4 Attachment(s)
2 meters..EME GS-31B amplifier
Hi all once more....This amplifier is the final one of this lot recently built for VK hams...It is the twin of the GS-35B 6 meter amp I posted yesterday but it uses a GS-31B tube...The GS-31B is rated at 1kw anode dissipation but otherwise the tubes are basically the same...The GS-31B tube construction allows the blocking capacitor RF choke etc to be mounted on a copper plate standoff so that there is minimal heat conducted toward the tuned output network which would detune the circuit...Other than that the amplifier is much the same as it's 6 meter counterpart..
The tuned output circuit consists of a coil and plate line..Tune control is a simple brass flapper capacitor securely grounded and the Load control is a simple copper disc connected to an N type socket mounted on an adjustable plate assembly...The tuned line represents a half wave circuit and extremely high RF voltage is present when the amplifier is being driven..However when properly set up and tuned it is quite efficient and gives about 400khz bandwidth with just a bit of peaking of the tune control...The load is set and forget and once the amplifier is set up into a 50 ohm load the Load capacitor is locked needing no further tuning unless the load impedance is other than the set 50 ohms..
The input uses a tuned T network...This is very critical to tune to get a flat 1-1 match as the input and output circuits are very interactive...Once the sweet spot is found no further tuning is required...
This amplifier is designed to run 500 watts of Data, JT65 etc for EME work...It only takes 25 watts drive so the digital mode load on most modern transceivers is minimal...At 500 watts continuous output the center post of the tube reaches only 87C and tube seals at that output reach 65C...I use a proper Infra Red Thermometer to take these measurements while the amplifier is under load...
For SSB this amplifier easily runs 1300 watts with 60 watts of drive..This is in line with the tube specs of 13db gain......There are no relays on the amplifier the owner using an external coaxial relay setup along with a Sequencer essential with his expensive antenna mounted preamps
These amplifiers are all very straight forward simple constructions...There are no inbuilt safety interlocks nor grid protection circuits...I work on the theory that those who use these amplifiers have a good knowledge of safety procedures when using this equipment...When these amplifiers are plugged into live mains there is a warning pilot lamp on the front panel to indicate mains on....The fan that cools the cathode circuit is wired directly into the mains socket on the amplifier and starts as soon as mains power is turned on..
This amplifier has been in use with the 6 meter unit for a couple of months now and the owner is very pleased with the results...In VK there is a 400 watt power limit on the VHF bands...A high power permit can be applied for through the ACMA...The operator must be able to prove that he has the knowledge and all necessary equipment to operate the station to the guidelines set out by the authorities..
Well, that covers three very interesting projects I have had the pleasure to build recently...I have included a few pics in this post to show the construction progress to the finished amplifier..
Enjoy...Bob..VK3ZL..
