Cheap eBay 2m Amps

[VK2XAX]

Hi Jan,

thanks for the updated layout.

There seems to be a few versions of this getting around and they don't all match up with the board/parts supplied.

I'm still waiting on my replacement parts but given the length of time, I'm not holding my breath !

regards

Tim

[VK2XAX]

Hi all,

I finally finished my 2m amp ( see pics) and set about doing some measurements.

For 30dBm in it produces 47.8dBm out @ 24v and 4.4A with a bias voltage of 4.2v. i.e. ~1w in for ~60w out

Pushing the amp until it just stated compressing sees an input of ~32.3dBm for ~49dBm out and 5.6A @ 24v i.e. ~1.6w in for ~80w out

Given this FET is usually run at 28v I tested it at that voltage and got 50dBm out for 32dBm in at 6.2A and 4.2v bias . i.e. ~1.5w in for ~100w out.

So this amp appears to be about ~59% efficiency.

CW tests showed no appreciable spurs with the 3rd harmonic ~37db down.

Attached results show the signal peak at 15.6dBm, an additional 32.2dBm needs to be added for the directional coupler and cable losses.

regards

Tim

[VK1JA]

Looking good Tim, considering the price on these things you can't complain !

Thanks for the update.

[VK4WTN]

great job Tim, you will have some fun using it now
regards Wayne VK4WTN

[OH2FTG]

Nice to see that some folks have actually built these chinese amps!
Mine is still floating around the desk in it's bag-o-parts form. Waiting for better documentation and inspiration, output relay and case.

[VK2XAX]

Hi all,

Still playing around with this amp I notice my driver amp getting a tad warm. Checking the input SWR of the main amp showed it to be 7:1 !!

Revisiting the circuit I replaced the 3 x 10n input series caps with a 20pF ATC. This droped the input SWR below 1.5:1 across the entire band and also boosted the power output by a few db.

While I was at it I replaced the 10k resistor in the bias circuit with a 10k NTC, this keeps the bias in check when the amp gets hot in multi minute key-down scenarios. Optimum bias voltage with a cold NTC is 4.4v.

regards

Tim

[ZL2IT]

Hi
My experience with these amps has been good so far.

I built 2 and run them in parallel via Wilkinson power dividers/combiners using a L & c configuration

This has been successful so far. Running with 28 volt DC am able to get 160 w output Plus on 144 mhz but I am limited to Power supply DC current which trips if I push it past that (should have bought a bigger one!)

I had 1 SMD cap fail (a decoupling cap) but I assume that it was faulty as I have had no further issues since.
Amp has been running all day on WSPR with no problems. RF Balance betwen amps is within 10% so I have not bothered to adjust to get a closer match
Efficiency about 55%, Output is clean on spec analyser >55db.

Heat sinking took a bit of searching but found a Chinese supplier who had a heat sink that fitted perfectly with good sized fins (150 x 69 x 36 mm) at US$26.40 for the pair.

All in all I found the risk worthwhile., should you destroy a FET they are cheap as used units

[VK3XYX]

Thanks to VK2XAX for the circuit diagram.
While trawling the net I found the attached circuit. One point to note, is that the 10ohm resistor and attach to the gate and not to the drain as shown. Other than that the input circuit looks a little odd.


Garry VK3XYX

[VK3XYX]

I was not quite happy with the performance of the low pass filter especially after running it through Elise. So I tried a 7th order chebyshev filter with 160Mhz cut off and standardised the components. Refer to the attached circuit diagram. The wire size for the 78nH coil is (as supplied with the kit) 1.25mm and 1.00mm for the 70nH coils. Capacitors are some ATCs that I recovered from old PRM80s model TM which has 1x 22pF and 1 x 39pF per RF board.
I have also included the Elise plot and the actual plot.
Stray capacitance and the fact the the SWR stipline is not 50ohm probably suggest why the trace is minimum at 268Mhz and then starts to rise again. I am not sure why the fall off is much sharper than that shown in Elise. (Second marker is at 432Mhz with -53dB attenuation)
I sure that once I get the whole thing finished and tested it will perform well.

