[Amps] Why people don't build amps.

[Roger]

Cecil Acuff wrote:
>
> ----- Original Message ----- From: "Roger" <sub1 at rogerhalstead.com>
> Cc: <amps at contesting.com>
> Sent: Thursday, March 25, 2010 2:56 PM
> Subject: [Amps] Why people don't build amps.
>
>
>> What are the sources for inexpensive transformers that will
>> do the job?  Not microwave ovens, those things get hot and are not
>> designed for continuous operation. OTOH if one is willing to put up with
>> their short comings they can be paralleled to provide power for a medium
>> size amp.
> ----------------
>
> Interesting post...I have some of the same questions.  Options for 
> inexpensive, reasonably sized plate transformers?
Carl came up with some such as pole pigs. Another would be old 
commercial repeaters that had amps built in. I'd guess some of those 
transformers would be good for about a KW PEP out. Of course the pole 
pigs are big and heavy, but there's no rule the transformer and PS has 
to be in the same cabinet.
>
> -----------------
>> But lets say you have access to one of those ultra linear 3CX or
>> 3CPX5000 pulls with fantastic IM in the minus 50 db range,  that can
>> comfortably run the legal limit using all of the drive from your 100
>> watt exciter.
> ----------------------------
>
> Can someone provide sourcing info on reliable, reasonably priced pulls 
> for home builders to build with?
> Would love to buy a couple Triodes that don't require sockets.  Would 
> even consider tetrodes.
>
This varies and although they show up on e-bay that's not a place I like 
to purchase tubes unless it's from some one with a good recommendation. 
At one time Jeff at Harbach (sp?) had some pulls. I think they were 
8877's but those dried up until he gets another source.  Another place 
would be http://www.wa8wzg.net/forsale.htm which has been posted before. 
But my knowledge of those sources is rather limited.

I may pick up a pair of 8877's or yc-156's and go from there. Or maybe a 
pair of 3CPX-800's for a 6-meter conversion.
> -----------------------------
>
>> Winding the tank coils is easy, but may require sacrificing some copper
>> tubing while learning, or you can find some one who is good at winding
>> coils.  Given the plate impedance, you should be able to find a design
>> to copy which makes placing the taps on the coils much easier.  Also
>> good hefty toroids for the 160 and 75 meter bands is now the way to go.
>
> ---------------------------
>
> I would be more interested in learning how to properly wind, measure 
> and tap using my GDO than shotgunning it.   I have used the copy cat 
> method and it works fairly well but I desire to understand.
I'd have to check, but I think the ARRL Handbook as well as the "Classic 
Amplifier Handbook" covers this.
> Are there books that describe the process for those of us who are 
> technical but not engineers?
Yup. they stay away from the Calculus and although not real simple, just 
take some study. It depends on how fancy you get with the band switching 
and segmenting the tank coil.80 - 10 is not bad using to sizes of tubing 
for a solenoid wound coil and a strap coil for 10.  It's common now days 
for toroids to be used for 160 and 75 with band switches from Multi-tech 
http://www.multi-tech-industries.com/rswitches.html
>
> What about no band switch and no tapped tank....just a large edge 
> wound roller inductor?  Maybe a good
Depends on how much power you want to run.  For me the original set up 
seems to take forever with a rotary tank coil.  Often they put a toroid 
at the one end for 160. You have to be careful as this arrangement will 
allow for a very wide range of frequencies and at least one of them is 
almost sure to hit one of the resonances for the plate choke.
> bit more expensive but time and loss of materials to trial and error 
> are expensive too.
When you take into account the cost of a custom band switch and the time 
to get the taps correct the rotary inductor with a turns counter isn't 
all that bad.  I just don't like all that cranking for the same reason I 
don't like to use vacuum variable caps.  If you look up Palstar they 
sell both the turns counters and the rotary inductor they use in   their 
AT-5K antenna tuner. That inductor is wound on a ceramic core.

As a side note, I've been using an AT-5K along with the DL5K dummy load 
AND the MFJ antenna analyzer to tune up without putting a signal on the 
air for the past few months. Some may find it lacking, but it's a whale 
of a lot more rugged and smoother tuning with better repeatability than 
my two MFJ989C's
>  
> ----------------------------
>
>> Now lets go back to that legal limit issue. I'm not condoning illegal
>> actions, but what is really more ethical, running a 2 or 3 KW amp with a
>> very clean signal,
>
> ----------------------------
>
> I'd much prefer to build and operate an amp capable of 2500W ICS at 
> the legal limit and not worry about splatter and stressing expensive 
> components. (or duty cycle)
Carl has me pretty well convinced that I want to try building up a 
3CX5000 due to the extremely low IM products (in the mid -50 db range) 
where the more common tubes are happy to be in the mid to upper - 30 db 
range
>
> Post raises several good questions for those of us that want to 
> homebuild but seem to have more questions about doing it right than 
> answers.
>
There are always more questions and the more I learn the more I learn I 
don't know.
One of my *old* computers (and I do mean old - it was my first personal 
computer)  has a big sticker on it that says: "Don't be afraid to ask 
dumb questions. They are far easier to answer than fixing dumb mistakes."

Tis a bit off topic, but that computer which is an Ohio Scientific C2-8P 
from about 1980 ran a single 1 MHz 6502 with 48K of dynamic RAM. It had 
a pair of dual 360K or  380K  8"  floppy disks (single sided) and cost 
$4,000 WITHOUT a monitor or keyboard. one 16K memory bank cost on the 
order of $400 (give or take a tad). My latest computer which uses a quad 
core Athlon runing 3.4Gig core speeds. That's 3,400 times faster on each 
of 4 cores than my old OSI was on one. It also has 4 Gig of RAM which is 
250,000  (quarter million) times more memory. The entire computer 
including a state of the art video card cost less than $1000 USD. 
Complete with wide screen HD, 3 ms  monitor was about $1200. Already I 
could build it with monitor for less than $1000.
> I have been inspired by the posts of the last several weeks...
Now that I'm really glad to hear.
> to the point where I started looking for suitable cabinetry for 
> building.  Purchased two 19" rack mounted chassis that had contained 
> 900Mhz radio transceiver systems from work.  Should make excellent 
> cabinets for home amp projects.  All aluminum, probably 18" tall and 
> 24" deep.  Will require some work to add and remove some internal 
> compartments but good starts for $25.00.
Remember too, that with an amp look carefully at how the commercial amps 
are constructed. Most are of a modular construction with a single bottom 
plate instead of building on the old 14" X 18" X 3" chassis like we used 
to do. The tubes mount on a sub chassis which mounts on the bottom 
plate. (vertically or horizontally depending on the tubes) The power 
supply and RF deck are separated by a partition that has one edge 
screwed to the bottom plate, one on the front panel, one on the back 
panel, and one on the top. They also use a two front panels. the first 
has virtually all the stuff going through the front panel mounted to it 
while the real front panel has the engraving and names on it and is 
typically fastened to the other front panel with 6 flat head Phillips 
screws.

Don't expect your first, or even second try to look like a commercial 
amp. Be happy if it works on at least one or two bands<:-)) If this is a 
new design for you and it will be, you may end up sacrificing at least a 
plate choke or two.

You might even want to build some metal working tools such as a metal 
brake.  This is where it's nice to have a friend who has a machine shop. 
<:-))  I get real satisfaction out of building things "that work"<:-))  
Now if I just had the airplane ready to fly<sigh>

73

Roger (K8RI)
>
> Cecil
> K5DL
>
>
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