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