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RADARS Linear project

A 'universal' PSU which can supply up to 2.4kV at about 1Amp. (3kV off load)
(photos from the not quite finished PSU)

Circuit Diagram (Adobe pdf - 1MB)

Up to 2.4kV at 1 Amp (0.4A continuous). 
6.3v or 12.6v heater supply up to 5Amp
Built-in bias supply and Ic, Ig and kV metering.
Size: 10" x 9" x 9"

In this PSU all delays etc are manually controlled, since the PA valves are so cheap to replace. 
Also, since the Gi-7B can be operated with heaters off at HT up to 3kV without damage,  it allows several 'linears' for different bands to be connected to the PSU in parallel - (provided only one is in use at any one time).

Top view showing kV, Ic and Ig meters and switching for Mains, Heaters and HV.
The multi-turn control pot is for bias adjustment.
The boards on the right control bias and relay switching.

Inside view showing the two HV transfomers and the heater transformer.  The aerial switching relay is mounted inside the box (bottom right)
The typical PSU switching on sequence is:
Mains ON, wait 10sec -  Heaters ON, wait 90sec - HT ON - Start Tx

The HT transformers used are obtained from scrap domestic microwave cookers. They will handle 600W continuously and provide about 2kV AC under full load. They do need a slight mod - easily accomplished with a heavy hammer and suitable punch!! (See Problems !)

To get full-wave rectification two HV transformers are used with their primaries wired in anti-phase.  This is due to one connection to the secondary winding being physically close to ground. Although this adds weight it does mean the PSU should run cool even at 1KW.
[Note: One side of the transformer secondary is normally connected to ground. Disconnect from ground and use as the 'centre-tap' near-ground connection]

An external 240v 8A Variac is used to control the primary of the HV transformers to give from 0 to 3kV max out. (hopefully 2.4kV on load)


Unfortunately the transformers from Microwave ovens have a problem.  They appear to have a very high series reactance - great for constant current supplies to magnetrons - but not too good for our purpose.
The PSU as designed gave 3kV off-load and only 1.9kV under a 300mA load.  Just about enough for the baby 400W linear - but not good enough for the next 'kW' HF project.

LATEST NEWS (10/03/03)- from Juha OH1MLZ.
He suggests a simple mod to the transformers which should make all the difference.  Apparently there are two 'magnetic shunts' built into the transformer which raises the series reactance. However they are easy to remove.  Full details are available on:  
but note - this is quite a big file (2.5MB)!!
Basically there are two small sections of laminated core wedged on either side of the centre core (and between the two windings).  Simply take a suitable punch + large hammer and tap them out, taking care not to damage the windings. (I used a large nail, ground the sides flat to fit the gap and flat at the end - two minutes later the job was done)
The two magnetic shunts can be seen here partially removed.

RESULTS - What a difference.... 2.85kV 50mA load - 2.4kV 250mA - 2.35kV 300mA. 
This suggests about 2.2kV @ 500mA continuous which should be ideal for a linear with a pair of Gi-7B
The transformers ran cool and no hum at 300mA.

Any more suggestions ? - E-Mail RADARS@radars.me.uk

Latest RADARS news.  Bryan, G3RLE happens to have a possible transformer in a bottom drawer!.  1640-1120-857-0-857-1120-1640 at about 1amp.  If it is working OK then a slight redesign to a full-wave bridge across the 1120-1120 taps should give us an even bigger PSU. 

Also - instead of using a string of diodes, Rs & Cs,  for the rectifier stack use a 16kV 1.2Amp stick, HD207, (again for Microwave Ovens, from CPC Ltd) for only about 6ea. brand new.
RADARS will be ordering some so if anyone wants any just let us know (RADARS@radars.me.uk)


If you do make your own HV PSU beware of the dangers. 
These voltages WILL KILL.   Just one touch and its all over!!