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Author Topic: Amp Plans to Build?  (Read 3125 times)

Posts: 382

« on: October 24, 2004, 07:25:13 AM »

I am thinking about getting another HF amplifier. I would really rather build than buy one. Is this really practical? I had one amplifier and got rid of it. I was not all that impressed with its construction quality anyway. Without discussing the brands, I was not impressed with the soldering and wiring, for example.
I built a lot of Heath Products when I was young, long before I got my first Novice ticket. I have also worked in electronics for a living and my wiring and soldering skills are very strong.
Short of designing an amplifier from the start, or borrowing from a proven design, does anyone have any plans out there for building an 10-80 meter amp? I don’t really want a kit. I would rather build it from the start.
Short of that, I guess I might pick up an old SB-220, for example, and go through it and make the improvements recommended and fix any construction problems I found in the process.
I guess I just have an urge to get back on the bench building something. Any ideas??


Posts: 3380

« Reply #1 on: October 24, 2004, 04:51:16 PM »

Ken -

First, there are a large number of amplifiers on eBay and at hamfests that have fallen into disrepair or abused (amatuer estates, ignorant operators, illegal CB or "freeband" usage, etc.).  This creates a ready supply of candidates for repair, restoration or raw parts (cabinet) -- I find that this can be quite rewarding -- and many of these are purchased for less than parts value.

Second, you need to decide whether you deisre a tube (glass or ceramic) or solid-state (bipolar transistor or FET) amplifier.  The differneces can be important to your fabrication skills and operation / usage.

Heat is handled in 2 completely different approaches.
Power supply (Voltage and current) designs are quite different in requriements and construction.

For new construction, and if you desire a tube design -- I would focus on ceramic based tubes.

The G3SEK control boards are outstanding for tube amplifiers
There are a number of inexpensive surplus Russian ceramic tubes on the market.  Lawson Summerrow W4EMF has been performing a number of converisons:

For slid-state deisgns - I would focus of Helge Granberg's work (Motorola Application Notes).  CCI kits that follow the Motorola Notes


Posts: 21837

« Reply #2 on: October 24, 2004, 05:00:45 PM »

The ARRL Handbooks of the 1960s and 1970s have dozens of time-proven kilowatt HF amplifier construction articles using a broad variety of tubes, most of which are still readily available.

Unless you have a very deep junkbox, this can be an expensive endeavor, probably more expensive than simply buying an amplifier.  However, if you're itching to build something, it sure is fun building amplifiers.  Four of my five kW amplifiers here at home are homebrew, and I have one commercial unit.

I use one "universal" power supply to power all four of the homebrew amps, since I used a center-tapped plate transformer capable of a great deal of secondary current (it's a Dahl hypersil wound 3000V, 1.5A CCS transformer that weighs about 75 lbs by itself), so I can produce either 2.2kV at an amp, or 4.4kV at an amp, with the flick of a switch (and a relay).   That allows me to use lower-voltage tubes like 4CX250s, 8122s, 8874s, 572Bs or whatever at 2.2kV, or higher voltage tubes like 4-1000s at 4.4kV.  With a primary 240V 20A variac, built into the power supply panel, I can crank the 4.4kV down to 3-3.5kV for 3-500Zs, 8877s and stuff.  It's pretty universal, but the darned power supply is 155 lbs and I'm getting a bit old to be lifting it much without a helper.

And that's the challenge!  The "RF decks" are pretty light and easy going, by comparison.

The SB-220's not a bad building block, though.  I don't have a 220 anymore (had a couple of them over the years), but the last one I had, I added a B+ output port to its rear panel so I could use its 3kV 500mA voltage-doubled power supply (which isn't bad) to power separate RF decks that didn't have any power supplies.  I'd bring out both B- and chassis ground separately to allow for external plate current metering, and used RG58 coax for this (B- on the center conductor, chassis ground on the shield), that worked well.  Worst case, if the coax shorted completely, everything still worked fine, I'd just lose metering.

73 & happy building!

Steve, WB2WIK/6


Posts: 382

« Reply #3 on: October 24, 2004, 05:43:44 PM »

TNX for the suggestions and the websites.
I had thought of buying a junker, if I found one, for parts. It's almost like home brewing from the get, and most likely cheaper.
I do want tubes, which I forgot to mention. I am just more comfortable with that warm glow. I like the dual 3-500Z platforms.
Maybe I will come by one in the future at a price I can afford to fix.

Posts: 9748


« Reply #4 on: October 26, 2004, 06:13:21 AM »

Most of the mods for Heathkits are not necessary, except for floating the control grids and using those silly RF chokes and 220pF capacitors that Bill Orr pounded into everyone to use, it was a really good KW input amplifier. Many of the mods center around nothing except some 6's idea of how every component problem in the world is a design error, and any arc is a parasitic...none of which are really the actual problem.  

The problem you'll find is since Eimac quit building glass tubes the other brands of 3-500Zs have very poor QC. The tubes outgass from internal elements or have seal leakage, and that causes an arc. They also have a tendency to develop grid to cathode faults due to poor geometry in the filament helice.

To compare tubes see:

You might consider getting something less expensive and more reliable than a glass a Russian ceramic triode or some 3CX800 pull-outs from MRI machines.

I agree the ARRL Handbook is a good source of information that is generally technically reliable.

Posts: 1

« Reply #5 on: November 02, 2004, 03:20:24 PM »

If you want to build an amp why does it have to have tubes?  I have built 2 1.2kw out solid state amps for HF and 1 600w out vhf solid state amp using the 'kits' sold by Communications Concepts.  I built many tube amps and I much prefer the solid state amps now.  They were also  much easier to build.
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