Re: External Amp

Howard Fidel
 

Nick:
You reminded me I left out an important piece. I added 10 ohm resistors in series with each of the gates. The circuit was oscillating at 120 MHz, which is probably why the IRF530s blew. I revised schematic is attached. For the 20 meter band, I made 2 air core coils I wound on a 1" D  piece of schedule 40 pipe using 9 turns 14 AWG wire. The pipe is just a winding form, not used in the circuit
That is about .9 nH each. The caps were 240 pF at the input at output and 360 in the middle.
I posted another schematic yesterday with the LPF designs for all the bands.
I adjusted VR1 in the Ubitx to get the correct drive level. Of course the uBitx's output is not uniform across the bands, so you need to set it properly for each band. I had to drive the amp at 28 VRMS to get the full output. I'm not sure what the input impedance is, so I can't measure the drive level in watts. However, at the same setting the uBitx was putting out 24.4 volts RMS into 50 ohms, or 12 watts.
Howard

On 6/10/2018 10:08 AM, Nick wrote:
This is good information. I also picked up the same amplifier and the IRF530s were quick to self distruct.

When testing into a 50ohm dummy load I was initially only getting about 10 watts. The resistors that came with the unit were 100 ohm instead of what I found in the online schematic of 150ohm . 

I made a low pass filter for it but I think the toroids are a bit small as they got really warm. I also made an attenuator pad at the input as I read that the pa does not seem to like more then about 2.5watts in.  The pad was a 220, 22, 220 with about 4db calculated loss. 

I found that with my o'scope the output waveform from the bitx was very distorted into the amp which could explain the low power and heating in my filter. In an attempt to figure out what was going on, I modeled the amplifier in ltspice .It did not work well as per the schematic I found. I had to increase the bias voltage and put a 10 ohm resistor across the two mosfet gates to get a nice input swr and a somewhat nice 70ish watt waveform out of it. This was simulated at 7.2Mhz with the ltspice model of the irf530 and all other components were ideal.

Since it looked good in simulation I attached the resistor and started increasing the bias voltage. I was not using a very small current limited power supply and while doing this there was a small puff of smoke and my supply voltage dropped to nothing. Oops...

I suspect not paying attention to the drain current may have been a problem. I think maybe they turned on and "shorted" at dc. Not sure if I trust the components that came with the kit though as they were in standard bubble wrap with no esd protection for the MOSFETs. They could also be fake MOSFETs.  

I don't currently have replacements available but I might try your suggestion of using the 520s in parallel.

I appreciate your post,
Nick


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