Introduction

Some ten years ago I acquired an N7ART "Audrey" 2 x 3CX800 432MHz Amplifier. I already had a 2600V 2A CCS PSU employing the Screen Transformer from the old BBC Band 3 Transmitter at Mendlesham, England. However it was impossible to move with less than 3 people. Thoughts turned to running the amplifier portable so thoughts turned to a more compact PSU; bearing in mind the following design considerations.

1. Moveable by one person (Lightweight)

2. Size to match Amplifier (17 by 12 by x")

3. Built in Voltage measurement using digital readout. Analog meter considered too delicate for portable use

4. Built in cooling fans for reliable operation

Components

Initially using a transformer from Peter Dahl was investigated but the weight was prohibitive. Browsing the web I came across an 8877 Amplifier by GJ4ICD that mentioned  a 1.8KVA toroidal transformer with 240V primary available from LinearAmpUK. The specification looked very suitable for the 432MHz amplifier when voltage doubling to provide around 2500V. Enquiries by email to Linear Amp UK revealed that it was still available (Carnhill VTT1360) . On my next visit to the UK I picked up one of the transformers and, much to the annoyance of the check in staff of American Airlines at London Gatwick, brought it back to the USA

Next consideration were the rectifiers. Two would be needed for the voltage doubler. Having successfully used K2AW's potted rectifiers for 10 years in the large PSU, these were again chosen

Smoothing capacitors were next on the shopping list. For space considerations a stack of electrolytics would be used. A visit to the local emporium in Dallas revealed a supply of 470uF 450V capacitors at a reasonable price. It was considered that six capacitors would give a suitable safety margin

Measurements revealed that a 17x12x4" case would be large enough to house all the components. A suitable chassis was ordered from K3IWK

Circuit

The circuit is shown in Figure 1.

TR1 has the following connections. brown & purple 240V primary. purple & grey 450V (1)  green & yellow 450V (2) red & orange 14V

The inrush protection circuitry works as follows.:-240V is applied to the unit through breaker SW1 which trips at 16A. This applies 240V to the primary of the transformer through surge limiting resistors R1 and R2. 240V is also applied to time delay relay module RL1. After 2S (the delay is settable on the relay module) 240V is applied by contact RL1/1 across RL2 which activates and shorts out R1 and R2 applying full voltage to the transformer primary.

VR1 provides regulated 5V to the LED meter module. The LED meter is programmed to be 20V "full scale" using the instructions supplied with it. R17 is adjusted so that 2kV indicates 2.00V on the meter.

R9 is a safety resistor to ensure B- does not float. N1 shows 240V is present, N2 shows the unit is active.

Components

R1,2	31R 25W	C1,2,3,4,5,6	470u 450V	RL1	240V AC coil time delay 2A
R3,4,5,6,7,8	25K 5W	C7	1000u 50V	RL2	240V AC coil 20A contacts
R9	1k 12W	C8,C9	1u 35V tant	SW1	DPDT 16A 240V breaker
R10	50R 50W	VR1	7805	TR1	Carnhill VTT1360
R11,12,13,14,15,16	100k 1W	D1,D2	6kV 1A	F1,F2	240V AC 4" square
R17	6100 (AOT)	BR1	100V 1A bridge	N1	240V AC Green Neon
		LM1	PM42S	N2	240V AC Red Neon

Construction

The following illustrate some of the construction points of the amplifier:-

The diodes are mounted on the chassis, top centre. The EHT sampler to feed the voltmeter is mounted top left, near the output connector.

Uncluttered front panel contains breaker, meter, 240V Indicator and On indicator.

The two fans are located on the inside of the back wall of the box. One sucks, one blows. A PET connector is used for the B+ connection. A TNC is used for the B- connection.

The capacitors are mounted on a 1/4" plexiglass plate with nylon hardware. The 50 ohm resistor at the output is stood off chassis with 1/2" ceramic spacers.

Layout of the slow start relays. The time delay relay module is next to the breaker. The 7805 regulator for the meter is mounted on the chassis.

The old and new PSUS with amplifier

Conclusions

Hopefully this article will demonstrate that it is possible to produce a compact EHT PSU.

Not to be reproduced without prior permission of the author

This page last updated 20-Jun-03