To: rsgb_lf_group@blacksheep.org
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 <cb09c941-4091-0b07-d1c3-67d4a64581a6@dctower.co.uk>
From: N1BUG <paul@n1bug.com>
Message-ID: <57ff16de-67c2-d7d9-ab41-cb63aba83fb9@n1bug.com>
Date: Tue, 9 Jan 2018 12:36:54 -0500
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Subject: Re: LF: Progress? with the PA
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Nick,

I was very interested to read your email. I am sorry that you are 
also having problems with this PA but at least it isn't just me. A 
few comments threaded in below...

> I've been, sort of, following this as I have a similar problem with the
> amp at <http://www.qrp-labs.com/ultimate3/u3mods/lfamp.html> I have not
> used the 2n4 cap to ground on the gate (yet) but I have added a series
> resistor of 15R to the gate prior to the DC blocking cap (or should I
> have put it after the DC blocking?)

I put my 15R directly at the gate.

> With out the 15R it was very fussy and would go into, what I presume,
> was oscillation - the voltage trace on my scope match was 'fuzzy' -
> looked like AM? with almost no provocations ... however lowering the
> supply voltage cured this.

That is similar to my experience. There are two different scope 
patterns I see when it starts acting up. Sometimes it will be one, 
sometimes the other.

One is as you describe, the voltage trace goes fuzzy, more so at the 
peaks than at the zero crossing. This will range from mild fuzz at 
the peaks to extreme where it is fuzz from the peak to the zero line.

The other is different. I usually have my scope set to fit about two 
full cycles (720 degrees) on the display. Sometimes the first and 
second cycle will be of drastically different amplitude even though 
both are crisp traces without any fuzz.

> I have just re-worked the matching unit at the aerial and now have an
> almost perfect 50R j0 match. so I know this is not a reactive issue

Have you tried it into a 50R dummy load? Mine behaves perfectly into 
a dummy load but not into the antenna, even though the antenna is 
tuned for 50R j0. I am thinking the antenna being reactive *off* the 
operating frequency sets off the oscillation.

> It still oscillated if I run it at 13.8V and drive of around 250mW (U3s
> 3 BS170s at 5V), drop the voltage to about 11V and all is well.

Yes, lowering the voltage makes mine more stable also.

> Increase the drive to about 800-900mW (U3s 3 BC170s at 13.8V nominal)
> and I can wind the supply up to a little over 12V before it goes wild.
> The bias is set to give about 75mA at idle.

Mine is far more stable when I run the bias down into the class C 
range (no idling current). For any given voltage what I usually do 
is start with the bias at zero (wiper of the pot at ground end), 
apply drive and turn the bias up until it goes into oscillation... 
then back off a bit below the point where it stabilizes again.

> If I reduce the bias & therefore idle current then the oscillation is
> earlier & easier to provoke,

Unless I am misunderstanding, mine is the opposite. It is more 
stable when the bias is set for class C (well below the point of 
having any idling current).

> if I go much over about 150mA the FET takes
> off in thermal runaway with the current going skyward with no change in
> bias setting - so a little under 100mA it is.

Mine does that too.

 > However if I give the amp a capacitive load it is much
 > happier ... and will take over 15V and no oscillation - that
 > ought to tell me something but I'm not sure what

My amp is no longer like yours as I have 4 x 10 turns on the 
transformer now, but yesterday I noticed it is happier operating 
slightly below the antenna resonant frequency than dead on or above. 
So I believe I am seeing the same thing.

I think the output transformer is at least part of the problem. 
Changing it in some way seems to have more affect on the instability 
issue than any other change I have made in circuit. I've also been 
told by those with the ability to model them that the low pass 
filters for this amp are terrible designs. The specifics went over 
my head.

Paul N1BUG