The G3NGD 400 watt PEP Output Linear Amplifier
A blown up photograph of the G3NGD 400 watt PEP Output Linear Amplifier. (Sorry about quality!)
Circuit diagram for the G3NGD 400 Watt P.E.P output (3·5MHz - 30MHz)  Linear Amplifier as used in 1978.

 
Note: the PL519 Valve has a heater voltage of 40 volts @300mA. In the circuit above, the three valves are connected in
series giving a total heater voltage of 120 volts. This is supplied by connecting a single silicon diode (BY100 type) in
series and fed directly from the 230 mains supply. In order to prevent an excessive heater current surge at 'switch on'
an  NTC Thermistor (CZ1) is also connected in series. When the valve heaters have reached their correct temperature, the Thermistor can be shorted out with a switch (S2)
The Cooling Fan used was one I purchased from 'New Cross Radio' in Manchester (Now closed down). It was an
ex-computer fan imported from the USA. It was a 110Volts input Fan and cost 50pence. My mains transformer had
a 110volt winding on it, and I fitted 2·2uF paper capacitor in series with the fan input in order to reduce its speed. 
The Linear Amplifier gave 400Watts PEP output on 3·5MHz, 7·0MHz and also 14MHz. On 21MHz it gave 300Watts
and on 28MHz 200Watts. The reduced power was due to using Television 'Sweep Tubes'  - (PL519), and also in
using a single Pi Network output. If separate plug in coils had been used for each band (especially 28MHz) the efficiency
would have been improved.
Note that a 2·2 Mohm resistor is connected across the 'Loading Capacitor' (Pi Network output). This was to discharge
the build up of static across the Loading Capacitor. It was annoying to hear the clicking noises as the voltage arced
across the capacitor vanes.
The Linear was only used for a few years. It was very reliable. It never gave me any trouble - not even TVI.
I eventually decided to remove it from my station, as I found that I could contact all my Radio friends using 100Watts.
One thing that the Linear Amplifier did to maximum effect, was to heat up the shack. Although I could see the Anodes
of the PL519's glowing 'cherry red', the Fan prevented them from blowing up. (Of interest, the PL519's were purchased
'second hand' at a price of 25 pence each.)
Below is a larger copy of the circuit diagram - just in case one needs to read all the data.

 
 
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G3NGD: June 2006