[Amps] Solid State Amps
km1h at jeremy.qozzy.com
Fri Oct 17 21:02:53 EDT 2014
Thanks Win, I was going to ask my #2 son who just retired from the USAF and
is living in Garmisch for the translation but he was already in bed. He
speaks German as a native and has decided to stay and work there.
----- Original Message -----
From: "Winfried Kriegl" <dk9ip at gmx.de>
To: <amps at contesting.com>
Sent: Friday, October 17, 2014 6:13 PM
Subject: Re: [Amps] Solid State Amps
It´s a perfect german translation, Carl :-)
73 Win DK9IP
Am 17.10.2014 23:56, schrieb Carl:
> Ive noticed over a wide swath of amp and other forums that SS is
> becoming more popular but seemingly misunderstood by most even more so
> than tube amps.
> The majority appear to be the newer generation of instant hams that have
> a problem understanding how to tune a tube amp, especially those models
> that are even marginal for experienced users.
> Also our rapidly aging older generation is having some problems tuning
> and even concentrating on tuning and destroy things in the process. The
> weight of a tube amp is another serious concern
> Neither group should be let inside a tube amp without training or
> assistance for their own good.
> Now the bad news....the lack of a quality SS amp at the price they want
> to pay. Yes, some have the money and there are a few choices but the
> majority appear to gravitate to MRF 150 amps that are being pushed to
> the limit witout any feedback circuitry and claims of "exceptionally low
> SSB distortion compared to tube amps".
> Yet nowhere are any IMD specs published that I could find.
> So for around $3K you get a 1200W amp, and you are warned about extra
> heat generation if the SWR is over 1:1! Hmm, it sounds as if they are
> brainwashing you into also buying an antenna tuner.
> As my Yiddish friends from Brooklyn used to say: "velkh a fun drek"
> Or from my German grandfathers: " Was fur ein Haufen Scheisse"
> Apologies for any translation errors as I had to cheat and use an online
> translator; couldnt remember parts of the phrase; its been 60 years or
> so (-;
> ----- Original Message ----- From: "Manfred Mornhinweg"
> <manfred at ludens.cl>
> To: <amps at contesting.com>
> Sent: Friday, October 17, 2014 2:24 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
>> 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.
>> Visit my hobby homepage!
>> Amps mailing list
>> Amps at contesting.com
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