W2DTC's HOMEBREW 3CX3000F7

GROUNDED GRID TRIODE HF LINEAR AMPLIFIER

click on any photo to ENLARGE

The 3CX3000F7 power grid triode with its 4000 watt plate dissipation loafs along at legal limit Amplitude Modulation, plus It only needs one D.C. power supply. The rugged 3CX3000F7 has a grid dissipation of 225 watts vs the popular 8877 which only dissipates 25 watts. The 3CX3000F7 (up to 30 mHz) is exactly the same electrically as the 3CX3000A7 (up to 110 mHz). The 3CX3000A7 requires a tube socket whereas the F7 is bolted to the chassis. The cabinet, PI-L tank and right angle drives were leftovers from a partially constructed Gates transmitter. With the major mechanical parts in place, the 80, 40 and 20 meter design was easy to build. In 2007 I added the 160 meter band, which is described near the end of this webpage. (Schematics are also included).

TUBE INSTALLATION:



The original Gates 4CX3000A socket and wiring was removed and a homebrew heavy aluminum template for the 3CX3000F7 was mounted on the Gates chassis. Installed the 3CX3000F7 by setting it's grid ring on the top of the homebrew template and holding the grid ring down with two heavy homebrew aluminum half-circle mounting strips. I had an Eimac SK-1406 tube chimney for a physically larger 4CX3000 so I made up the space difference by inserting a ring of teflon between the tube and the chimney and "glued" it in with GE Silicon Kitchen and Bath caulk.

PLATE CHOKE INSTALLATION:



Added a homebrew plate choke, glitch resistor and homebrew parasitic choke (plate circuit schematic at bottom of this webpage with wire size and turns count). Full power tests on the 3CX3000F7 were conducted on the popular AM frequencies of: 3885 kHz, 7290 kHz and 14286 kHz with no signs of instability. (NOTE: Internet research revealed that no parasitic choke was required with this grounded grid triode and I removed it when I made my 160 meter modifications). See 160 METER MODIFICATIONS below.

CATHODE CIRCUIT INSTALLATION:



Installed the 3CX3000F7 filament transformer, two 30 amp filament chokes in parallel, the RF drive coupling caps and installed eleven 6 amp diodes between the center tap of the filament transformer and ground to set the bias at -7.7 volts for a no drive idling plate current of 325 mills. The original 1000pf coupling caps were replaced by 6800pf (yellow ones) for a little better match to the cathode as noted by less reflected power. Since there is still some reflected power with the 6800pf capacitors it suggests that an even greater value would be better. (NOTE: 1. Internet research revealed that the bias diodes were unnecessary and were removed. 2. For 160 meters the two filament chokes were replaced with a higher inductance choke). See 160 METER MODIFICATIONS below.

ANTENNA PROTECTION CHOKE :

If the plate coupling capacitor should short out, this 40 uHy choke from the ouput PI-L inductor to ground (same as being across the output coax connector) will short the B+ and cause the high voltage fuse to blow.

HOMEBREW HIGH VOLTAGE POWER SUPPLY:



Built a full wave bridge, capacitive input, Peter Dahl transformer based 5000 Volt power supply into a standard 3 foot Bud cabinet (Schematic at bottom of webpage). A soft-start delay was built using a 30 second solid state cube (ABB SSAC Solid State Timer part number TS14130) in series with a 60 amp contactor that shorts out a large wire-wound start-up resistor. For diodes I paralled two sets of 14KV K2AW Silicon Alley rectifier diodes and mounted them on a 5" x 7" x 2" heat sink. The high voltage fuse between the two standoff insulators was made from a single four inch piece of #30 magnet wire. If there is a high voltage short in the 3CX3000F7 compartment, the fuse material explodes and disappears (this feature was tested). High voltage wire between the power supply cabinet and the amplifier has a 30KV rating and is encapsulated in shielding stripped from LMR-400 coax cable, which is grounded at both the amp cabinet and the power supply cabinet. The power supply has a rating of 5000 Volts at 1.7 amps CCS with a 133 MFD 8100 volt bank of computer grade caps inside a Bud cabinet. The power supply cabinet and the 3CX3000F7 RF Deck cabinet are connected by Scotch 25 Electrical Grounding Braid which also connects to the common point station ground. Click HERE to see my other high voltage supply.

GARAGE LOCATION AND REMOTE MONITORING EQUIPMENT:



Outside view of both the 3CX3000f7 linear, the power supply and the remote viewing wattmeter. (The top section of the 3CX3000F7 amp cabinet has a switchable inductor and a vacuum variable for tuning out the 2nd harmonic; however, the 2nd harmonic is so far down that I keep it in the switched-out position). Even though I have an oscilloscope at the operating desk in the ham shack, I added an inexpensive oscilloscope and RF pickup loop in the garage to save me those back and forth trips while tuning up. The wattmeter reading camera has a 900 mHz transmitter. Video output from the remote 900 mHz receiver plugs into a video monitor in the ham shack.

POWER UP TESTS: Tests were conducted at 3.885 mHz with the following setup (refer to block diagram below): The AM driver transmitter was 100% modulated by a 1000 cycle audio tone. The input and output waveforms were sampled with RF pickup couplers and oscilloscopes. After the initial highest power test, with amplifier loading controls adjusted for linear operation, the 1 KW Navy dummy load (DA-75/U) was given a cooling off period, then the rest of the power output tests were completed.



Block diagram of test setup prints best in landscape orientation.



NOTE: When the final tank circuit was tuned for the highest RF carrier, using Bird wattmeter readings, significantly more output power was obtained than the data shown below; however, when modulation was applied, the oscilloscope showed distortion. The final tank circuit was retuned for true linearity with an AM waveform on the scope and the data (shown below) was recorded. The plate voltage was 4500 VDC. Input and output power in linear operation:

25 watts in---- 375 watts out

50 watts in-----750 watts out

100 watts in---1500 watts out

200 watts in---3000 watts out

300 watts in---5600 watts out

400 watts in---6000 watts out



Since the output power is fifteen times the input power, I set the AM driver transmitter for 25 watts of carrier (100 watts PEP) which yields a 375 watt carrier and a clean 1500 watts PEP. Another nice thing about the extra 3CX3000F7 tube plate dissipation is that I don't have to retune the amp to move around the band, even when the antenna feed presents a less than desired SWR reading.

SCHEMATIC DIAGRAMS:



A B C

A: Schematic of the 3CX3000F7 plate circuit (print in landscape orientation).

B: Schematic of the 3CX3000F7 cathode circuit (print in landscape orientation).

C: Schematic of the 5000 volt 1.7 amp plate supply (print in landscape orientation).

160 METER MODIFICATIONS COMPLETED IN 2007: