I personally have gravitated towards 2SC2879's, MRF120's, and MRF455's for
the car... the 2SC2879's are 14 VDC devices and are super dirt cheap in
quantity.... even from a high-buck place like RF Parts.
A buddy of mine, K0UM is in the process of building a 48 VDC MRF150 amp for
his car... using a voltage quadrupler of a simple design (now I say simple,
knowing full well he owns a power supply design company). The higher
voltage devices have better efficiency.
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
Email: bill@wjschmidt.com
WebPage: www.wjschmidt.com
"If computers get too powerful, we can organize them into a committee; that
will do them in." -- Bradley's Bromide
----- Original Message -----
From: "PA3DUV" <pa3duv@planet.nl>
To: "Dr. William J. Schmidt, II" <bill@wjschmidt.com>
Cc: <amps@contesting.com>
Sent: Tuesday, March 29, 2005 12:40 AM
Subject: Re: [Amps] Solid State HF High power linear amps
> Bill,
>
> Do you use the 48 V devices in your mobile amp application?
>
> Dick
> PA3DUV
>
> ----- Original Message -----
> From: "Dr. William J. Schmidt, II" <bill@wjschmidt.com>
> To: <craxd1@ezwv.com>; <amps@contesting.com>
> Sent: Tuesday, March 29, 2005 7:45 AM
> Subject: Re: [Amps] Solid State HF High power linear amps
>
>
>> Hence the Pi-L circuit for tubes to clean them up too...
>>
>> There are a lot of misconceptions about solid state amps on channel. I
>> have built a many of them... and I will never back to tubes for power
>> levels
>> less than 5kW. Here are a few comments that may shed some light on the
>> mystery of the solid state amp:
>>
>> 1. "Require hard to build power supplies"... Yeah, I stopped building
>> power
>> supplies years ago. Why? Because there are thousands of surplus power
>> supplies on the market for almost nothing. Case in point... transistors
>> like MRF150s and MRF154/ MRF157's require 48 VDC. About 50 amps for full
>> power. It so happens that you can buy surplus... continuous duty... cell
>> installation power supplies that are 25-50 amps for less than $50 each.
>> You
>> will be awfully hard pressed to build or buy a power supply for a 3-500/
>> 4cx1000/ 4cx800 for that price. I think some of you have already tapped
>> this market?
>>
>> 2. "Solid State amps splatter without a filter". Well, yes they
>> probably
>> do need a filter to meet the FCC harmonic suppression requirements. Your
>> tube amp had the advantage that the filter and the transformer is the
>> same
>> device. Can't do that with the transistor design. But, what's so hard
>> about a 3 section filter? I can typically get away with eight 3 section
>> filters for 1.8-30 MHz. I have a trivially simple circuit that
>> occasionally
>> samples the frequency, divides frequency by 2**18 to give 229 states...
>> that
>> can be fed to a look-up PROM... to select one of 8 filters. About 8
>> parts
>> (or two if you use a PIC) gets you the automatic band switching for $15.
>> Does your tube amp have automatic band switching (well a few expensive
>> ones
>> do!)?
>>
>> 3. "High SWR Toasts the transistors unless you have protection
>> circuits".
>> Really? I have a mobile amp... 1.5kw output continuous.... can operate
>> into a dead short indefinitely and has no high swr circuitry. Just like
>> tube amps, if you design the amp properly... with conservative specs...
>> able
>> to handle full reflected power... it is not an issue. If you insist on
>> using an swr shutdown circuit... you can use the line section in the
>> output... four op amps to control the bias circuitry. $5 Automatic...
>> and
>> it resets itself!
>>
>> 4. "Transistors are expensive". I don't know where you all buy your
>> transistors, but I've collected hundreds of MRF150s at an average price
>> of
>> about $8 each. The MRF154's are more compact... are really four MRF150s
>> on
>> a single die... but I don't use them because they are too expensive. I
>> think the new APM mosfets show great promise here... I priced out a pair
>> of
>> the 300 watt jobs at $38. So $72 for a cool kW. Not quite the $50 for
>> the
>> 4cx800... but then there is that darn tube socket to purchase..... and
>> that
>> little complication of taking it mobile. There are so many RF transistor
>> choices, I almost don't know where to begin.
>>
>> 5. "Need matched pairs of transistors". Nope. Hogwash. Take the
>> MRF150s... four in a push-pull circuit. Just individually bias the
>> transistors... set the bias for identical current through each... and
>> they
>> are good to go.
>>
>> Unfortunately, I don't really have the time right now to design a 2kw+
>> PEP
>> amp with the APM transistors. I want to... it's on my list of new
>> projects.
