15Watt Linear

Here is the schematics and PCB pattern for my 15watt linear.

15watt linear component placement

15watt linear pad placement

Descriptions, History :-

I had made this linear by accident. My 5watt cw tx/ DSB tx was not giving me signal reports. I was worried ! What is the  problem ? Then it occurred  to make a single device linear . One ckt was interesting . Put queries in the Indian Homebrewers forum at Facebook. And lots of reply came. VU2RON, VU2ESE, VU2DEV and lots more friend jumped in.

Finally tried it with IRF510 but it went burst with 25volt dc power supply. Then changed the IRF840/IRF640. Both worked fine. vu2ese had suggested me to put some 200pf at the drain. VU2DEV suggested making a Pi-LC matching. He even pointed out the calculations in the ARRL handbook , which he had gifted me. But, I settled for transformer & toroids. Ron hinted on impedance transformation of this broad band transformer.

Then made it. I used Small pig nose baluns bought from tpsonline Bhopal. Put 6 togather. 3x2, Glued with water pipe glue and it worked. It did not heat-up. The biggest problem with the ferrite is that it heats up and freq response depending on the size and material. So, we have to take care of this problem.

I used in the input a resistive 3db pad. This db-pad solved my problem for loading the exciter, proper match etc. It was happy happy solutions. With pad some fraction of input power is wasted in heat. Its okey.

I put LED to indicate power supply presence. I put a protective diode 1N4007. Not shown here. The VCC line is a big one.  Instead of deriving 12volt from the pcb I used the LM7805 voltage regulator. The simple preset is used on the pcb to bias the gate of IRF. At gate a small value resistor 5 ohm is used to stop parasitics. If you have ferrite bead please use that also. Now, a days we don't find ferrite bead esily.

Filter is classic bilateral 50 ohm three pole. Simplicity. Actually 7 pole filter should be used for more spectral purity.

The relays for changing over the antenna etc are not used here as it was meant to be a beacon / CW tx.

Components & Schematics:-




vu3inj linear
 

Setting up the linear:-

this requires some care & instruments.
Put Digital Multimeter into 10A range. Set the current to about 100mili amp. Now key the exciter. Make sure there is RF. This will increase the linear current to about 1.2 Amp. Use the dummy load or dipole antenna. VU2PTR digital VSWR meter will indicate the 15watt. So, this is the set-up. Nothing more. Mind you the preset tuning is little trickey. Slight increase in the bias and power jumps from 5 watt to 15watt. At one point of time you will feel that heatsink is too much hot but there is no power increase. buck down a little... don't set it for constant 1 amp. at max 300ma is also good. Do little experiments but just care for heat....

This linear has been never tested on SSB. If I ever do, will let you know.My approach was use whatever is on hand. Till date I have made two linears and both work wonderfully. One is with me and one is with VU2TOO.

Heat sink:-

It is one of the most important factor in this linear. You ought to use a biggest heatsink you can find. Old CPU heasink is okey. I also use the fan.

On Air test:-
On one two occasion I asked for listening service from VU3INK now (VU2JYX ), VU2DEV and two more hams. I am thankful to you Kishore for being with me. My signal was good. consistently 55 to 57. In this I had not tuned my antenna at all. SWR was around 2.5, So, the signal reports are not bad.

Will update few more fotos of linears and corrections within two days.

That's it folks the wheel has been re-invented.

Use it and don't forget to customize the bells and whistles ( relays, heatsink , box etc)



 

Homebrew RF Power Amplifier

Thank god i have reached to this point after having the real linear amplifier.

I am going to edit and re-edit this page for sometime till i make a linear myself.

vu2ptr_linear

At present trying to use the linear made by vu2ptr to increase my ham radio life and to be on air urgently.

Later on i will make another linear  of this or any other design (push-pull or single ended FET)  and put the process here. ,

If anyone wants to buy ready made linear or its pcb, toroids etc please do contact vu2ptr directly. He also makes vswr meter, lc meter , frequency counter etc.



