What turns me off to Solid State is the fact when you need replacement
transistors, they most likely have become obsolete. I'm tired of it. Same with
displays. You need to throw the radio away because the display that went out
is no longer available. Screw all this new stuff. I'll use my Collins and
Drake rigs with a L-4B, L-7, SB-220, etc. At least the Chinese 3-500Z tubes
hold up. Anything using 572Bs these days are disqualified from being used in
my shack. I'm off this "buy a new rig every year" Merry Go round.
Lee, w0vt
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: amps@contesting.com
Sent: Friday, October 17, 2014 1:24:13 PM
Subject: [Amps] Solid State Amps
Dear all,
Dan touched the subject of solid state amps, and Louis was quick to state that
most hams would prefer a good tube amp. If you ask me, the performance/cost
ratio will dictate what hams finally prefer, rather than any philosophical
concepts.
So, what we need to finally move tubes out of the ham realm (except for those
who really love tubes, of course), is making solid state amplifiers that are
better and less expensive than tube amplifiers.
And the best approach to do that is _not_ by porting tube era technology to
solid state devices, nor is it to keep building copies of Helge Granberg's
designs forever. These approaches simply produce a poor performance/cost ratio,
when taken to the 1500W level.
Let's see what the weaknesses of solid state amps are:
- Heat. Solid state devices simply are very small, and don't tolerate extreme
temperature. So, a high power, class AB, solid state amplifier will ALWAYS be
problematic in terms of cooling. It will need large heatsinks, fans, heat
spreaders, and careful design of the thermal aspects, just to start becoming
viable.
- Fragility: RF power transistors are usually run very close to their absolute
maximum voltage spec, close to their maximum current spec, and at or even above
their rated thermal capability, with the heat sink system used. Any problem
like
non-perfect SWR, relay glitches, etc, and their survival depends 100% on
excellent protection circuitry. Tubes instead are so forgiving that in practice
they don't need protection circuits in most cases, or some tubes need simple
circuitry to protect against excessive screen or grid dissipation, but not much
else.
- Poor linearity: Both bipolar and field effect transistors are less linear
than
tetrodes and pentodes, and while better than triodes, they don't have enough
gain to use them in grounded base/gate configuration. So, they depend on
negative feedback or other external means, to arrive at good IMD specs. Many
designers still don't grasp this concept well enough, and try building solid
state class AB amplifiers without negative feedback, getting horrible IMD
performance.
Now some people have tried, and are still trying, to solve these problems by
brute force methods: Use lots of transistors, on big heatsinks, run them well
below their maximum specs, use UHF transistors at HF to get enough gain that
allows using lots of negative feedback, and put in complicate protection
circuits. The results of these efforts can work reasonably well, producing
amplifiers that are instant-on, no-tune, reliable, and about as large and heavy
as tube amplifiers - but the solid state ones tend to be more expensive, done
that way. And often the implementations are simply wrong and unsafe, for
example
by relying on an SWR sensor placed between the low pass filters and the antenna.
What we need to do, my dear friends, is something totally different. For
starters: Forget class AB, because it's too inefficient, and forget Granberg's
push-pull configuration, because it has no inherent protection features and
needs problematic transformers.
Instead of Granberg's design, we need to place our RF power transistors in half
bridge or full bridge configurations, with effective antiparallel diodes. This
configuration eliminates all risk from overvoltage. Then we need to run our
transistors in switchmode, _not_ in any linear mode, to get rid of the heat
that
causes so much trouble. Then we add simple current sensing with quick shutdown,
to protect against severe overcurrent situations. We need to take the highest
voltage transistors we can, up to a level of 400V or so, to get rid of the
ultra
low impedances that result from low voltage operation, and which are hard to
handle. And instead of a broadband transformer (not very easy at the kilowatt
level), followed by relay-switched low pass filters, we should use
relay-switched resonant matching networks. That's no more complex than the low
pass filters, and the resulting Q is low enough to pre-tune these networks to
each band and then forget them.
And then, of course we need to add circuitry around the amplifier block, to
obtain a linear transfer function despite the switching operation of the RF
transistors. This can be done by RF pulse width modulation of the drive signal,
power supply modulation, bias modulation, a combination of two or three of
these, or any other method. This is far more complicate than a traditional tube
amplifier, of course, but it uses cheap, small, widely available components,
and
so it's inexpensive to implement.
The result would be an instant-on, no-tune, small, lightweight, silent, highly
efficient, reliable _and_ inexpensive legal limit amplifier.
Anyone actually developing this concept to market maturity can put all existing
ham amplifier manufacturers out of business. A scaring thought - for them!
Do you notice the logic in this? Going from class AB to a switching mode
achieves several important advantages:
- Cooling becomes very much simpler, cheaper, and silent.
- Power supply requirements are drastically cut down, producing advantages in
cost, size, weight, etc. A 1700W power supply can power a 1500W amplifier.
- Power consumption is reduced a lot, an important selling point in many
countries that have expensive electricity. Maybe not in the US, where it is
almost free.
- The transistors needed are very much smaller and cheaper than those needed
for
class AB, due to low dissipation requirements.
- A good active linearization circuit can produce far better linearity than
class AB with 10dB of negative feedback, and even better than that of tetrodes.
And the difficulties involved in this approach:
- Finding ways to get around the limitations of present-day RF power
transistors, in terms of voltage-dependent internal capacitances, slew rate
limitations, and high voltage handling.
- Summonning the determination to do all the detail design work, and break free
from the idea "if Granberg did it that way, that must be the best/only way".
Any idea, anyone?
Maybe we should start a collaborative open project, developing this thing! The
final goal: A solid state amplifier no larger nor heavier than a typical HF
radio, that can produce solid legal limit output in all modes, with no time
limit, with good IMD performance and high reliability, a total parts cost
around
$500, and selling to those who are too lazy to build it, for around $1000.
I'm just waiting for the right transistors to show up, and then I will do it
myself. With the transistors I know right now, I would get up to the 40m band
only, or at most to 20m, but not to 10.
Manfred
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http://ludens.cl
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