G4FPH's Amateur Radio Pages - Henry 2000D-27 Amplifier Project


This project started as so many do with my spotting a few interesting looking pieces of surplus equipment at a radio rally. If you find yourself with one of these units, the following notes may help you to get it going.

The project is still very much work in progress. If you are interested, drop me an e-mail for more details.

The Henry 200D-27 started life as the RF heart of an industrial plasma generator. The amplifier was made by Henry Electronics of California, USA. Yes, the same Henry as makes a nice range of amps. for the amateur market!

Recently I have started dabbling with external anode valves. They really do not have soul (see pictures elsewhere on the site), but sadly look like the future (in as far as any exists!) for valve RF amps. Over the years, I find my attitude has changed from a position of advocating only glass triodes in grounded grid to one open to the merits of external anode triodes and even latterly tetrodes. Must be getting old!

RF Deck

The Henry 2000D-27 RF deck is a zero-bias, grounded grid amplifier, designed for spot freqency working on 27.12 MHz (an Industrial Scientific and Medical [ISM] allocation). It uses a single Eimac 3CX3000A7 valve, and is designed to run 'dead key' (amateur translation = long contests and rag chews) at 2000W CW output. The output tank is a Pi-L configuration, with fixed values for C1, L2 and variable L1 (tune) using a shorted turn arrangement and C2 (load) using a flapper capacitor arrangement. I think there may have been a few different variants of this product, one using a 3CX1500 valve (1500D-27?) and possibly another with a 3CX1200 (1200D-27?). I guess your choice depended on how much RF you wanted to generate! Its possible that there were also some amps. for the ISM allocation in the 13 MHz band? Click on the thumbnails below for bigger pictures.

Other Parts You Will Need!

Accompanying the amp. chassis is a metering/control panel, heater transformer, supplementary axial fan, HT supply and solid state CW driver. I suspect the CW driver that I have is not the right one for the amp. (it is a 50 Watt unit that runs off 28 Volt DC - the 3CX3000A7 would I think need at least twice this amount of drive for 2 kWatt output). The thumbnails below show the general arrangement. I do not have the original HT supply - I suspect this was for 3 phase mains anyway, so no great loss as my shack has only single phase supply. Click on the thumbnails below for bigger pictures.

The metering/control panel has a dedicated meter for cathode current (0-2Amp), plus a multimeter for grid current (0-1 Amp); HT voltage (0-10 kVolt?); and filament voltage (0-10 Volt AC). The two other meters read forward power (0-2500 Watt) and reflected power (0-250 Watt) courtesy of a Bird sampling line containing two standard inserts. Click on the thumbnails below for bigger pictures.


The blowers require 115 Volt AC and so a dedicated transformer was installed in the rack to run these. There is a microswitch and vane arrangement in the mouth of the main snail blower attached to the bottom of the RF deck, clearly designed to indicate 'air present' and feed 115 Volt AC back to somewhere (meter/control panel?). I made no use of this feature. The supplementary axial fan (560 c.f.m.!) will probably not be needed for Amateur service. I intend to put a speed controller on this and punch a big hole in the top of the rack.

Filament Considerations

The filament transformer is single phase 230 Volt primary (with taps down to 190 Volt). Although the UK mains is now supposed to be nominally 230 Volt, in practice it seems to be little changed from its previous nominal 240 Volt - the supply companies just widened the tolerence! As any over voltage on the filament reduces life expectancy of the valve (can be dramatically), resistors were installed in the transformer primary circuit to lose a few volts. The current on the supply side of the filament transformer is around 1.8 Amp, so two 3R3 100W resistors in series proved just the job. Incidentally, the heater voltage test points on the RF deck measure the AC appearing across the feed throughs on the rear of the chassis. The crucial piece of information you need to know is that the filament choke drops about 0.45 Volt when the valve pulls its rated filament current of 51.5 Amp, so aim for 7.95 Volt AC on the filament test points. The RF deck contains a rectifier circuit that drives the filament voltage position on the multimeter.


As it comes, the valve is run at zero bias. This produces a standing current that might be considered excessive, some 350 mAmp at a low-ish HT of 3.6 kVolt. Nice and linear but, in addition to a heater power of close to 400 Watt, we are now adding another 1 kWatt of hot air! Great for cold Winter days in the shack, but as I was experimenting in the Summer and had no more clothes to remove, I decided to make a bias board and incorporate the necessary keying and meter protection absent in the original design.

Metering/Control Panel

The metering/control panel contains active electronics to drive the power meters, plus a relay, presumably to start the HT supply and take account of whether the blower is suppliying air to the valve or not. I could not be bothered to trace out these parts of the circuit. I have been promised a circuit though, so watch this space!

The cathode and multimeter wiring is straightforward and can easily be traced straight back to the Jones plug. Both of my meter/control panels had a burnout on the HT voltage position of the switch for the multimeter - some design fault here, I think. The switch is a standard three position, two-pole wafer so a replacement part may be obtained easily. For the time being I have just bent the burnt contact out of the way as I already have a Voltmeter on the HT PSU.


The pictures below show the underside of the RF deck before and after installation of the prototype keying/bias/meter protection board. I opted for minimal change to the RF deck. Two new holes were drilled in the side wall to accomodate the filament meter rectifier board (previously mounted horizontally right in the middle of where I wanted to put my new circuit board); the grid meter test point connector was removed and the hole enlarged to take the bias voltage adjust pot (fully adjustable cathode bias of course OM!); the earthy end of the safety choke across the RF output was moved to free up a hole in the central divide which was enlarged to feed the keying relay wires back to the rear connectors. Click on the thumbnails below for bigger pictures.

I didn't bother with over current protection for the grid. When you have 225 Watt of grid dissipation to play with and a 100 Watt max. drive source, who cares!

Does it work?

Yes it does, and how! Its a pity that conditions on 10 metres are so poor. Getting the amp going proved fairly straightforward. I have two decks, one slightly cleaner than the other. My plan is to make one a single band amp. for 20 metres, the other possibly bandswitched for the low bands. At present, I have the input and tank circuitry as original for testing.

At the time of testing, I did not have a very good HT supply available. Prior to any of my mods., the following results were obtained with 3.8 - 3.5 kVolt on the anode. (I wish I could remember how to do a table in html!)

Drive (Watt)-----Ig (mAmp)-----Ik (mAmp)-----Output (Watt)





One link worth checking out is that of Ken, W2DTC, who has done a similar job on a Gates unit. Ken's page is here!

If big valves interest you, you may be glad to know that Eimac has now finished putting its definitive 'Care and Feeding of Power Grid Tubes' book on-line as a series of PDF documents

More soon!

Copyright 2004 G4FPH - even the spelling mistakes.

Last modified: 7 November 2004