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Re: 50W PA Input SWR 3 and up! What can I mess with?

 

Andy, I tested my 20 Meter 50 watt PA using the NanoVNA and found approximately 2:1 all the way out past 50 MHz. I didn't check higher in frequency. When I unkeyed the amp, the SWR went to infinite across the spectrum as expected because the diode switch (D1) opened the RF path to the attenuator. I think it would be beneficial for you to check it both keyed and unkeyed to make sure diode D1 is properly turning on and off. You should not have to add any more attenuation than what is already there. And you should not have to modify the input circuits. This amplifier is usually?a solid performer as designed.? If you are sure the attenuator is properly built and you have checked connections with an ohm meter, and you are sure all the enamel wire is properly installed then you should look closely at the operation of D1.

73 ... Ron

On Wed, Feb 14, 2024 at 6:29?AM <natereik@...> wrote:
On Wed, Feb 14, 2024 at 12:04 AM, <kontraptioneering@...> wrote:
it looks to me that there's a very happy frequency that this amp wants to operate, which implies that somewhere, this frequency of 10 Mhz is being set by a certain combination of components - capacitors, inductors, etc?

Could it be C2 or C3? And I don't know about changing the number of turns on T1, but maybe? I could experiment with a few things I guess.? Any suggestions for starting points from the crowd? I've attached the schematic for convenience.?

This isn't the first time this has come up here...I'm curious as well.?

I don't know much, but it is interesting to me/ I wonder why 1 uf caps are used on the input here. On the 10W linear PA kit, it goes straight into BS170s for the first stage, and then some 0.1 uf caps before the IRF510s....... And the input transformer is different.? ?
The WA2EBY also uses 0.1 uf on the input.?

-Nate N8BTR
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