MiniPA-70 and MINIPA-100 step by step

The difference between transmitting with 5 or 12 watts of power in practice will not reach half a unit "s" in a remote receiver, however, that can be the difference in critical conditions to achieve a QSO. But the mcHF behaves much better emitting with its nominal power of 5 Watts, that's why a "help" in the form of linear amplifier is a better option when necessary. MINIPA-70 or MINIPA100 is a good  and unexpensive "help".


My recommendation is first separate the supports of the transformers T1 and T2 (cut them from the printed circuit) and then solder the components of small size. Leave the fan and PTT connector for the end. These connectors prevent solder other nearby components and can easily melt due to excess heat.


In this way the transformer T2 is built. The two small metal tubes are the half turn of the primary. Use a 100 Watt flat tipped soldering iron to build T1 and T2 and solde their brackets to the printed circuit board. Contrary to what is believed, the high temperature decreases the time used for soldering and prevents collateral damage, such as the detachment of the printed circuit tracks. Use resin for soldering and tin abundantly first the parts to be solded.


The three turns of the secondary winding through the inside of both tubes.


The same happens with the transformer T1.


 The half turn is constructed with the two tubes.


The two coils of the secondary pass through the inside of the two tubes.


This amplifier is a symmetrical "push-pull" stage. Pay attention to build it with perfect symmetry. Locate T1 and T2 centered on the hole left in the printed circuit. 


The 100 Watt option uses a double MRF9120 Mosfet. This double Mosfet has the Source electrodes attached, for that reason it only has 5 pins. The Source electrode has a small cut in the corner. The Gate electrode is rectangular. This component does not require an insulating washer because both Source electrodes are connected to metallic body and grounded to radiator. In the 70 Watts optión the mosfet are IRF530N with TO220 case, and have the Drain electrode connected to the metallic body, so they need the insulating washer.


Apart from the Mosfet used, the 100 Watt version includes a resistance in parallel with the secondary of the transformer T1.
The adjustment is very simple: it is enough to regulate VR1 to obtain with 12 Volts, a consumption of 1 amp without input signal. If you observe a non-linearity, current "bias" can be adjusted to a higher level. In most cases, with 1 Ampere of current at rest is usually enough.



Do not connect this amplifier to an antenna without first connecting a low pass filter. Below you can see a video with the description of the operation of the 70 W version.

VERY IMPORTANT NOTICE:

  1. Some people who read this post contacted telling me about their bad experiences. For this reason, although it is common sense, I must warn that before connecting the linear amplifier to a power supply for first time, make sure that the cursor (pointer) of the potentiometer VR1 is set to the ground side, that ensures the initial condition of zero bias before adjustment. If initial bias level is too high and you do not have an adequate heat sink, the thermal death of the power mosfet is guaranteed.
  2. Using forced radiator ventilation witn fan is highly recommended. 
  3. The use of a laboratory power supply with limited intensity, for example, at 2 amps during the initial bias settings, is also a good option.
  4. Bear in mind that under normal operating conditions, current peaks of up to 12 - 15 amps at maximum power can be obtained. 
  5. An AB class amplifier has a performance not exceeding 70%, which worsens in the higher frequency bands. In these conditions, if you are providing 100 watts in antenna, approximately 60 - 70 watts are dissipating in the form of heat in the final mosfets. Imagine the heat given off by a 70 Watt incandescent bulb.






One recommendation: if you are looking for a customized aluminum radiator, here it is. 

https://www.ebay.com/itm/Silver-Tone-Aluminium-Radiator-Heatsink-Heat-Sink-100x50x30mm-I1D2/253631205303

In ebay or aliexpress you can purchase an up-tp-date low pass filter, which shares the filter for nearby bands. Another option is to build your own. Up to 100 Watts.

https://es.aliexpress.com/store/product/Assembled-dc-12v-100W-3-5Mhz-30Mhz-HF-power-amplifier-low-pass-filter/2132223_32705199905.html

Comments

  1. Muy buenas tardes Alvaro Excelente su articulo. lo armare e instalara en mi MicroBitx

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    1. Me alegraría mucho saber que te sirvió de ayuda. Un abrazo.

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    2. Great article. Wish I'd seen it before going through the tedious steps with my Ohm meter to reverse-engineer the amplifier, and blowing out the FET in the process!
      One question regarding the DIY kit; why is there no resistor provided for terminating the T1 secondary as there is in the MiniPA ? I believe it's 11 Ohms, and would expect it to be rated for the 5Watt maximum input power. Without that resistor, the input VSWR will be excessive and 110 Ohm feedback resistors will be unloaded as well.
      Again, great job,
      Bob

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    3. Did you take any steps to verify balance of the two output transformer ferrites? I understand that the mix can be +/- 20%. Possibly OK if the tubes are from the same batch.

