Guys, what you describe here is transformer saturation! Most likely using
too small core.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Manfred Mornhinweg
Sent: Friday, May 04, 2012 5:22 AM
To: amps@contesting.com
Subject: Re: [Amps] Tubes vs. Solid State
Bob,
> My problem area was the extent of harmonics generated within the
> device. H3 was within a dB or two of fundamental energy levels and
> H5 only marginally better.
That typically happens when your output network isn't correctly done.
There is an incredible amount of equipment, including HF ham transceivers
comemrcially made today, that have incorrectly implemented power amplifiers,
due to their designers not understanding of the basic principles under which
transformers operate.
> A serious problem. My output arrangement focused largely upon a 1:9
> coax wound RF2000 from RF Parts as used in the Granberg designs at the
> 1kW level.
Granberg apparently was the one who "invented", or at least popularized,
the wrong output network. Several of his papers contain the mistake, but
others do not. It seems to me that he really didn't understand this issue,
at least not when he published those old papers.
How are you feeding the drains? If you are using a bifiliar choke, designed
in such a way that it can act as a balancing autotransformer, then that
should be fine, and you have to look elsewhere for the reason of the high
harmonics. But if you are using two individual chokes, then that's wrong,
and if you are feeding the drains through some sort of center point on the
transformer, then there is a pretty good chance that it's wrong too!
Typical symptoms of the incorrect output configuration are: Extremely high
distortion (harmonics, IMD), horrible waveform at the drains, that includes
peaks well above twice Vdd, low efficiency, low gain, and a sort of gain
breakpoint: Up to a certain power the amp is easy to drive, and from that
point up it gets suddenly very hard to drive further.
> Harmonics were not a consequence of transformer saturation
That could hardly ever happen at HF. Before you saturate a ferrite core at
HF, you will melt it down with the losses!
But DC saturation can happen, in very tricky situations, specially if you
have hugely more inductance than needed.
> No problem in a single frequency amp but I am way short of clever
> enough to figure out a scheme which will handle that over 5 octaves.
Use either an output transformer that has a true center point, or a bifiliar
choke to supply power. Note that the typical RF power transformers made from
two ferrite tubes, with a single-turn primary, DO NOT HAVE A CENTER POINT.
The junction of the two metal tubes is NOT a center point! Using this
junction as a makeshift center point causes endless trouble, and many
amplifiers, based on some of Granberg's designs, contain exactly this
mistake.
With transmission line transformers, a center point is usually also
unavailable, but some transmission line configurations can have one.
The basic point is this: Class B or class AB push-pull amps MUST, I repeat
_MUST_ have something that provides balance around a true center point. It
cannot work in pure differential mode, because each FET conducts for half of
each cycle, and is in high impedance during the other half cycle. You cannot
draw current between one transistor that is on and another that is off!
That's why balun or balbal type output transformers only work correctly in
conjunction with a bifiliar feed choke that provides the center point.
Class A push pull amps do not have this restriction, and can work well in
pure balanced mode.
So, check your feed arrangement, maybe that's where your problem is!
Manfred
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