Making linear:-

RF amplifier design:-

Now, organizing the available info for use. It becomes very hard to choose from, but generally all are derivative of the wa2eby irf 510 push pull linear amp design.

Toroids, FETs & other things.

On hardware front I have toroids, irf840,irf510, power supplies, dummy loads so that is not the issue.

But will take time to first pack my existing things and put into the boxes. thats priority...........

The single ended linear:-
Most probably i will start building the singe ended IRF510 based linear. It will same as the done before but with more power and more current and proper toroids. I think air core toroids does not drive the other stages properly due to some reasons. Don't use the air-core toroids in the power amplifier. simple ferrite cores (slugs) can be used upto the 1watt.

when in doubt where we turn for help ? THE ORIGINAL OEM

HERE IS WHAT vu2RM HAS TO SAY. (My- copy- cut- paste- tech)

PUSH-PULL AMPLIFIER FOR 14 MHz
By: S. Rama Mohan Rao, VU2RM

    After working many years on my RM-96 with IRF830...etc. in the final PA, I have tried the same tried the same circuit for 14 MHz transceiver. The same broad band is used with IRF830. It has given an output of 4watts or so. Then, I have gone deep into the subject of VMOS & TMOS power FET's. I have decided to use these devices with Low CISS & COSS (input & output capacitance) values. I have finally chosen to use IRF510 devices. This has a maximum Voltage (VDS) of 110 V.

I have built a push-pull stage because of the following reasons:

*      When a tuned circuit is used in the final tank circuit with a single stage amplifier, the quality of the signal sounds good.
  *
      Nevertheless, with a single stage broad band it is not the same quality.
*

      I felt that this difference is due to the flywheel effect of the tuned circuit.
    *

      During the technical discussions with some of the home brewers, it is learnt that the)/also found the same when the wave shape was seen on the oscilloscope.
    *

      The positive and the negative half cycles are not the same
    *

      Therefore, I have decided to make a push-pull amplifier.
    *

      Also for the reason that as we are using broad band amplifier, there are bound to be some harmonic generations, due core saturation of the final toroid and various reasons.
    *

      In push-pull these can be avoided largely and therefore less spurious outputs and better quality.
    *

      Keeping in view al/these points the following circuit is evolved and it is working satisfactorily. 

Experimentation made on this circuit:

    After building this circuit, I found, one IRF is getting hotter than the other. I made all checks I know, but it is observed that one gate is getting more voltage than the other. Then I reversed the secondary windings. Now, the other gets more heated. So evidently, IRF has nothing to do with this unequal drive voltages to the gate. I tried it with an imported toroid. No luck got fed up and thought of introducing negative feedback to equalizes the gain of each FET. So, tried combination of RC network, which is a general practice. About 100 Kilo ohms in series with 150pf capacitance from drain to gate. Now both the positive hail cycles and negative half cycles are equaled and the defect of one FET getting hotter than the other vanished. The quality also improved. Even I operated for one or two hours the heat sink is not getting heated up. i am using 22 Volts supply on the drain and it draws about 2 Amps. (i.e. 44 watt DC input). It is giving a power output of 25 watts into a 50 ohms antenna as measured on a Daiwa power/SWR meter. Further, there is no reflected power from the PA stage. So, RF filters are not used.

    I have not built a 21 MHz RM-96, but looking at CISS value of IRF510, I have a strong belief that it can work on 21 MHz also. So this will be a good choice for 7/14/21 MHz power amplifier. When one Ham from North India told me that he is using IRF840 in 14 MHz and is getting about 20 watts output, I immediately got hold of IRF840 and replaced the IRF510 with IRF840. The power output is just 4 watts. I have increased the voltage from 22 V to 40 V, the output power went upto 10 watts. However, the heat sink is getting hot indicating that the efficiency is Low. Therefore, I left the PA like that, settling with IRF510 only. Happy, go lucky, now that is all my experimentation on the push-pull power amplifier.