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    4. jerryk1234 Well, I never thought that there could be much difference between the characteristics of the two ferrites. I imagine a quick way to check its characteristics would be to wind three or four turns, make it as accurate as possible on both toroids, and measure the inductance on both. Whenever I have mated it has been with mosfets. https://www.firstwatt.com/pdf/art_matching.pdf

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  2. I love your article because I was going nuts trying to find one for the MINIPA100/70 I just curious if you need the bandpass filters on the amp if you are feeding it with a pre band passed signal from a transceiver. I'm feeding the 5mw output from the 6 Band HF AM FM CW transceiver model "New 6-band HF SSB Shortwave Radio Shortwave Radio Transceiver Board DIY Kits Set"

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    1. I strongly suggest you use four 7-pole Chebyshev or Butterworth low pass filters, following the same mcHF scheme. They are calculated to attenuate from the second harmonic in all amateur radio bands from 3.5 to 30 MHz. Coincidentally using T68-6 ferrite cores the theoretical calculation gives the same number of turns as with the T37-2 cores used in the low-pass filters of the mcHF. You can use the same values in the capacitors. You can check it out for yourself here: http://toroids.info/T37-2.php

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    2. pamam, a perfect linear amplifier would not need an output filter... if you ever find one, let me know, though... :) In practice, the easiest way to get nonlinearity out of a linear amplifier is amplitude distortion, so any signal whose power exceeds the linear range will need filtering. Any time you're relying on input amplitude to maintain linearity - for example, a satellite repeater and upconverter - the best-practice recommendations tend to be pretty conservative. You use something like a 1 dB compression test to find the output amplitude for which 1 dB of increase in the input yields less than 1 dB increase in the output, and then back off your input by a factor of 2. I don't think you want to do that on a small amp like this. I would personally do exactly what Alvaro recommends and get another output filter. Having extra filters after your earlier stage won't hurt anything, and in fact will probably save you some power burned in your last stage amplifying useless harmonics.

      By the way, Alvaro, thanks - I was looking for exactly the kind of writeup about this kit that you have provided.

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    3. This is a class AB amplifier. Each MOSFET approximately linearly amplifies one half wave of the input signal. There is a distortion in the "crossover" area that is difficult to avoid unless "paired" MOSFETS are achieved. It is enough that the gains of both are a little different for that distortion to occur. Since every waveform can be decomposed as the fundamental summed with its harmonics in various proportions, it is clear that any form of non-linearity is transformed into harmonics. A possible improvement to the Minipa-70 and Minipa-100 would be to have independent gate biasing for both MosFets , although this would complicate the circuit.
      have nothing to do with the designer of the Minipa-70 and Minipa-100, I am just a user who has built several. I'm glad I was able to be of help to you, I only intended that.

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  3. I find the skimpy, incomplete instructions obscure and frustrating. The photos do not match the accompanying descriptions. For example the secondary winding details on the two transformers are not shown although the description implies otherwise.

    And how difficult can it be to find someone who is fluent in English to review instructions for five minutes and correct such obvious errors as "welding" instead of "soldering".

    The instructions are terminated after the notes regarding the transformers. The majority of the amplifier remains to be assembled but there are NO instructions for assembly.

    I'm very unhappy.

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    Replies
    1. Sorry for the inconvenience. Initially I used "Welding" in the sense of "tinning" the parts of the broadband transformers and copper wire. Then I used it wrongly to replace "soldering". It is already corrected.
      The Chinese manufacturer does not include instructions to assemble the amplifier, so my intention is only to share my experience with this assembly.

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  4. Thank you for interesting article.
    Do you know, how to modify MINIPA 100W SSB (with MRF9120) to have output power adjustable (for ex. 10%, 25%, 50% and 100%) giving on PA input constans driving signal from TX?
    Regards
    Mario

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    1. To get preset output inputs below the maximum, I think it could be achieved by varying the supply voltage, or by varying the power of the input signal.
      Varying the feeding voltage of the mosfets is complicated and can dissipate a lot of heat if a linear regulator is used. But limiting the input power can be done with resistive voltage dividers that maintain the input impedance around the desired 50 Ohms.
      By the way, I have operated the Minipa-100 with 24 volts using the Mosfet MRF9120. I think broadband transformers cannot handle much more power than 100 watts.

