ZL3SV - 1KW Legal Limit RF Linear Amplifier project

A few years ago the Govt put the power level up to 1KW PEP output power. Before that the limit was 500 watts so my 200 watt Kenwood TS480 that had been doing a great job was only about 4db below the limit so chasing 4db was hardly worth the expense so I erected a very large antenna array but that is another story and another article.

When chasing the DX my 200 watt Kenwood got very hot at times so when the power level was increased to 1KW I decided it was time to see what I had in the workshop from the days one of my companies designed, build and maintained commercial Radio & TV stations.

I designed this Ham Radio amp for 30 watts of drive from the Kenwood so it could run cool and I selected parts that are over rated for the job as I wanted a safe, long term reliable amp that can operate at full power on SSB, AM, Digital or FM without a time limit and with no stress.

I have used many solid state amps and whilst they are more efficent they are not as reliable or robust as a tube Amp. Many of the amps on air that cause splatter are a result of people trying to run them above their rated limit and or over driving them. This amp runs at less than 30% of the ceramic tubes capabilty so it is very clean, very stable being grounded grid and very linear.

As you can see by the photos most of the parts are used and over engineered. I used a new mechanics tool draw set as a case as it had the wheels etc, was well made and I could use the draws for different parts of the amp. I left some draws out, had new front panels made up and powder coated the same colour and its worked out fine. It also has a safety lock system so all draws are physically locked and micro switches on each draw so if any of these draws are open the amp is dead and an alarm sounds and LED's flash to show the draw that is open.


Specs - View Diagram

Maximum output power: 1kw - Limited by circuit breaker

Drive level: 30 watts for full output

Modes: SSB, CW, AM, Digital, FM

Frequency range: 160 metres to 10 metres

Input tuning: LDG ATU

Output tunning: Manual Pi tuner with variable Inductor

HT Power supply: 5200 volts DC

Weight : 145 kg

Power consumption: 2400 watts max at 1kw RF output on CW.

Limited by 10 amp breaker and 3 pin wall plug capacity

The Photos below show the progress

ABOVE - RF Filament choke inside a die cast box with grid current shunt and blower on top

ABOVE - RF deck - The blower sucks air from under the amp through 2 stainless grills and feeds it into the die case box

that can only exit via the tube and then out the rear of the amp via another earthed grill - It is very important to have double earthed grills

ABOVE - Output Manual ATU, Antenna relay and filament transformer deck

ABOVE - The power supply has 48 x 6amp, 1000 volt diodes for the bridge and 20 x 385uf capacitors at 380 Volts

The transformer is a modified pole transformer so it has a very low impedence so voltage stability is very good.

Above - The capacitor is a 1000 pf Vacuum variable 20kv capacitor about the size of a rugby ball - The 2 output C1 capacitors are 1000pf at 15 kv

I re located the RFC & two output C1 Capacitors to the top as per the other photo 3 above

ABOVE - Looking down on the tuner Deck. The two welding type plugs take the filament voltage to the tube from the filament transformer deck.

- 7.5 volts at 51 Amps,

ABOVE - Control deck - Safety switch monitor - SWR with remote Coupler on RF Deck - LDG Input Auto tunner

NB Only 12 Volts items in this draw

All other draws have micro switches so the amp can not be turned on unless all draws and the safety lock are closed


ABOVE - The finished 1 KW amp - Photo taken just before I fitted the knob on the vacuum varible capacitor .

Top left digital meter is HT KV, below is HT Amps, then grid volts in milli volts and below grid current in ma, then the control supply volts

and below filament volts - Bottom left is the Roller Inductor with turns counter

The power supply is in the bottom draw

I have used it on air and it works fine. Because we run on solar power I do not use it very often.

At 1 KW RF out it draws about 9 amps at 240 volts so it can drag our batteries down very quickly.