One other thing that does concern me is that the ferrite in the input transformer is a material that is somewhere between 43 and 61 which would provide very poor performance at 2m. There is an photo on hellocq (http://www.hellocq.net/forum/attachment ... d8.jpg?274) of what looks like an early version of the amp which uses a conventional (at 2m) 9:1 input transformer.

Any one have any ideas on the subject?

Garry VK3XYX

[VK2ZIW]

BMARC club has 4 x Pager transmitters with 100W 28V PAs. 148MHz.
A straight forward module with Motorola transistors in the output.
The p/s, signal generating and modulation section is junk.

At the clubhouse, they are clutter.

Anybody interested?

[F8GGD]

Hello,

sorry for my broken English.

Here the schematic for PA 400 to 470 mhz

http://115.28.16.44:81/file/3392.pdf

73 F8GGD

[VK2LK]

Nice work guys! This has me very interested now.

I think it's time to build my first amp now...

[F8GGD]

bonjour,
I began the manufacture of a PA 70 cm,
but documentation is insufusante and missing component.
The MRF186 is used.
73 F8GGD

[VK2XAX]

Hi all,
I've just completed the 70cm amp and it works OK too.

Once again there are many layouts available and obvious mods to the component values. This was evident in mine in that all components were supplied for two possible variants ! At least they were all the right values

Initial test show this amp is not as efficient as its 2m brother, and, like the 2m unit, its input SWR is 7:1 ! Changing the input cap from the string of 10n to a 20p ATC and moving the tiny trim cap to the position closer to the FET dropped the SWR to an acceptable value.

I also added the 10k NTC to the bias circuit as I did with the 2m unit.

Now that F8GGD, ( thanks mate!) has found the original schematic for these things, the components supplied make sense. A bit of tweaking should make these perform better than they are.

Current testing gave these values...

5w in 82w out @ 24V 7.2A

5w in 100w out @ 28v 7.7A

1w in 28w out @ 24v 3.7A

So not too bad. I believe it should be possible to drop the input SWR further ( its still nudging 2:1) and replacing the two ATC's on the output strips with trimmers should increase the efficiency and output of the amp.

cheers

Tim

[VK2ZIW]

Our BMARC club has 4x 100W 28V Pager units, 148.025MHz
Easy to work with RF PA module, rest is junk.

Location Glenbrook clubhouse, in 19" racks 1m high.
Actual unit is 4RU.

Any interest?
bmarc-secretary@bmarc.org

[F8GGD]

Good evening,

what is the bias current to be applied for use in SSB and CW ?

merci beaucoup,

and 73 from COGNAC city IN95UQ

[VK2XAX]

Good evening,

what is the bias current to be applied for use in SSB and CW ?

merci beaucoup,

and 73 from COGNAC city IN95UQ

Hi,

For these amps we set a bias voltage which is 4.2 - 4.4v on the gate.

You can measure that at the junction of the 100n cap and 50ohm resistor that feeds the gate or directly on the gate with no RF applied.

regards

Tim

[LA9XNA]

This is my first post on the formum.

I just bought one UHF and One VHF of these amps, whil waiting for packages to arrive i have been doing som preparations on how to build them.
Here is some information that might be handy.
http://g4fev.atspace.com/mrf18670cm.htm
http://g4fev.atspace.com/mrf186linear.htm
It seems like the input coax is a bit to short in the original building description.

[VK1JA]

Another version has popped up on eBay, listed as a 170w FM 80-180Mhz amp for abou $70-80.

Not sure if it's linear (ok for SSB), let's see what we can dig up.

If they are 170w and linear, two combined would be nice.

http://ebay.com.au/itm/331866135126

[VK5TM]

Considering this comment in the description, it's one I wouldn't touch with a 40ft barge pole.

Kit parts needs its own debugging, and installation procedures photos, debugging instructions, I have limited time does not provide advice, do not understand those who buy with caution