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  5. This comment has been removed by a blog administrator.

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    1. This comment has been removed by the author.

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  6. Just an observation - you need at least 4 turns on the output transformer to achieve 100 watts at 13.8 VDC - 3 won't get you there at 13.8 volts (typical radio power supply). 13.8 x 3 (winding ratio) x 2 (push pull) = 82.8 VDC - Watts=Vpk^2/100 = 82.8^2/100 = 68.5 Watts. 4 turns doing the same math will provide 121.88 Watts - which matches the device (MRF9120) pretty well - it's rated for max power dissipation of 250 Watts and working power of 120 Watts. Change the output transformer to 4 turns out and you'll be able to easily get 100 Watts into 50 Ohms at 13.8 VDC. Be sure to use at least a 7 element Chebyshev low pass filter - these amps produce a lot of trash - square wave out. You can even buy a good low pass filter kit at Communications Concepts Incorporated for $15. Here's the link to the 40 Meter version: http://www.communication-concepts.com/fl1-40/

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    1. A few months ago someone recommended to me in a chat the need to add one more turn in the secondary of T2 to obtain maximum power, as well as the use of a thicker conductor. I tested it with 4 turns and the result is exactly as you say. The truth is that I also noticed an increase in the SWR at the secondary without overheating.
      I have also tried to exclusively power the mosfets with 24 volts and obtained almost 130 watts at 14 Mhz, on a dummy load, with a notable overheating in MRF9120 Mosfet. But the signal was somewhat degraded and the bias adjust become critical.
      My low-pass filters are 7 pole and 4 units, following the same scheme as in mcHF, but using toroids T68-6 ferrite cores and 2 KVolts. ceramic capacitors.
      I'm still doing tests. Thanks for your contribution

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    2. I have tried out what K6ESE said about needing 4 turns when running ~13v. Driving my MiniPA100w with a ft-818nd I did indeed get 100w on 40m where are when the output transformer was 3 turns was just over 50w -- yes I have a low pass filter auto selecting the correct filter based apon the band data from the yaesu portable.

      Look

      https://youtu.be/cOQ1_3HzQfA

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  7. Hi, I purchased a ready made unit from aliexpress. Connected the LPF and tested the amplifier. My SWR almost doubled. Disconnected the LPF , the SWR has lowered a bit , but still not within range. Still above 2.
    Did not understand " the pointer of VR1 has to be on ground side initially". Please share the exact steps.
    How is the LPF to be connected. - The OUT of the LPF goes to LPF IN on the Minipa and the IN of the LPF goes to the LPF Out on the Minipa. Is this the way to conect
    Or
    LPF OUT needs to be connected to Minipa LPF OUT and LPF IN needs to be connected to LPF IN on the Minipa.
    How does the antenna matching happen. I am using a long wire folded terminated antenna. I use UBITX to connect to Minipa.
    Thanks
    Rajesh, Mumbai, India
    VU3RHU
    73

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    1. Hi, Rajesh.
      The low pass filters must be connected to the output of the amplifier. There is a two-pin connector on the board for this function that usually goes with a wire bridge or jumper, that is the right place to insert the low-pass filters. I recommend 7-pole chebyshev or butterworth filters. I made mine following the system that the mcHF uses but using diferent toroids. These filters are "symmetrical", they do not have a defined input and output, they are simply inserted between the amplifier and the antenna. No need for a filter for each band, with 4 low-pass filters you can cover all amateur radio bands. The SWR meter must be installed between the output of the low-pass filter and the antenna.
      The cursor of the VR1 potentiometer must be on the ground side, it is to start from zero polarization before making the adjustment.

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    2. Hi Alvaro, thanks for the reply. Have been testing the Minipa by connecting it to UBITX, my SWR has almost doubled. What needs to be done. So also checked the voltage across C2 , it reads as 4.65volts. The VR1 gives me values of 4.8 to 4.5 volts, hence kept at 4.65 as an inbtween value.
      But yet to make a contact using the Minipa as the SWR is high. Please advise. 73

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  9. Hi, Alvaro,
    I just assembled MiniPA 70w from China without input signal or idle consumtion current is 1A, that is normal?

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    1. Yes. 1 Amp. is recomended bías idle current.

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    2. If you don't test with a heat radiator, the mosfets will hit the thermal rush and will be destroyed.

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  10. Hi! My MOSFETs aren't flowing current with 3.1 V biased... Tried to max the VR1, which gave me about 3.2 V, but still no current. Took the MOSFETs out of the amplifier and tried with a separate power supply. Had to increase voltage to 3.2 V in order to get a current of 0.5 amps.

