During the summer 1997 I completed my new GU-84b tetrode power amplifier. I used my previous 4CX250R PA power supply for this (Picture 1). Because there have been a few requests for construction information, I decided to write this article. Please do note that this is not made for first PA project builder and I will not take any responsibility about errors of this document. Picture 1 (at right): GU-84b amplifier and power supply Click on bordered pictures for zoom

GU-84b Tetrode

GU-84b is the military version of the Svetlana 4CX1600b. GU-84b is made for bigger anode dissipation and current. It also needs more powerful blower than 4CX1600b (picture 2, below).
Picture 2: GU74b(x2), GS23b, GU84b and 4CX250R
GU-84b socket has air chimney and cooling air is then feed through grid cabinet to anode. Svetlana socket SK3A can also be used but in that case cooling air is feed first to anode cabinet. Socket cooling is important and should not be forgotten. The both sockets have a built-in Screen capacitor and therefore it is possible to connect the cathode to the ground. GU-84b socket has also 0.5W resistor for anode current measurement purpose. NOTE: NOS, Old Stock or long time unused tube shouldn’t be connected to plate and screen supply without first carefully wakening it up. I would recommend at least 24 hours of heater time before applying other voltages.		Picture 3: 4CX1600b & SK3A

PA Working Conditions & Cooling

The power output with my power supply is about 1500 W in linear service. The efficiency is about 65 %. Plate voltage is 1900 V @ 1.2 A. The tube needs also 350 V on screen and –84 V (-120 V or more on rx) on grid (G1). Filament uses 27 V @ 4A. GU-84b maximum anode dissipation power is 2.5 kW. With this power the tube needs really big blower that can give 75 mm H2O pressure and 2.5 m³/min airflow. With 1.6 kW dissipation tube needs 30 mm pressure and 1.6 m³/min airflow. With my own experience at 1 kW output level and HSCW (high speed CW) in meteor scatter mode the output air temperature (20ºC room temp) will change from 30ºC®50ºC within one 2.5 min TX period. The blower is in separate box and cooling air is feed in the 90 mm aluminium tube to PA. On TX the pressure is 32 mm and on RX 20 mm in the input cabinet. Smaller fan could also be used as can be seen from air temperature.

Power Supply

The filament PSU is 27 V @ 4 A DC with current limit circuit. This feature is good for a soft start. I use a L200 regulator and a pair of 2N3055 transistor for this circuit. The filament resistance is 2.5W when it is cold and it takes 3 seconds until current is below 4A. The tube manufacturer recommends that cooling should remain up to 5 minutes after filament is turned off. The idle anode current is less that 200 mA during TX when G1 is -84 and G2 350V. On RX G1 voltage should be at least -120V, otherwise noise will decrease RX sensitivity. Please be very careful that anode voltage is not disconnected before G2. More valuable information for PA and PSU is found from http://www.svetlana.com.

PA construction

The plate is a ½ l coaxial resonator and grid an LC circuit. Before building I recommend visiting SM5BSZ homepage to see what Leif says. The input is directly from there and the Q-value is decreased with the resistor. But still the gain is about 16 dB. There is no need of neutralizing the PA with this construction. The output circuit is based on the W9QXP (now W7QX) 2 m and 70 cm PA. Even though Jerry says that ½ l is too big for 2m PA. I didn’t agree with Jerry and decided to give a try with this mechanically larger but simplifier construction (Picture 4). Aluminium is not so expensive, I thought.
		Picture 4: Plate Line Construction

At first I had difficulties to find the right length of the anode resonator. With the help of Jukka, OH1FF I found the correct resonator length. Because of this the cabinet is about 200 mm longer than needed and there are extra holes for tune and load capacitors. However now it is possible to change the tube to 4CX1600b. It needs 75 mm longer resonator.

Picture 5: Plate tune & load capacitors

Output load and tune are both capacitive because more simpler mechanics than inductive version (Picture 5). The efficiency is now a little bit worse with this construction. Amplifier cabinet is 1 mm aluminium. The 100 mm diameter anode resonator tube is made from this same Al-plate and connected with Al-rivets. Of course copper or aluminium tube would have been much better choice. I just didn’t find any. The 1 mm thickness is not too strong so I have to use aluminium profile to make enclosure more rigid. I used very thin household Teflon to lead the cooling air out. This 0.15 mm PTFE sheet is normally used for baking underlay.

Picture 6: Input circuit (note the non-inductive resistors in the center of the picture)

Picture 7: Input circuit view #2

Tube parameters

GU-84b and 4CX1600b maximum values are presented in the following table.

	GU-84b (GY-84b)	4CX1600b
Anode voltage	2.2 KV	3.3 KV
Anode current	2.0 A	1.4 A
Anode dissipation	2.5 KW	1.6 KW
Frequency	250 MHz	250 MHz
Screen grid G2	400 V	350 V
Grid G1	-150 V	-150 V
G2 dissipation	30 W	20 W
G1 dissipation	1 W	0.1 W
Filament voltage	27 V	12.6 V
Filament current	3.7 A (3.4-4.0)	4.4 A
Filament/cathode warm-up time	3 min	2.5 min
G1 Cin	102.5 pF (90-115)	86 pF
Anode Cout	20.5 pF (18-23)	12 pF
Cgp	0.2 pF	0.15 pF
Transconductance	71 mA/V (50-92)	50 mA/V
Anode diameter	98 mm	86 mm
Anode temperature	200 °C	250 °C
Air flow 1600W (flow/pressure H2O)	1.6 m³/min, 30 mm	1.0 m³/min, 10 mm
Length	111 mm	109 mm
Weight	1.4 kg	0.75 kg

Input Circuit Tuning

The input circuit was a little bit complicated to tune than output. Jerry W9QXP uses 50W resistor input that needs no tuning at all. I did try this but with poor result. I couln’t get more than 600W output with 100W drive. I decided to use a traditional LC-circuit. Here are some hints how to pre-tune it.
·	Install the tube and connect blower and filament on. Note: disconnect all other wires. Connect 8KW resistor between G1 and ground. This resistor simulates normal operation conditions: 90V @ 11mA.
·	Use a multimeter to measure DC voltage between G1 and ground over the 8 kW resistor.
·	After 3 minutes heating time connect drive for 144 MHz and change tune and load so that there is maximum voltage in G1. The tube works as a diode and rectifies all RF. Do not touch to anode or G2 because there are very high voltages.
·	Input SWR should also be near 1 at the G1 voltage peak settings. The maximum needed 80V is reached with drive level 30 - 40W.
·	Finally the input adjustment is made with normal working conditions with SWR meter. After this there is usually no need to change input tune or load capacitor values.
Finally
This amplifier has been in use since June 1997. There has been no problem with it. In my mind the construction is simple, even though it uses a lot of aluminium. I can recommend this project without any reservations.