4 813's ON THE DESK TOP
A 10 - 80 METER DESK TOP KILOWATT AMPLIFIER
Here are a few pictures of my four 813's desktop (G2DAF) amplifier built about
18 years ago while living in the outback here in Australia. Luckily, I brought most parts for the amp with me from America when I immigrated as it seems that some items are either hard to find here in OZ or very expensive.
Originally the amplifier was wired in the typical "grounded grid" configuration. Some problems were encountered with low output of 700 watts maximum on 80 meters and very low output of 200 watts on 10 meters which I believe was contributed to large amounts of interelectrode capacitance in parallel when wired in GG, but I could be wrong. (comments/advise anyone?)
After some discussions with Doug VK6ER and being inspired by his nice 4-1000 multi band amplifier wired in passive grid G2DAF configuration, I decided to give this configuration a try. Doug warned me "those bottles will take off like rockets". And they do now! Output is 1000 watts into a 50 ohm dummy load on 10 and 15, 1100 watts on 20 and 1200 watts on 40 and 80 meters. The amplifier is also using a high wattage, non-inductive, passive resistance of 50 ohms rather then the typical value well into the hundreds of ohms and the band switching on the input has now been eliminated.
HV is 2800 VDC with no load and drops to 2500 VDC under full load. Screen voltage is derived from sampled and rectified RF from the input/passive grid resistor. K2AW HV diode modules, HV electrolytic caps, Passive grid resistor and TR relays etc. are in the under chassis.
The chassis is 12in.(H) x 17in.(W) x 14in.(D) or approx. 305mm x 430mm x 355mm. The weight of the amplifier is 68 lbs. or approx. 31 kg.
Those interested in reading about the G2DAF configuration can find the whole original article published in RSGB Bulletin, April 1963 in .PDF file format (720kb) on a few different internet sites.
SCHEMATIC #1 SCHEMATIC #2 SCHEMATIC #3 SCHEMATIC #4
TOP (reverse view)
SCREEN DIODES & G2DAF GRID RESISTOR
SOME RECENTLY DISCOVERED B&W PHOTOS
AND WHAT I TESTED THE OUTPUT OF THE AMPLIFIER WITH.
THE RESISTOR STACK OF THE HOME MADE DUMMY LOAD. 5 RINGS AT 10 OHMS EACH, IN SERIES AND SUBMERSED IN TRANSFORMER OIL. THE RESISTORS WERE PUT IN A LARGER 1 GALLON PAINT CAN NOT SHOWN IN THE PICTURE.