 1100-1200 ohms. Plate impedance is found by taking the plate current (at maximum) times 1.8 (constant), divided by plate voltage, the result is the approximant plate impedance. However this does not account for strap inductances or capacitance. Once the final plate impedance is known, the tank circuit values for each band can be found in William Orr's radio handbook.
I purchased (8) 1000µF @ 450V computer grade electrolytics (see the above picture), which will yield 125µF @ 3600Vdc when placed in series. Using the capacitance chart in the Radio handbook, I will need an HV filter capacitance of about 120µF. A single 150K ohm 2 watt metal film resistor will be placed across the screw terminals of each cap, for bleeder/equalizer purposes. The final plate voltage will be about 2000Vdc @ 1A. continuous. Each 250B draws a maximum of 250mA of plate current, so the plate supply will be nicely match for the set of (4) tubes. Above you can see the Peter Dahl Co. Hypersil plate transformer, rated at 1600Vac RMS secondary, with 220-240Vac primary. I measured the secondary voltage with my Fluke true RMS reading meter & Fluke HV probe, and it read 1650Vac with 240Vac on the primary. I'll be using a full-wave bridge rectifier set I removed from my Harris RF-103 project.
Power supply deck, original Henry part.
Above is a picture of the lower power supply deck, now stripped of all parts, it had suffered a little damage over time, as seen by the sag in the middle. This is OK, I plan to scrap it and design and build a new frame from aluminum sheet and angles.
Above, (seen nearly actual size) is a picture of a MeanWell® switched mode 24Vdc, 0 to 6.5A power supply. This PSU will supply all the control voltages for relays, LED indicators and lighting, as well as the tube heater current. Since the 4CX250B is an indirectly heated cathode tube, the heater/filament can be operated with AC or DC current from a simple PSU, or transformer with relatively low voltage insulation. These tubes normally run with 6.0V @ 2.6A. on the heater. My plan is to use this 24Vdc PSU with the tube heaters all in series. The voltage will be divided across each heater, but the total current should not exceed 2.6A. Tests confirmed this. The advantage to this set up is that the same PSU can fill several needs. DC current tends to be less harsh on filaments/heaters than AC current. Tight regulation and in-rush control can be assured with this PSU as well. The disadvantage to using a series set up; if one tube has an open heater, the amp ceases to function, and finding the defective tube could be troublesome. I have a plan for finding the open tube quickly, more on that later.
The regulated power supply for the screen and bias will be provided by a set of boards from G3SEK. These boards also provide functions like T/R relay sequencing, warm-up delay, over-load protections, meter outputs, and many more. Stop by the web site and check them out.
Display
Some of the unique features of the display and indicators are that all will be LED or LCD. No incandescent lamps are to be used.
Above is a picture of the panel meter I will be using. I chose LCD panel meters rather than analog mechanical meters, for the reasons that the LCD meter has an accuracy of 0.5%, as opposed to analog meters 5%. These LCD meters will work well enough during tune-up and testing. During SSB modulation these, just like all analog meters will not yield any useful readings. The panel meters are back-lit with a nice green glow. All other indicator lights will be colored LED type.
This project is currently on standby status, until my other amp project is finished.
Copyright © 2007 M.A. Erickson, KK5DR. All rights reserved.