Linear amplifiers

Hello all I have a 818 but being the current situation with the solar minimum is somewhere close I have decided to get a little Chinese 50w linear .

I have seen a few pop up on ebay …

What are people opinions ? When they describe to be used via the acc. Can anyone explain how this is wired up?

Is the + & - on the linear go to the battery and then just a switch wire … or where can I obtain such a lead from please .

Regards matt 2E0FGX

Learn Morse, costs nothing and will give you a signal 30W louder than SSB with that 50W amp. :smile:

Have you looked at all the pictures of what you will get if you buy that item? 3rd picture should answer your questions.

You will need a short cable to connect the 818 antenna socket to the amp. One end needs a PL259 the other needs either a BNC or a PL259 and you’ll need 50cms-1m of RG58. You can make that yourself in a few minutes.

The amps are not bad, they do have filtering. They’re a reasonably robust design. The chances that it will really be dispatched from the UK are non-existent especially as the text says you have to pay import duty. If it shipped from the UK there’d be no duty. Expect to pay VAT and a handling charge . Typically the handling charge is £8.25 (was the last time I paid) and VAT is 20%. Maybe another £30 to pay on the £140. You may be lucky and escape the charges but HMRC is getting better at catching Chinese imports now.


I was waiting for this reply.

I also considered getting one of these direct-from-china amps for a while (and I AM a CW op). You may have already seen this:

I was also considering an HF Packer Amp, which is more expensive but a known quantity.

I’ve hauled my FT-857 on to a few summits when I had need of more power. If you get the MX amp from eBay, please post a follow-up review!

Good luck,


I felt duty bound to give it! That review seems reasonable and fair. Not sure what the signal is like if you don’t have 13.8V. Probably you need a 4S LiPo for the amp and a 3S LiPo for the radio. The PA device is rated for 28V operation, so 4S LiPo is OK… you’ll get a few more watts.

At £140 ($180US), they’re a bit too expensive in my opinion.

I have one. It works fine with the 817. I set the 817 to 2.5w output and that drives the amp fully to its max output. Set the 817 to 1w and you get 20 from the amp, or 0.5w at the 817 and get 10w from the amp.

Antenna cable as described by Andy. I wish they’d used BNC connectors.

PTT cable from the ACC socket on the radio - some suppliers provide that cable and others don’t. Look up the pin number on the DIN socket to provide for keying the amplifier from rx to tx. It is labelled PTT on the manual but it actually means “hook the Earth return line of the relay of the external amplifier to this pin; when the 817 goes to Tx that pin is grounded so the amplifier is keyed onto tx”.

Using ssb with a speech compressor (Sotabeams lists them) has a similar effect to amplifying the output of the 817. Using cw also provides benefits over ssb, as you will read here in many threads.

Using cw with the amplifier gives you a killer signal! And using a compressor with the amplifier also works very well, but makes the whole setup sensitive to RF feedback, so take care with end fed antennas which usually result in a “hot” microphone. (It’s not a fault in the amplifier, it’s a fault in the antenna system. )

73 Andrew VK1DA/VK2UH

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I think the HF packer amp is discontinued, the designer has pretty much decided to close the operation down. Website gone, mailing list closed, no stock left.

Edit: wait, it seems Virgil is still posting to the mailing list and considering what to do. See hfprojects at He may still have for sale the compact version of the amp, with the plug-in filters.

Andrew VK1DA/VK2UH

That is a shame. From the research I had done, it was a great amp.

The other one I looked at is the Hard Rock 50, which does appear to be in production.

More expensive than the amp from China, but again it’s a known quantity with good support.

When I enquire (from Germany) on the UK eBay site, I get a delivery date of 5th to 12th. of December - which to me would suggest a UK warehousing service (or at least a European one). The company behind the sale is almost certainly located in China.

Be careful with suggestions to run the amp at over 13.8 volts - although the PA transistors are rated at a higher voltage, the BIAS, RF-sensing or PTT circuits may not be.

As Matt has a 2E0 call sign he is only allowed to run 50 watts PEP output, so care should be taken not to exceed this level. The review suggests whistling in the rig driving the amp indicates an output of 50 watts on the SWR/Power meter. This is probably therefore nearer to 80 watts PEP which is more than allowed on an intermediate licence in the UK.

