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JFET Power Amplifier oscillations

Roger Knopp
Intellectual 270 points

Hi!

I am being a bit frank here and is actually asking you skilled guys why my JFET Power Amplifier oscillates.

I don'r know if Ti manufactures BF245A but I hope so otherwise it is a slim chance of getting help :)

BF245A is my favorite JFET and I have built a "copy" of the famous Williamson Tube Amplifier from 1947 using BF245As.

As "Power tubes" I am using 2SK175 right now but have found more linear versions which I will use in the future, the 2SK135.

I have kind of come to the conclusion that the warm tube-sound isn't actually because of the tubes, it seems to be because of the output transformer which shortens out even harmonics.

So I have built a JFET pre-amp with MOS output transistors in a push-pull arrangement around a transformer which I have wounded and glued manually using laser-cut FR2 and a transformer core made by the best C-Core transformer manufacturer in the world, Lundahl Transformers.

The first use of my amplifier is intended to be as a guitar amplifier which my current data actyually shows work fine.

However, I have hopes for more.

Right now I have a FULL POWER bandwith of only 20Hz-5kHz, here I get 8W output.

But at 20kHz I only get 1W output before the signal turns realy weird, it almost looks like a frequency doubler!

I have measured the signal at MOS gate and simuoltanoulsly  at MOS drain but at 20kHz they do not look the least a like which makes me understand nothing.

I am attaching some pictures of this and more.

I really hope you may want to help me even thou this really is not related to any of you fine products.

Kind of desperate as you can see :)

Best regards, Roger
PS
High output current AND say 1kHz works, high output current AND 20kHz does not work (please see AND as a logical operator).

Open Loop, 1kHz, Upper: 0,5V/DIV (Input), Lower: 5V/DIV (Output), Z=3,9 Ohm


Closed Loop (BF=12dB), 1kHz, Upper: 0,5V (Input), Lower: 5V (Output), Z=3,9 Ohm


20kHz, Open Loop, Upper: Ug at Power_MOS (sensitivity=unknown but not important because signal is "good"), Lower: Ud at Power MOS (sensitivity probably 5V/DIV), frequency is doubled :)