    What to do? The VR1 doesn't output voltage above 3.2 V...
    Thank you!

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    1. What mosfets are you using? MRF9120? One amp of quiescent current is achieved approximately with the VR1 potentiometer in the middle of its travel. If you are using the IRF530N mosfets, I recommend that you select a pair that has the quiescent current as similar as possible, the gate electrode voltage is the same for both. Paired IRF530Ns are not shipped in Chinese kits. What's more, I have bought some very cheap IRF530Ns on Aliexpress and they turned out to be fake.

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  11. Greetings everyone, please tell me what value VR1 has. Thank you.

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  12. The IRF 530s that came in the kit failed very quickly, source to drain shorted.
    Replaced with ones from Langrex, UK are working beautifully.
    Get genuine replacements from a reputable supplier.

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  13. Superb information ! Thank you for writing it up and making it available
    EA7KNW

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  14. hi everyone. I have the 200 watt version of this amp which uses a different output transistor, but has the same basic circuit. The bias circuit is exactly the same as the 70 and 100 watt versions. I recent discovered that the bias pot VR1 is open circuit and need to replace it. Does anyone happen to know the value. In my amp R7 (resister in series with VR1) is 900 ohms. So I assume a 1K pot will work just fine.

    KD1MU

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    1. So I replaced the pot with a 2k .125 watt from mouser. I am now able to get 2.7 volt bias at the gate. I have a 24 volt amp and am 24 volts at the drain. I am not getting any bias current (measured at the supply). I assume that means I blew out the LDMOS. I was getting 1 amp bias before the pot went.

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  15. Alvaro,
    Thank you for taking the time to create this write-up. As a novice, I understand the frustration of assembling a project and not having it work. There is precious little information on this kit and I've found my expectations were too high for this project. Right out of the box, the salvaged MRF9120 FET that was provided was faulty (shocker). I had a pair of IRF510s on the shelf and wanted to see if it would work (just to see any amplification). I was unable to get the circuit to work and was wondering if this is even possible. I transmitted on 2 watts into a dummy load with bias set at 2.7v. The MOSFETs still test good and I was just hoping for any guidance. I also have some IRF530s on order, but with COVID hindered delivery, it may be a while. Thank you in advance.

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  16. I did find and use some MRF9120 on ebay ... but that was 10 months ago ...

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    1. Thanks Seth7. I, too, saw those on eBay. I've read on the internet (so you know it's true) that a lot of these MRF9120s from China are counterfeits and they range anywhere from $18 to $48 with no way of knowing its authenticity. At those prices, it defeats the purpose of buying a "cheap" alternative to higher quality and listed amplifiers. I've always been one who needs instant gratification and waiting for the slow boat from china to deliver possibly fake MOSFETs that may or may not work doesn't satisfy my needs lol. Hence the more available IRF530s. (IRF510s are out as I've already let the smoke out on the ones from the shelf)

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  17. It's true ... many many counterfeit, the ones I bought worked, and yes ... I expected them to be junk but got lucky. There are some that are usable as fake ...

    We're in an economic war ... buyer beware

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  18. mnipa70 keeps blowing the FETS as soon as i apply 13.8Volts. But only one FETS blows. Do you have any suggestions on what to trouble shoot?
    kc5mol@riocomputer.com

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    1. With the fet out .. check bias voltage. Check resistance from gate tab to swiper of bias pot to verify symmetry ..

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    2. Кто нибудь пробовал альтернативу IRF 260?

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  19. what is the purpose of that resistor in parallel with the secondary on the 100w version, and what is it's value?

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    1. Роль резистора вторичной обмотки ограничивать амплитуду раскачки. Советую всем менять IRF530 на IRF260 (покупал паяные на али, меньше вероятности нарваться на подделку) Резистор параллельный обмотке не ставить. При 13.8 мощность 80, при 24 можно вытянуть и 180. Эксперименты с этим транзистором не так страшны.

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  20. Переменный резистор следует сразу заменить на более качественный, уже не один радиолюбитель взорвал мосфеты из-за плохого контакта. IRF260 выдержали 200ватт при ошибочно забытой антенне.

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  21. Dears,
    I have a problem with the amplifier kit (with MRF9120)
    1. When trying to set the Idq, from 240mA it starts oscillating.
    2. Randomly oscillating also with less Idq
    3. The feedback resistors are to hot
    Any idea?
    73 Richard OK8WW

    ReplyDelete

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