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GS-35b Linear Amplifier
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GS-35b Linear Amplifier
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Pictures of my GS-35b legal limit linear. Up and running on May 8, 2007.


RF deck: 17" x 12" x 3" chassis,  tube, socket, plate choke, tuning capacitor, 75 meter inductor, loading capacitor. Underneath: filament transformer, filament choke. 


Rectifier and filter assembly (fits on top of the control chassis): two (2) 17" x 12" sheets of Plexiglas(R). The bottom sheet is held above the control chassis by 1/2" nylon spacers. The top sheet, with eight (8) 3" diameter holes, is held above the bottom sheet by 2" nylon spacers. Eight (8) 900 uF 450V filter capacitors, eight (8) 10k ohm 20 watt bleeder/balancing resistors, a bridge rectifier made from twenty four (24) 3A 1000PIV diodes, with a .01 uF 3kV capacitor across each diode, two (2) 100 ohm 25 watt glitch resistors in parallel, and a 2A fuse.


Control chassis: 17" x 12" x 3", a 240 volt AC supply on/off contactor/relay, a step-start contactor/relay and 1 second RC delay circuit, two (2) 10 ohm step-start current-limiting resistors, a thermal fuse on one of the step-start current-limiting resistors, a grid bias, metering, and control board, and a 16 VAC transformer for the control circuitry. The HV rectifier and filter assembly fits on top of this chassis.


Here is the RF deck with its front rack panel attached.


Here is the control deck + bridge rectifier/filter assembly (mounted on top) with its front rack panel attached. You can also see the meter panel (more out of focus than usual).


Meter subassembly attached (temporarily) to the rf deck. The transformer is a relatively small (1500 VAC 500ma CCS) temporary plate transformer... being used while I wait for the 2400 VAC 1.5A CCS transformer to arrive. With this temporary transformer, I can operate at ~160 watts output at carrier with ~10 watts of drive from my Johnson Ranger (2000 volts at 250 ma on the plate of the GS35b). When I get the larger transformer, I plan to operate at ~375 watts output at carrier (3000 volts at 400 ma on the plate of the GS35b). This corresponds to ~31% efficiency at carrier and 62.5% efficiency at 4x carrier (the peak rf level that corresponds to 100% modulation)


Here it is, assembled, in a 20 rack unit Raxxess economy rack (particle board + laminate veneer).


A bit of a "rat's nest"... but I'll fix that later.


This is the new plate transformer that I purchased from Tom Hand, W4WDS. It is a beautiful work of art! 240 VAC input, 2400 VAC CT @ 1.5 amps CCS output. It also has an extra pair of output taps for 2150 VAC CT. Since I am using a bridge rectifier, the center tap is not connected. Tom shipped it to me in a crate, which was placed inside of a box, which was placed inside of a second box! The wooden base is the bottom of the shipping crate. The particle board on top is a protective cover I made up to keep my fingers away from the input and output terminals. One of these days I'll replace the wooden cover with one made of Plexiglas or Lexan(R). With this transformer, I operate the amplfier at 375 watts output at carrier (with 20 watts input), 100% modulation, and excellent linearity. I measure the linearity by observing, on a dual trace oscilloscope,  the rf envelope at the output of the EF Johnson Ranger driver superimposed on the envelope of the output of the amplifier. If I put the scope into the mode where one input is on the x-axis and the other is on the y-axis, I see a nice straight line when I modulate. Most of the time, I just observe the superimposed waveforms vs. time. That way I can see the output of the Ranger and the output of the amplifier at the same time. 

Below is a simulation of a few power supply waveforms. Note that the filter capacitance is 112.5 uF (900uF/8); the transformer secondary resistance is 45 ohms (9 ohms intrinsic to the transformer + 36 ohms added in series with the secondary, using power resistors). The load is drawing 400 mA of average current.
The red trace is the voltage across the filter capacitor. It varies between approximately 3220 volts and 3240 volts... corresponding to a 20 volt triangular-shaped ripple at 120Hz frequency. I.e., the 120Hz ripple is about 0.6% of the average voltage... which corresponds to 44dB of ripple attenuation.
The light blue trace is the 0.4A load current.
The medium blue and light green traces correspond to the current through the diodes of the bridge rectifier on alternating half cycles. Note that the peak current (for this value of secondary resistance) is roughly 3 amps, and the current waveform is roughly trianglular in shape (the base of this triangle is roughly 27% of the width of a half cycle),



Here is an update view (June 24, 2007) of the back of the amplifier. Things I added since the prior photo was taken: 1) A DPDT antenna switch, to allow me to easily bypass the amplifier. 2) A small "Bud box" containing a 15:1 current transformer, whose secondary is terminated by a 50 ohm resistor. This allows me to observe the input current waveform to the cathode of the tube. It is interesting to see the actual waveform, which shows the cathode current flowing for only part of the rf cycle, and increasing abruptly when the cathode is driven negative with respect to the grounded grid. 3) A high voltage fuse at the point where the B+ attaches to the plate choke, following the approach that Ken, W2DTC has published on his web site. It consists of a 3 inch long piece of #30 copper wire, suspended between two nylon insulators. There is also a 2 amp conventional fuse on the B- side of the power supply. 4) A Plexiglas cover, hanging over the back of the RF deck . 5) A 5 megohm auxiliary bleeder that is used to drive a 1 mA meter for measuring the plate voltage. The current from this bleeder also passes through a circuit I use to monitor whether the high voltage has bled down, before opening the Plexiglas cover. The circuit drives a red LED, which stays illuminated until the high voltage drops below 100 volts. 6) An antenna tuner in series with a 4MHz band pass filter, to provide a nice match between the EF Johnson Ranger and the input to the amplifier. The 4MHz band pass filter blocks harmonics of the drive signal, produced at the non-linear impedance at the input to the amplifier, from traveling back toward the Ranger's output tank circuit. Those harmonics are reflected by the filter, back into the amplfier's input. 7) I used four (4) ferrite cores and seven (7) turns of #14 insulated copper wire to make an output choke, which will blow the fuse if the plate coupling capacitors short out (preventing high voltage DC from appearing on the output of the amplifier). Finally, I changed the position of the tank coil to make it easier to change (for operating on other bands); and I added a shorting jumper that I have used to operate the amplifier on 40 meters. To operate on forty meters, I attach the crocodile clip to one of the coil's turns, which reduces the effective number of turns from 15 3/4 turns to 9 turns.

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