What used to be a G2DAF HF amplifier !

If you want to build an amplifier using a tetrode but don't want to mess around with  screen and grid powersupplies the G2DAF concept is for you!
HV and filament voltage required, just as in a grounded grid amplifier. However, the screen (G2) voltage is derived from the input RF and the grid (G1) is at DC ground.
Check the circuit description below.

However, since a few years I have converted the amplifier to a normal tetrode amplifier with fixed voltages on G2 (900 Vdc) and G1 (-105 Vdc). This did not significantly change the performance of the amplifier, it  just made it a little easier to drive. The G2DAF configuration worked flawlessly for >15 years. But as always, at some point I get the urge to change things - just because....  :-)


The shack, my QBL HF amplifier to the right. The bottom panel hosts the PSU for filament, screen and grid voltages for this amplifier. Above that is a 4CX350a amplifier for 144 MHz and at the top my modified LK-450 HF amplifier                                



The QBL 5-3500 tube

Check some detailed pictures, circuit diagram and tube specs

Anode compartment Anode plus tank circuit 160-10 mtrs
Anode compartment 1 Anode plus tank circuit, side panel view
Anode compartment 2 Anode plus tank circuit, top view
Bottom view Input circuit, relay pwr supply, screen bypass
Screen bypass 1 Screen (g2) PTFE sandwich bypass, close view
Input Input PI-network, relays switch different bands
Input 1 Input PI-network plus input resistors
Schematic diagram  G2DAF style QBL 5-3500 schematic diagram
Input circuit, coils Schematic of input circuit using 9mm dia coils with trimmer cores
T/R switching T/R switching using RJ-1A relays
QBL 5-3500 specification sheet PDF with all tube data. (1Mb)
Check out SM3PXO and SM3BDZ (SK) spectrum analyzer measurements of a G2DAF amplifier


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