A 400 Watt Linear Amp
for 20 - 15 - 10 meter bands using four QQE06/40 tubes.
My previous linear amps work well. The first one used
four PL519 tubes and managed over 400Watts and the second one used two 813 tubes and makes 800 Watts pep on 20 mtrs but starts to fall off a little at 15 mtrs. At 10 mtrs it is even less efficient and struggles to make 400 Watts. The 813 amp linearity is copy-book perfect, not quite so for the PL519 amp.
Linear number three then will use QQE06/40 tubes and certainly will show no signs of giving up at 30 Mhz.
A three band linear is being attempted using four such tubes for the 20, 15, and 10 meter bands.
Information for this project is gleaned from several places but notably PA0FRI who suggests the QQE06/40.
VK3NX who used three of the tubes in parallel for a 50Mhhz linear.
The tank coil design software by F1FRV is superb and makes that part of the job a lot easier.
Lloyd, ZL1LA has also afforded much help and encouragement.
It is clear that some experimentation will be needed.
Since April 2012 when this page was posted this amplifier has been completely rebuilt because performance was disappointing. I could not derive sufficient screen voltage from the recitified input RF as was attempted.
It is only left here because most of the internal parts are re-used.
The four QQE06/40 are mounted in the usual manner as shown and with due regard to the earth plane alignment with that in the tubes. The antenna relay, an input load resistor on copper heatsink, and ant' sockets can be seen. The resistor is made by Diconex and rated 500W 50 Ohms. These two main assemblies are held in place using rivnuts for easy removal. Space is availed for the small coils associated with the input circuit.
The tank circuit assembly holds the home-brew tuning C and coils. The ceramic band switch and the load C can be seen. The switch shaft extends to the front panel using a plastic rod since it is close to the coil. The top coil section is the 10 mtr part, the middle bit is added for 15 mtrs and the lot is used for 20 mtrs. Ceramic tiles were cut for the end plates of the tuning C. Cheap plastic is not believed any good at high RF voltages. NB The turns on the 10 mtr coil part have been reduced since taking this pic..
Here the thing takes shape. The left hand "RF" side shows the tank assembly in position held to the base with four rivnuts. The tube assembly (2 tubes shown) is also held with rivnuts and raised up using short tubular spacers to bring the anodes up towards the coupling C whilst availing space underneath for components and wiring. The plate choke is wound on a solid ceramic former to 44 uH with one only resonance at 33Mhz. Some turns could be removed...but it should be ok for the bands chosen. The tube access is closed off with an ali plate at completion.
The toroidal tranny is seen sitting on a felt pad to ease any chafing. It provides both the htr and anode voltages. This right hand section will hold all supply parts and the relay delay options for soft starts and inrush protection.
A modified relay has been made to increase spacing and hopefully will handle the 1100 volts that it will need to switch. That armature will be held down to provide about 1/4 inch contact gap with the 24 volt relay pulling in at about 20 volts.
Yes - it seems to work well.
The front panel minus artwork and a meter switch.
A bit premature maybe - but it has to be done. I was lucky to find two broken verniers but which made one good one.
More to come - I am not sure when!
I may even produce a circuit or two.
With the power supply and delays all working I felt the front panel could go on. Tune and load labels should have gone above knobs of course....so I screwed up!! Let's hope there's nothing worse.
Meter dials will need re-writing when calibrated. Also a meter switch to go in.
The PCB on top hides the toroidal tfmr. It holds the rectifiers, the voltage doubler, a 12v regulator for a 555 30 sec timer, and a 1 sec timer for two step C charging. The B+ smoothing caps are under the smaller pcb which holds the bleeder resistors and a glitch R.
The relay at bottom right is a 1 sec inrush delay to short the 50 ohm series R seen below the terminal strip.
The long horizontal terminal strip connects the toroid wires and allows appropriate series/parallel jumper options for HV and LV.
To reduce contacts arcing a 10n and a 1K are in series across the contacts. (It does allow a path for unwanted 200 volts to develop on the B+ line but this should not cause damage during the 30sec tube heater warmup.)
The one left hand "mains" switch switches on the 230 volt ac supply and initiates the relay sequence. 1 second inrush delay across 50 ohms, then 30 second delay for heaters to operate, then a 1 or 2 second slow start for the capacitor surge.
After that the ready lamp displays.
I am hoping to use this on 10 meters for at least a couple of weeks and before the sunspot numbers disappear again.
, is the rebuild in a new housing but using most of
the major sub assemblies. The rebuild adds another toroid and
voltages for the grid bias, screen grid and 24v relays.
It also adds time delays.