>> I know it can be done... and I'll bet cheaper than any tube design
>> including
>> the power supply. Maybe this summer...
>>
>> Sincerely,
>>
>> Dr. William J. Schmidt, II K9HZ
>> Trustee of the North American QRO - Central Division Club - K9ZC
>>
>> Email: bill@wjschmidt.com
>> WebPage: www.wjschmidt.com
>>
>> "If computers get too powerful, we can organize them into a committee;
>> that
>> will do them in." -- Bradley's Bromide
>>
>>
>> ----- Original Message -----
>> From: "Will Matney" <craxd1@ezwv.com>
>> To: <amps@contesting.com>
>> Sent: Monday, March 28, 2005 7:03 PM
>> Subject: Re: [Amps] Solid State HF High power linear amps
>>
>>
>>> Yup,
>>>
>>> Plus the solid state designs splatter something awful without a filter
>>> on
>>> the end. Most amateur amps have a switchable filter on the output
>>> (switchable between each band). Also, if you have a high SWR condition,
>>> a
>>> transistor is generally toast unless you have a bunch of safety
>>> circuitry
>>> to prevent it. A tube can handle the beating with no ill effects for a
>>> period of time. Some of the large ones I've seen using up to 32
>>> transistors had to use welding cable to supply the power due to the very
>>> high current involved. Most transistors for this frequency range (2-30
>>> MHz) run on 12-18 Vdc and just a few higher. The transistors were really
>>> designed to be used as a single push-pull stage even though they can be
>>> split and combined.
>>>
>>> If it were me, and were going to start a new one, I'd build using a
>>> couple
>>> of 4CX800's.
>>>
>>> Best,
>>>
>>> Will
>>>
>>> *********** REPLY SEPARATOR ***********
>>>
>>> On 3/28/05 at 12:13 PM David Kirkby wrote:
>>>
>>>>Prasad Agrahar wrote:
>>>>
>>>>>Group
>>>>>
>>>>>Once a very active ham, I have been inactive for many years.
>>>>>
>>>>Same here
>>>>
>>>>> I have
>>>>>started dreaming about getting back into this exciting hobby. Signs
>>>>>are clear that the day is fast approaching. I was a home brewer and I
>>>>>will still be one. My wish is to gather parts to build a decent HF
>>>>>high power linear soon.
>>>>>
>>>>>That made me wonder whether I must focus on building a tube (valve)
>>>>>amp or is there any other choice? Tubes are not made any longer. Tubes
>>>>>that are available in the market are old leftovers and soon, there
>>>>>will be no more.
>>>>>
>>>>>
>>>>There are still made. There are numerous manufacturers still in
>>>>business, with Eimac the best known, and probably the largest.
>>>>
>>>>>I noted that discussion on this group is mostly about tube (valve)
>>>>>amps. But it is fact that all commercial rigs are all fully solid
>>>>>state and with transistor finals. If so, why no one talks about
>>>>>building high power transistor amps? Is any one working on home
>>>>>brewing project? Is it the cost of transistors or is it the technology
>>>>>that makes it difficult?
>>>>>
>>>>One issue I see is that for very high power you are going to need
>>>>either:
>>>>
>>>>a) Lots of transistors
>>>>b) One tube
>>>>
>>>>The chances of you finding the matched transistors on the surplus market
>>>>at sensible prices is very low. So you will probably have to buy
>>>>multiple devices at full commercial prices. There are numerous tubes,
>>>>like the YC156 (5kW) on the surplus market at very reasonable prices. I
>>>>think the cost of transistors/kW new will be more than the cost of
>>>>tubes/kW, but I have not priced them up.
>>>>
>>>>There are very few solid state designs for multi-kW published in amateur
>>>>journals. High power is usually obtained by combining the output of
>>>>various modules (more complexity). Motorola had some application notes
>>>>on reasonably high power. No doubt there are application notes from the
>>>>manufacturers of transistors that describe ways of getting high power. I
>>>>think you will find the design will be complex.
>>>>
>>>>Tubes are more rugged.
>>>>
>>>>High voltage power supplies for a few kW are easy to build, whereas 28 V
>>>>or so at 100's of Amps is no trivial task.
>>>>
>>>>There seems to be lots of good reasons tubes are more attractive for
>>>>amateur use at high power.
>>>>
>>>>--
>>>>Dr. David Kirkby,
>>>>G8WRB
>>>>
>>>>Please check out http://www.g8wrb.org/
>>>>of if you live in Essex http://www.southminster-branch-line.org.uk/
>>>>
>>>>
>>>>
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>>>
>>>
>>>
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>>
>>
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