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A600 PA project : Problems and solutions.
The use, copy and modification of all info on this site is only permitted for non-commercial purposes,
and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

==>> Always FIRST press F5 <<==

My original amplifier PCB V2 showed the following :

1. A to low parallel impedance was measured over balun T5 (possibly damaging Q2).
2 When switching-"ON" the external 50Vdc supply, resistors R31/32 injected an up to +17V DC-pulse onto the FET gates.
3 Q1/2 outputs were not forced to balance, and chokes L1 and L2 could show DC saturation.
4 The output of TR1 was floating, not forced to balance.
5 The amplifier bandwidth was not limited to the used frequency range.

After a lot of E-mail traffic between ON9BOG and me, again testing and again damaging another Q2, the  below described modifications were developed. They resulted in (20210125) :
-  A robust amplifier.
-  Less needed drive (1Wrms).
-  Efficiency at 400W SSB : 48%.

TIP : By adjusting both FET gate bias voltages for each FET Idq = 0.9 Adc, the best amplifier linearity was achieved (see NXP data =>). And NOT driven to a distorted 600W output, but to a clean (and legal) 400W SSB output.

After eight modifications, my PA fulfils my expectations. Performance is reliable,
as long as drive power is max 1W, and not causing to high output power combined with an LPF set to a wrong frequency band.


Modifications.

REM: see also modification F, G and H.


Ver. 20201018. E1. 17.5 uH.
This balancing transformer is replacing L2 and L3.
Both winding halves are connected in series

Better transformer.
Ver. 20201217. 1.72 uH. See changes to C8 and C9.

E2.

                    Modifications to prevent damages to Q1/2
                     in the order of importance 
                               
Ver. 20210320.
A.  The attenuator shunt is not needed anymore. 
The Maximal needed drive power showed to be about 1Wrms.
When using a 100W TRX and a 20dB 100W input attenuator, the drive power is limited to 1Wrms in order to protect Q1/2 and to prevent IMD (this 1W will mainly be dissipated into the 11 Ohm gate resistors).
F.  Improve output balance. Replace balun T5 by a better higher-Z Common Mode Choke. Wind fourteen (14) turns RG316 on a 29mm #31 FairRite core 2631801202 .
D.  Prevent the injection of a DC pulse on Q1/2 gates when switching-on the 50V power supply. Connect 10uF 25V tantalum capacitors with minimal short wires each in parallel to C4, and to C7.
C.  Replace C3 and C6 each by a 47nF 400V film capacitor. 
E.  Improve the output balance further by replacing chokes L1/2 by one balanced transformer (TNX to ON9BOG) 
-
Wind 3 turns 6x2.8 mm (1.5mm2) twin speaker cord on a 8x50mm Ui=300 ferrite rod (Conrad orderNr. 1565952 ). Total inductance about 1.72 uH.

Then tune this transformer to about 7.5 MHz resonance as follows :
- Solder at Q1 between drain and source 330 pF MICA .
- Solder at Q2 between drain and source 330 pF MICA .
B.  Force input transformer TR1 output to be in balance :
Connect the center of the secondary of T1 for RF to the PCB ground plane.
- Connect 10 n + 100 n + 1 uF film capacitors in parallel. 
-  Remove a little outer insulation at the center of Tr1 coax screen.
- Connect this paralleled capacitor trio very shortly between this center of T1 sec. and the nearby GND connection of T1 primary. 
G. Extra safety measures :
 Limit the bandwith of Q1/2 to about 50MHz, thereby reducing the risk of RF instabilities (possible tendency to VHF oscillations). 
-
Loosen the gate pins of both Q1 and Q2 from the PCB   !! Static precautions !!
- Solder a 3.3 ohms 0.25W metal film resistor between Q1 gate and R8. 
- Solder a 3.3 ohms 0.25W metal film resistor between Q2 gate and R9. 
H. Hard limit the input power :
-
Remove the original D6 ( was +-12vpp).
- Solder a surge diode CDSOD323-TO5SC (is +-6Vpp) on its place at the connections of the secondary of Tr1.