Ham Radio Forum

160m linear amplifier with mosfets low output at some frequencies

hi, i have construct this linear amplifier 500w with 12 mosfet irfp360 for 160m band:
http://tzitzikas.webs.com/linear500w.jpg (schematic)
The manufacturer and other amateurs who have construct the follwing set, claims that the linear
provide in output at 110vdc 450-500w carrier (13A current), at 55vdc 100w and at 75vdc 200w carrier and the
requirements for r.f driving power are only 2w (Also he claims that is broadband at frequencies between 1000-2000khz)

The PLL which i use to drive the linear 500w, consists of:
pll
oscillator: http://tzitzikas.webs.com/oscillator-1.JPG (schematic)
modulator: http://tzitzikas.webs.com/modulator-1.jpg (schematic)
linear 2-10w with 2 irf640 mosfets: http://tzitzikas.webs.com/irf640-1.JPG (schematic)
the block diagram is pll-oscillator-->modulator-->linear 2-10w with 2 irf640 mosfets-->linear amplifier 500w-->dummy load

50ohm 1kw
I do all test in dummy load 50ohm/1kw.

I took these measures:
at 1450khz at 65V(voltage to drains of irfp360): 103W output power current 4.8Α and driving power 2w
at 1700 at 65V 60W output power current 3.3Α (i don't change the driving stage settings (like voltage e.t.c)
at 1200 at 65V 183W output power current 7.8Α
(at 1850khz! the output power was only 50w!)
swr=0 (all frequencies)
After, i connect only the linear with 2 irf640 (driver) to the dummy load and i took these measures at 20v (voltage to drains

of irf640):
1431khz 2w
1700khz 1.9w
1200khz 4w
So, you can see that the driving power was increasing when the frequency was decreasing.

After these measures i gave to linear 500w, 3.6w of driving r.f power at 1450khz and it gave only 130w at 65v
(current was 5,5Α and the modulation in this test wasn't very good- AM modulation).

So, it is clear that at lower frequences linear 500w, and the other circuits give the maximum r.f power.
The designer claims that this linear amplifier is broadband for 160m and mw band (1000-2000khz)
I would like my linear to give 185w at 65vdc at high frequencies between 1400-1900khz.
What changes do you propose to do at linear amplifier 500w, linear 2xirf640, modulator or oscillator
to give at output 185w at 65v? Maybe some changes in transformers or capacitors??
Moreover, the one heatsink was colder than the other one.

some photos from my constructions:
linear 500w:
http://tzitzikas.webs.com/linear_photo_1.jpg

thank you very much!

tzitzikas wrote:So, it is clear that at lower frequences linear 500w, and the other circuits give the maximum r.f power.
The designer claims that this linear amplifier is broadband for 160m and mw band (1000-2000khz)
I would like my linear to give 185w at 65vdc at high frequencies between 1400-1900khz.
What changes do you propose to do at linear amplifier 500w, linear 2xirf640, modulator or oscillator
to give at output 185w at 65v? Maybe some changes in transformers or capacitors??
Moreover, the one heatsink was colder than the other one.

1) Were you ever able to make the amplifier work correctly at the higher frequencies?

2) Of course, the difference in temperatures between the two heatsinks suggests that the amplifier circuit isn't balanced. How much difference in heatsink temperature did you have?

Randolph