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Re: [Amps] Solid State Amps

To: Manfred Mornhinweg <>
Subject: Re: [Amps] Solid State Amps
From: "Col. Paul E. Cater" <>
Date: Fri, 17 Oct 2014 14:48:42 -0400
List-post: <">>
Great post Manfred-----I am looking forward to reading all the replies.


On Fri, Oct 17, 2014 at 2:24 PM, Manfred Mornhinweg <>

> 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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