GS35b 6m Amplifier

           

Here are some pictures of my GS35b amplifier for 6 meters I built back in 2005. 90W drive delivers about 1500W output. This amplifier was designed to run 24/7 key down for heavy contesting and high duty cycle WSJT modes. The output network is a Pi-L to suppress the second harmonic and a T-network is used for the input. I based it on K8CU’s 6m amplifier and pulled some ideas from other places like the ND2X QRO site. It was very easy to build using a drill press and hand shears. Cutting metal was 90% of the work. There are a lot of pictures on this page. Please allow time for them to load.

 

This is not a SMALL amplifier! It’s built for continuous duty.

Power Supply

The plate transformer is a Peter Dahl unit rated for 2750VAC at 1.5A CCS. Input voltages are selectable for 220VAC and 240VAC. A step start provides a 2 second delay to limit in-rush current. All parts in the power supply were derated by 50% for ample margin. A medium sized box fan moves air over the bleeder stack and the 50 ohm glitch protection resistor. The plate power supply cabinet makes a nice “table” for the RF deck to sit. Overall the amp stands about 3 feet high and reminds me of one of those big floor model Henry amplifiers.

Control logic is from the triode control board. The HV supply cannot turn on until the filament has been on for three minutes. After the time has expired, the control circuit turns on the HV supply.

 

RF Deck

Brass strap and screws are used for the anode connections. A Teflon chimney not shown here is held in place with a hose clamp. Layout was not critical since a lot of room was available. Connections were made as short as possible. The Dayton blower forces air through the bottom the chassis and out the top. The tank coil is made from ¼” copper tubing. Brass tabs were soldered to the ends of the coil to allow for screw mounting. Screws were not spared in constructing the walls and lid to have excellent RF shielding. I have ZERO TVI problems without a low pass filter on the output and meet spurious output levels at full power output. Harry, WA4OFS (SK) machined my tube grid ring mounting socket that is bolted to the deck.

 

Tuned Input Setup Notes

Setting up the tuned input without powering up the amplifier is easy. With the tube in the socket, solder a temporary 110 ohm non-inductive resistor with an 18pF capacitor in parallel to the tab where the input connects to the tube cathode. Hold the other end to ground. Hook up a MFJ 259 analyzer or equivalent to the input. Adjust the input variable capacitor and squeeze/spread the inductors to obtain a 1.1:1 VSWR. This should not be a sharp dip. Remove the temporary resistor. The input impedance of the tube is higher on 50MHz than the published value of 35 ohms for an HF amplifier.. It’s closer to 90-100 ohms. This may take some experimentation to get it right.

 

Operational Data

Plate Voltage   3800V

Plate Current   850mA

Grid Current    260mA

Idle Current     150mA

Watts Input      90W

Watts Output  1500W

 

 

Inside the Anode Compartment

 

Bottom of the RF Deck showing the G3SEK Triode Board and Tuned Input Box

 

Inside the tuned input enclosure. The filament choke is near the bottom.

 

Another view of the bottom showing input cover in place and HV wiring attached

 

Close up of the G3SEK triode board and associated wiring

 

Associated Links

W4ZT GS35b Website

ND2X QRO Russian Tubes

G3SEK Triode and Tetrode Boards

GS35b Datasheet