[Lowfer] Re: Amplifiers for 136k

[Paulus]

Hello all,

I am just now building and testing the Quarter KiloWatt Lineair
Amplifier based on the design of G0MRF.
He has the 3C90 Toroids, PCB and Powermosfets (no IRFP450A at this
moment) available (look under Kit Index).

There are more designs and lots of info on G0MRF's Website:

http://www.g0mrf.freeserve.co.uk/index.htm

With 4 IRFP450A (PushPull), at 50 volt, with about 100 milliWatt drive
at 50 ohm, the 50 ohm output is about 60/65 Watts at an current of 2
Amps. (I dont have an heavier Power Supply at this moment but this
output is fine for me)
The IMD is about -25db (with Psk31), this is the best I could achieve at
this outputlevel.(max 60 watt RMS).
For HF this IMD would be an little bit short, but I think that the
signals on 136 kHz are often in the QRPPPPPP class, so I think this IMD
will do.

This Lineair is running fine from about 30 kHz to about 800 Khz.
All looks very stable, no running away of the current, no strange
behaviour, but I am using an good heatsink, and have made an simple
temperature-compensated 8-diode (1n4148) stabilised voltage supply for
the DC voltage on the gates of the Mosfets. These diodes are temperature
coupled to the 4 Mosfets. 
The Quiescent current is per side (per 2 Mosfets) about 100 milliAmp.
With this current I got the best IMD figure with Psk31.

When buying PowerMosfets do buy the double amount ( 8) so you can
measure them to have about the same Quiescent Current. That is needed
because in the design there is 1 trim-potentiometer for the DC
gate-voltage for 2 Mosfets.

For this 60 Watts Output the Output-Toroid (3C90  58 mm diameter) has
primary 9 turns (diam 1,2mm)  and secondary also 9 turns (diam 1,0 mm).
For testing I made an lot of taps on the secondary to find the best tap.
With more output the drain to drain impedance will go down and the
secondary turns have to go up. In the diagram of G0MRF the output is
about 250 Watts....(primary 9 turns and secondary 27 turns)

At the 60 Watts Output level the Output Toroid does not even warm up...
(so could be smaller in diameter)

The Input-Toroid is an 3C90 15 mm diameter with primary 27 turns 0,56 mm
and secondary 2 x 11 turns 0,8 mm.(modification from DJ8WL)

The DC Feed Choke is an 3C90 25 mm diameter Toroid with 10 turns
Bifilair
1,0 mm, like it is in the original design.
This DC filtering seems not enough (at 60 Watts output there is still
about 2 volts top-top 136 kHz on the DC), so will add an extra choke
(Toroid) soon.

The output signal looks about OK on the scope but has clearly still the
odd harmonics in it.
With Lowpassfilters I am sure the Outputsignal will be an nice sinus.
For testing this I put (at 136 kHz) a 10 NF at the output to ground and
indeed the output was almost a good sinus.

The next thing is building these Lowpassfilters and I will make three
sets:
For 73 kHz, 136 kHz and 500 kHz.
At this time only 136 kHz is allowed here, but perhaps the 500 kHz will
come in due time.......and the 73 kHz would be nice to say an short
hello to an G-station...

After his I will have to build the antenna's some time hi......


Cheerio now es 73

De Paul, pa0ocd