Re: Oscillation problems Bitx20a


Howard Fidel
 

HI Fokko:
I have one of those amps running now on 20 meters. I just wanted to let you know that I had some issues with it. When I built it, the amp self destructed, and was probably oscillating.  I found that the IRF530s were running very hot, as well. I didn't have any in my parts bin, but I do have IRF520s. If you put 2 IRF520s in parallel, you get about 20% more capacitance then the IRF530, however you get much better thermal performance and can use a smaller heat sink. You also need to add LPFs for each band. I will be writing up something on this soon and posting it.

Howard

On 6/6/2018 3:07 AM, Fokko PH0KKO wrote:

Thanks, great tricks. The low pass filter is on the same board but hidden behind a vertical wall. The ratios on my board are about the same as you describe: built closely together at the input but a bit more spread out towards the final stages. So I can still put some vertical double sided copper board between the last stages and connect them with thin coax through holes. Time for some experimenting, that is one of the fun parts of our hobby.

In my case L3, T1 and L9 are reasonably close together so I put L3 and L9 at a 45 degree angle to T1 hoping they would affect each other less. Maybe some pieces of board between them as well? I have enough of the stuff for years to come.

I bought one of those cheap 20 dollar Chinese linear amps on Ebay, the MiniPA70 class AB. With about 2.5W in it should give me around 40-50W out on 40 meter. So if I can get a nice clean 2.5W signal out that would be good enough for me.

@Farhan: yes I am using a DIY dummy load, about 51 Ohm 40W. That should be good enough.


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