73 Ed.

I have one of those amps. Great little amp for my KX2. Others here in Alberta run them as well. They run nicely on a 3 cell LiPo 5000 mAh battery, get alittle hot on a 4 cell. I have used mine numerous times with a 4 cell and a voltage reducer.
Normally output is about 30 watts which I find is about the sweet spot for battery life / QSO’s.


I was waiting for the Learn cw hi hi !

Luckily this one comes with the cable , many have not when seen other links to them .

Matt 2E0FGX

I will be doing tests with a power meter and dummy load to see what I get on output before hooking to a antenna . But I would more than likely use 2.5w input .

I will be doing my Advanced licence next year also .

Matt 2E0FGX

CW is but one of many ways of getting your 5 watts heard more widely, and all much more fun and VFM than a linear amp.

Vertical antenna with elevated groundplane
MFD or portable beam for VHF
Speech compressor for SSB
Sought-after summit ref
Keeping abreast of propagation and selecting band accordingly

Etc etc

Been on 5w throughout SOTA. It’s more than enough - even on SSB. When the bands are open, they’re open. When they’re closed, they’re closed.


With the 817/818, speech compressors are worth considering. Virtually no weight, and no extra batteries need for a useful improvement in performance.

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Richard last few time operated ssb on 40 or 20 I noticed was harder than in the summer due to conditions . Guessing just gives your audio more of a punch ?

Matt 2E0FGX

A good point, a speech compressor gives an improvement of several dB in talk power, making the 817/818 equivalent to about a 20 watt rig. I must hint for one for Xmas!

As an aside, isn’t it a pity that the CW brigade can never resist proselytising for their religion?:grinning:

I can recommend the inline speech compressor for the 817 family radios. I bought a DYC 8x7 at Friedrichshafen (I think Ed was there when I punted) and it makes a big difference. I did have to fiddle with the level pot on mine (listened on another radio and tweaked for max smoke/minimum distortion). Tests show that it can make a signal that you would only listen to just to get the summit ref and report into a signal that it’s possible to ragchew with not much effort.

I think the design of the compressor is slightly flawed - it is difficult or impossible to set the output level to match the level of the unamplfied/compressed microphone, so a test for compressor on/off is never fair. The pot controlling the output of the preamp is effectively a compression level control and the second pot is… the (other) actual compression level control, which changes the compression level internally in the compressor IC. The difference is probably seen when there is background noise, which will be too high if the preamp output is too high, negating the noise gate function somewhat.
I turned the amp gain pot down to about 25% and then set the compression level so I could hear the difference in a receiver without wincing at the sound of the audio…

Hi Matt, I use a similar HF amplifier that you can buy either as a kit or ready built board out of China - the same board is sold as both a 70w or 100w PEP output board (depending upon supply voltage). This one you have to add an LPF board to and put in your own switches and a case etc. So not as simple to get going as a MX-50. Getting back to my point, on CW mode - key down with 2.5w of drive gives 70 watts carrier out (so at least 100w PEP) on 40m (less on 20m etc). So i think you may find you need to set the drive to 1.5watts to keep the output under 50w PEP. This will also depend if there is an attenuator on the front of the amplifier of course.

Good luck with the Advanced test when you get to it. as someone who took the C&G RAE back in 74 I never went through the steps - the exam was the exam and needed about 6 months of training - which if you look at the three levels of UK licences is probably about the same time needed there.

As regards SOTA you won’t be running move than 50 watts in any case, as to make any noticeable difference you’d probably need to go to 200 or even 400 watts and the extra battery capacity and larger components in your antennas that you would need would be restrictive.

The jump from 5 or 10w to 50w is the major one for portable operation.

73 Ed.

Cheers all for your input . I may try the speech compressor first and see how I get on . Is it pretty much plug and play ?

When going inside a radio I’m pretty clueless so I leave it to those who know what there doing when comes to adjustments with pots etc .

I will be running the 818 of a 4s lifepo4 which I have started to build from 18650 cells . Will be 12.8v . Not sure on the ah as all the original wraps on the cells was removed but I know they was more than 1200 mah so at the minimum will be 4.8ah . Will be charged with a hobby charger at a charge rate at 2a .

Matt 2E0FGX

Sadly not. When in series, the capacity is that of the smallest cell, so 1200mAh in your case.

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