Hi again all...This is one of a pair of amplifiers I built recently for a ham in VK3...His main interests are EME work and playing with the high GHZ frequencies...He wanted amplifier's for 2 and 6 meters that would reliably withstand the hard wear that the data modes put a tube through but straight forward without bells and whistles..
As I have already designed a few of these amplifiers I was able to put this 6 meter amplifier together for him..The amplifier is built around a GS-35B Russian 1500 watt anode dissipation pulse triode....In my experience these tubes are rugged and reliable and only a fool will damage one..
From the pics you can see the simple construction...I machined a socket from 12mm thick aluminum stock and bolted it to the chassis floor...The tube slots into the socket and is secured with two simple "keepers"..The RF deck is sealed from the cathode sub chassis and is pressurised by a 5.5 inch Papst 12-28vdc fan..These fans are big and produce 285cfm...The air blows into the RF deck and evacuates through the massive copper tube cooler, through a Teflon chimney attached to the top cover..
The Pi-L output circuit uses a 1-10pf vacuum capacitor rated 12kv..I used an old WW2 150pf capacitor for loading...The cathode circuit uses a tuned T network as seen in the pictures...Ht power supply as a simple voltage doubling circuit using 6A10 rectifier diodes and 8 500pf 390mfd electrolytics...The amplifier uses one of my custom made Toroidal transformers..This unit is rates at 2kva and gives about 3800 vdc peak on the highest tap..I use the simple relay resistor step start system in all my amplifiers..The amplifier bias is developed through a small bias system developed by Tony King W4ZT (SK) ...I always bring the bias set control out to the front panel so that the correct idle current can be selected for the mode of operation...The input and output relays are standard off the shelf units....The output relay is a tiny 24 volt AC unit with two sets of 20 amp contacts with no internal wires..AC relays work quite happily with DC and this particular relay has never let me down in many amplifiers..
A couple of notes of interests for anyone home brewing using these Russian tubes..These tubes like the 8877 exhibit around 50 ohms driving resistance into the cathode..I have seen many times where hams have used these tubes and omitted to use a proper input matching network to the cathode...Usually they will then complain that the tubes are very inefficient and need heaps of drive....No short cuts can be taken and a properly tuned network is essential to get the best from these tubes...I use T networks on both my 6 and 2 meter amplifiers and trim them so that when properly adjusted to 1-1 input maximum output power is achieved ..With careful attention to details to both input and output networks the stated 13db is obtainable...It only takes 75 watts to run these tubes to 1500 watts output...Many state that they need 100 watts and I know exactly why..
These Russian triodes don't have the gain of a 8877 but for $120 they are excellent value and don't need all the complicated costly grid protection circuits..
Enjoy the following pictures..
Bob..VK3ZL..
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Originally Posted by KA9MOT
