[Amps] Tubes vs. Solid State

[Manfred Mornhinweg]

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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