Saturday, December 29, 2018

Thoughts on the Hardrock-50

Parts, panels and rear panel board, front panel board, QSK board, main circuit board

I finished building my Hardrock-50 (with QSK option) about a week ago and have been using it daily since then. Actually, I'm not quite through with it since I still have the built-in autotuner to install. The instructions recommend successfully building the basic amplifier first, before adding the autotuner, so that is what I've done.

Once the tuner is installed and configured I plan to make a video of the amp's operation using both keying modes (PTT and carrier-operated relay keying), with rigs that have a PTT out and others that don't. For now though, some thoughts:


Why the HR-50 and not the Elecraft KXPA100?

I had initially considered Elecraft's 100-watt amplifier but ruled it out for a variety of reasons (including an issue with the ordering process itself):

1) HR-50's compatibility with a broader range of QRP rigs
Even the boards contain instructions!

A unique feature of the HR-50 is its ability to use a portion of the incoming RF to actuate the T/R keying. Most simple QRP rigs do not have a PTT out signal and would therefore not be usable with Elecraft's amp without some modification of the rig.

QSK capability is given up when operating the amp in this mode but that is a fair price to pay in order to be able to use my old HW-9, QCX, Hilltopper and other such rigs with this amp. Like many hams, I have a small collection of QRP kits assembled over the years - the ability to use them as 50-watt radios is quite appealing to me.

Radios that do have a PTT out can be used with this amp and retain their QSK capability. This includes rigs like the KX2, KX3, FT-817, etc. These particular rigs can also be configured in such a way as to cause the HR-50 to follow band changes. This requires an optional plug-in circuit for the DB9 jack on the rear of the HR-50 and, for the KX2, a single TRRS cable going from this plug-in to the KX2's AUX jack.

Info on the HR50 paired with the Elecraft KX2 is here.

2) Cost vs. effectiveness

Driving a 100-watt amp to full output with a 5 watt radio represents a 13dB increase in output power.
Completed amplifier board
Going up to the 50 watt level represents 10dB of this 13dB gain. Put another way, 10/13ths of the benefit of raising a 5 watt signal to 100 watts can be achieved with a 50 watt amp that costs less than half the price. Spending double only gets you a further 3/13ths boost.

The main use of my HR-50 will be with the Icom IC-705 which has recently become my do-everything rig: CW, digital, park activations, 6m monitor, etc.

3) Portable operation/battery life

The other use of the HR-50 will be for portable operation, activating parks, etc. where I'll most likely use the KX2 on CW.  Band conditions recently have been very poor, especially for QRP operation - newer battery chemistries now make portable operation with 50 watts almost as convenient as 5-watt operation was a decade ago. With the HR-50 only drawing 140 mA during receive, many CW contacts can be made with a small and lightweight 5AH battery. This would be less true with a 100 watt amp.

Building the Hardrock-50

The build, with QSK option but not (yet) the autotuner, took 8 hours. A large part of this was winding & preparing the toroids/binocular transformers and preparing various interconnections of small coaxial cables.

Panel and QSK boards completed

The manual mentions that many builders seem to have difficulty with T3. This is a binocular core with 10 turns of small wire on each side of the core. Then, two additional windings of one turn each, are inserted into the core as it is mounted onto the board.

Completed toroids awaiting installation
For me, this was easy - my difficult part was T2. T2 is also a binocular core with only four turns of very stiff #18 wire.

As much as I hate to admit it, it took me three attempts to finally get the windings compact enough within each side of the core for the wire to fit through for the fourth turn. This one component probably took me half an hour!

Almost all of the electronic components are pre-installed and, in addition to the parts mentioned above, the only parts for the builder to install are relays, a few trim pots, various headers, four MOSFETS and associated hardware, jacks and front panel buttons & display.

Instructions are clear and well illustrated with many color photos.

Panels being attached

Alignment of the completed HR-50 is a simple process requiring only a multimeter to adjust the bias of each MOSFET. Once that was done, the two halves of the case were screwed together, the serial number sticker was applied to the ginormous heatsink and the kit was finished.

Unbeatable support

First RF

The Moment of Truth. With the new amp placed downstream from my Anan-10e transceiver and a WM-2 QRP wattmeter, and an antenna tuner and dipole antenna on its output, I fired up the works on 40 meters. The PTT output on the transceiver was attached to its corresponding input on the HR-50. The front panel menu on the HR-50 showed the keying method chosen (PTT), the band selected, temperature (selectable C or F) and supply voltage.

So far, so good.

I set the transceiver's output to two watts and keyed it. The HR-50's display then showed my SWR (1.0) and power out - 61 watts!

Two watts in provides 61 watts out on 40m

I made a few contacts with FT8 on various bands and the HR-50 exceeded my expectations. A few days later I attached a dummy load to the HR-50's output and, with the driving transceiver's output set to two watts, made the following measurements on all bands 6-160 meters (60 meters is available only with the built-in autotuner installed - later) with a supply voltage of 14 VDC:

6 meters - 56 watts
10 meters - 65 watts
12 meters - 67 watts
15 meters - 71 watts
17 meters - 65 watts
20 meters - 43 watts (5 watts drive needed for 50 watts out)
30 meters - 44 watts (3 watts drive needed for 50 watts out)
40 meters - 61 watts
60 meters - 50 watts (measured after installing ATU option described here)
80 meters - 68 watts
160 meters - 60 watts


I built the HR-50 over a two day period and it was a refreshing change from the type of construction of a typical QRP transceiver. For those who don;t like winding toroids, they can be purchased pre-wound for $30. This will shave 2 hours from the build time but I think I value my completed kit more due to having accomplished all the steps myself.

