ShortWave   QSK Linear Power Amplifier

The workhorse

The active element in the amplifier is an 4CX1000A tube from EIMAC this tube is a ceramic tetrode,many other popular designs make use of triodes  (eg 3CX800 ,3-500ZG.)

Input Circuit
This is the input PI filter,it is build with standard power relais
It is controlled by DC switching voltage to switch bands,in this way
one could use a microprocessor to allow for automatic band change.
Each stage consists of an inductor and 2 capacitors .
Notice on the right 2 BNC connectors 1 as input from the TR switch
to other going to the cathode of the tube (via a coupling capacitor).
When the tube is used in the common grid configuration (as it is the case) the impedance
at the cathode is in the erea of about 100 Ohms.
Use of this PI-filter to match the transceiver's 50 Ohm impedance has the advantage of acting
as an extra filter to reduce spurious signals .(compared with the use of wide band 2/1 balun).

QSK TRX switch

This is the QSK TRX switch ,it is build with 2 vacuum relays
from KILOVAC ,these are very fast acting relays capable to withstand
5000 V and a maximum current  of 15A !!!
Notice the coax at the left going to the input circuit described above.
The relays normally operate at 28V DC ,but to furher reduce the acting time,
the coils are put in series with a power resistor,and fed from the mains via a
diode bridge and a power mosfet ,driven by an opto-coupler.(see picture below)
You can clearly see the input(TRX)  and output (ANT) connectors .
Always keep connections as short as possible to avoid problems.

This circuit controls the TRX relays as described above.

Top: the power resistors connected in series with the relay coils.

Front right :the opto-coupler ,and in the background the power MOSFET.

Bottom tube cavity

This is a view as seen from the underside of the unit with some
plating removed.
At the left is the air inlet coming from a large and noisy centrifugal fan,
it makes sure tube never runs hot (the max temperature of the tube
should never exceed 250 °C,or tube sealing could fail ..)
The cover (now removed for this photo) should never be removed,
since the forced airtstream would stop cooling the tube.

At the right you can see the filament choke,bifillar wound with thick insulated wire
This choke acts as a high impedance to allow input signal to couple to the cathode,and
at the same time to feed the filament of the tube.(6V 9A).

Filament power supply

This is an ordinary PC power supply modified so it no longer gives 5V /15A but it delivers now
the required 6V  5%  tolerance voltage to operate the tube.
A small soft-start circuit has been build into the unit to avoid thermal shock of the tubes filament.

I have never seen another amplifier using stablised filament supply.
When using this kind of ceramic tubes ,care should be taken not to exceed the 5% tolerance
of the tube specifications filament voltage.(Notice that with the 10% tolerance on the mains line,this
save margin is never garanteed when using an ordinary AC transformator)

Screens power supply

This pcb provides VG1 and VG2 voltages.
Both voltages can be adjusted as needed.
VG1 can be controlled to chose the required operation class.(class AB ,class A).

VG2 is fixed at 325V and maintains at this value for VG2 currents from -40 to +40mA
variation is no more than 10V.
The circuit is consists of a combination of voltage shunt regulator build with power
MOSFET's rather then with zener diodes.
VG2 and VG1 currents are measured and go to the protection PCB.
When protection activates,the tube is in cut-off  (VG1=-120V ,VG2=0V).
  

Protection is active when Ia ,Ig1, Ig2 gets above the limits ,or the heat up time (4min)
is not elapsed yet. 

This is another picture showing some of the feed-trough capacitors and some de-coupling.

You can also see an overvoltage protector (500V type ) this should protect the Vg2 supply
in the case of an arc inside the tube.

User for the Anode supply...only payed 2$for this solid 86Kg Tranformator.

Experimental setup (caution :deadly when touched ..3500V 1A )

See the capacitors,10x 1000uF/400V ,and some soft start stuff.

A look in the inside.

You can see load tuning ,vacuum plate tuning ,tube
rf choke ,and pi-filter coil assembly (dump parts from Rohde & Schwartz).

This is the cooling fan.

73's Johan