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/
>>>
>>>
>>>
>>>_______________________________________________
>>>Amps mailing list
>>>Amps@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>>
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