Completed HR-50

I am very pleased with everything about the kit, from customer support to final performance, and I look forward to operating it with a variety of rigs on both FT8 and CW, at home and afield.

Knowing what I know now, I would buy this kit again, without hesitation. And no, that's not an advertisement, just an honest assessment. I paid full price for the kit (as I do for everything mentioned on this blog).


  1. Big congrats John! Excellent job and nice review. I reall appreciate your tests as same as the building comments. I fully agree with you. The kit looks cool and valuable and it seems to be nice option for all those QRP rigs like ATS, SW+, KX3, K1 etc. Enjoy it John on the bands! 73 - Petr, OK1RP

  2. Thanks Petr - I plan on trying it out on the simpler rigs soon and maybe an outdoor trip once our harsh (LOL) Texas winter is over.
    73 - John

  3. Nice review John. I had an early Hard Rock 50 and I can add the support is everything they claim. I had an initial problem and after some back and forth trouble shooting it still had problems. They suggested I send it to them and I had it back aligned and in working order very quickly. They are a first rate company.

  4. Hi Tim - I had to email them a question too, on Christmas Eve. I told them not to rush to get back to me given the date but I had an answer within two hours. A first rate company that probably doesn't get the publicity it deserves.
    73 and HNY Tim.

  5. Another review of the HR-50 was written by AD5X:

    Phil mentions significantly less output power than mine - the reason for this is that HobbyPCB updated the main amp board and the bias parameters for the MOSFETs. They used to be adjusted for 50mA of bias current each; the new instructions call for 150mA each. This significantly increases output power.

  6. I came across the HobbyPCB folks at Dayton the year after they won the QST amplifier contest. I told them I thought it was a great idea making it into a kit, and wished them all the luck in the world. I think I also remember telling them that Elecraft support was the gold standard as far as I was concerned, and that they needed to strive for great customer support in addition to a great product. Don't know if I can take any credit, but it sure looks like they followed through.


    -- Dave, N8SBE

  7. I'd say they definitely followed through, and did so with a kit product that has no real competition. A truly unique gem and, at this point in the solar cycle, a needed one for many otherwise-QRP ops thinking of stepping to the Dark Side!

    73 - John AE5X

  8. I have built and use regularly 2 HR-50 amps with ATUs, #1130 and #1392. Love them both. They work flawlessly on my ANAN 10e and KX3.

    Would my amps be able to be set to the upgraded bias? I have no trouble getting the rated 50 watts out with less than 2 watts drive but a few more watts makes a difference sometimes.
    PJH, N7PH

  9. Nice to know I'm not the only Anan-10e user! I don't think there were any changes to the circuit to allow the higher bias to be set but I would sure contact HobbyPCB before increasing the current to the MOSFETs. I think you would see the most difference on 6m.

    73 - John AE5X

  10. Thanks for this posting ... it was one of the factors that convinced me to purchase the HR-50 and ATU. The two kits arrived the other day and now have (thankfully) something new and enjoyable to occupy my time during these shelter-in-place times.

    One question, if I may: I've started winding the toroids. This is an activity I actually enjoy, having done many of them for 3 different Elecraft kits and several QRP Labs kits. This is the only time, however, that I've seen it recommended to scrape off the enamel prior to doing the solder "ball" technique; in the past, I've used just the solder "ball" technique, including for Elecraft's 100 watt KPA100 amplifier.

    Any opinion on this? What was your experience?

    Thanks very much.

    Best -

    Bruce K1FFX

    1. Hello Bruce,

      I've recently learned that there are some types of enameled wire that use heat-resistant enamel. I don't know if the wire in the HR-50 kits uses this but perhaps they want to cover all bases in case their supplier changes what is sent. This would prevent them from having to change the instructions, or worse, not learning about it themselves until they start receiving emails from buyers that point to poor solder connections of the toroids.

      Regardless of what method is used, it's always best to take a few extra seconds and measure continuity of the prepared windings with an ohmmeter.

      73 - John

  11. Hi all,

    I'm really interested in this amp, but I'm more concerned with current draw than maximum output. Is there a kind soul out there that would publish current draw and output numbers at 1 watt input? I have searched high and low for those numbers without success. I found AD5X's review posted above most helpful, but the low input current draw still evades me.

    Thank you!

    1. I emailed Jim, based on previous comments on how gracious he is, and he replied with a very detailed answer! Thanks Jim!


      The amp I have has the QSK board fitted which probably draws 200-300 mA more than an amp with a T/R relay and my bench supply is 13.8V but that has very little effect on the DC current. The drive was set to exactly 1W:

      Band    Output    DC Current
      160       25W       6.4A (the QSK board introduces some loss on 160, an amp with the T/R relay will have higher output)
      80         40W       6.8A
      40         39W       6.8A
      30         28W       7.2A
      20         40W       6.5A
      17         46W       6.6A
      15         40W       6.2A
      12         38W       6.4A
      10         34W       6.8A
      6           38W       5.3A

      This is a statistical sample of one and to be honest I don't know what the standard deviation is on the current. Your mileage may vary!

      Jim WA2EUJ

  12. Hi John,

    Any updates after using the HR50 with your IC-705 for a while? Are you happy with the QSK? I'm about ready to push the BUY button after reading your review, but ya can never ask enough questions :) Thanks Scott ka9p

    1. I still like it and think it is the perfect companion for the IC-705. It is also power-efficient as amps go. My 12V 12AH battery powers the 705/HR50 for about 4-1/2 to 5 hours of heavy use.


    2. Great John thanks, your blog on this rig is wildly helpful. 73 Scott

  13. I just ordered one to pair with my relatively new Hermes Lite 2 Plus. I can use it standalone or to drive my Mercury IIIs to near full output, I try to limit it to 1KW or less.