Hi all....It has been a while since I posted one of my amplifier projects...I decided to up the power stakes in a new amplifier for 6 meters and put this unit together using a Russian GS-35B 1500 watt anode dissipation triode...I have experience using these tubes in my water cooled 160 meter amplifier and have found the Russian triode an exceptionally rugged and reliable generating device...
My first job was to source a suitable HT transformer compatible with the tube....I ended up ordering a custom made hefty Toroidal unit with multiple secondary taps for flexibility so that it can be used with other tubes...The pictured transformer was the result and after testing I ordered a bulk lot for future projects..
The actual amplifier project was straight forward using my past experience setting things out so that servicing is easy....The amplifier has a hinged front panel allowing access to the control circuits, step start and bias circuits etc...
The RF deck contains the tube, plate choke and a Pi L output circuit....I used a new Jennings 1-10pf vacuum variable 25kv neutralising capacitor for plate tune, a 1/4 inch silver plated inductor and a new 200pf German kit capacitor for loading....The L coil is a 5 turn 10 gauge inductor....The amplifier uses a hefty Russian coaxial relay for output...
The input is simple consisting of a Tunable T network...The filament tranny is Toroidal, multi tapped primary so that I can set the heater voltage to 12.6 volts..
The serious stuff is in the HT power supply of course....The rectifier consists of 22 6A10 1kv 6 amp diodes and 10 470mfd 450 volt electrolytic capacitors...I used the 1425vac tap on the HT transformer to give me around 3700vdc.....Now this is serious stuff to have on your desktop and if it catches fire don't pee on it...That will hurt...Keep the kids away from it also..
The amplifier cooling is done with a large heavy 6 inch Papst 7450ES Metal Bladed fan...The RF deck is pressurised the air being drawn in through the HT PSU..The air exits through the tube cooler and Teflon Chimney on the top cover....Air pressure shows .4 inch on my manometer, more than satisfactory..The cathode compartment is sealed from the RF deck and is cooled by a smaller aux fan..
I used all anodised aluminum for the amplifier case....I carefully ground away all surfaces at the joins where the surfaces came together...The case came together beautifully and looks very nice and is completely RF tight..All earthing points were cleaned both sides to make sure of good contact...
I finally finished the project last week and tested all electrical functions ensuring everything worked as designed...It was time to apply a bit of RF and see what happens....I set the amplifier up with two Bird watt meters and a Bird 1kw dummy load and connected up the Yaesu 897....First job was to run 10 watts in and set the input T network to 1-1 vswr...Then I proceeded to tune and load the amplifier in increments...Everything went beautifully and the final outcome was better than expected....Something to note using these triodes, the input needs to be finely tuned as you increase power to get the most efficient transfer of power and grid current...I gave the amplifier a few days of hard running using CW fast dots at 1kw output...30 minutes on and 5 off....It has never missed a beat and power output doesn't vary....The poor Bird dummy load doesn't like it though and started to protest eventually..
The final figures were 1600 watts carrier output with 95 watts of drive....The amplifier "sweet spot" is 1200 watts with 60 watts drive....The efficiency starts to drop away at higher power mainly due to the HT transformer limit..
I designed this amplifier as a 1kw output unit to suit the new 1kw power limit being introduced in March this year....I think I have achieved all I set out to do...
Messing around with this stuff is fun and sometimes terrifying if the XYL walks into the workshop and says BOO....Being a cantankerous old fart and at my age only having a short attention span I often wonder how I havn't electroplated myself..(:-> However I love doing these jobs and I am already building Mk2 of this amplifier and another 2 meter version...
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