Modifications to the
W1SL 144MHz Power Amplifier
By David Anderson GM4JJJ
There follows a few notes
showing the modifications I made to the Classic W1SL 144MHz
Power Amplifier design. For important background and other
details please read the References.
Download the original article
in Adobe Acrobat PDF format (470K)
In place of the expensive Eimac SK600 bases you can use the
surplus "NATO UHF" Bases (the ones with the Y shaped clamp
on the top) at 144MHz, though you will have to make some
adjustments to the dimensions of the anode line. I measured
the value of the screen bypass capacitor in the bases at
3.4nF. There is a variable capacitor fitted to the grid
spigot which can be removed to reduce the height as it is
not required. Also remove the white screen wire from the
screen pin allowing the screen to be fed directly from the
base in the grid compartment in the usual way.
These bases were available as surplus in the UK from :
Sinequanon (www.sinequanon.co.uk)
PO BOX 262
LoughBorough
LE11 1ZJ
UK
Tel:+44 (0)1509 238088
The Grid circuit is unchanged from the original article.
Neutralising tabs are also as in original article.
See the following drawings for details of modified anode
circuit etc. This was necessary because the valve bases are
a larger diameter and need to be spaced further apart,
hence the length of the line has to be shortened. The other
nice thing is that the lines are straight for the full
length, not needing to be bent up. Valve changes are very
easy with these valve bases, no need to unfasten the anode
line clamps! Bases are really cheap, I paid £8 for a pair
in about 1983. Price in March 1999 was £18 for 2 bases,
still quite reasonable.
In class AB1 the amplifier needs less than 1 watt of drive
for 600 watts PEP output.
For EME use I can reduce the bias to allow class AB2
operation and with 5 watts drive get 1 KW out on CW. Get a
transformer that will give about 1600 volts AC at 800mA for
the EHT. You need +350V stab for the screens for SSB.
(+300V for CW class AB2).
Transformers - I have found a very helpful
firm that can supply custom wound transformers at
reasonable prices.
(Thanks to Bob Carpenter, G4BAH for updating this item)
Alphabet Electronics Ltd.,
Hacche Mill,
South Molton,
EX36 3NA.
Tel. 01769-572055
Fax. 01769-573946
Steffan Nutt is most helpful.
For example:-
1500VA
Primary 0-230-240-250V 50Hz Interwinding
screen
Secondary 0-6.3V @ 8A, 0-18V @ 2A, 0-125V @ 100mA,
0-350V @ 150mA, 0-1600V @ 800mA
Size 150mm x 130mm x 143mm
high
Price: April 1997 £135 plus £7 carriage plus VAT. Delivery
2 weeks.
I particularily recommend that you read the chapters on PSU
designs and interlocks in THE VHF UHF DX BOOK
Neutralising the Amplifier
Here is the method I used and found most straightforward
and safe.
1. Disconnect EHT lead and remove Screen Voltage (pull the
screen fuse).
2. Apply Blowers, Heaters and Grid Bias.
3. With a small amount of RF Drive at 144MHz to the input
circuit, sufficient to show some reading on the grid
meters, measure the Output Power on an RF millivoltmeter
connected to the RF Output connector.The RF millivoltmeter
must be terminated in 50 ohms.
4. Peak the Grid and Balance the Grid (using the Grid
meters).
5. Peak the Anode tuning for maximum power on the RF
millivoltmeter.
6. Adjust the neutralising tabs for minimum power on the RF
Millivoltmeter.
7. Repeat 4,5,6 until lowest amount of power at stage 6 is
obtained.
References
1. "New Ideas for the 2-Meter Kilowatt" by Thomas F.
McMullen Jr, W1QVF and Edward P. Tilton, W1HDQ, QST
Magazine, February 1971 (ARRL).
2. THE W1SL 144MHz POWER AMPLIFIER by John Nelson,
GW4FRX.
THE VHF/UHF DX Book, 1992, DIR Publishing, ISBN 0
9520468 0 6
McMullen later obtained the callsign W1SL and that is the
name that this design of amplifier has taken. Thomas
McMullen, W1SL died in 1993.
Translation of English to American
Valve = Vacuum Tube
Anode = Plate
Heaters = Filament
EHT = B+