all my amplifier projects,
started out as a "what if?" For several years I had been using an amp
with two 4-400A's in parallel, triode connected, in a grounded grid
mono band 40 meter
chassis. The chassis was an experiment that was never quite finished.
It used cross flow cooling similar to the Heathkit method and proved to
be very reliable. Although I could get 1500 watts out with the pair of
4-400A's, it took a lot of drive at the anode voltage I was running. At
this level of output, I was seeing some output compression. After with
this for a while, the "what if" process started up and the idea of
three tubes in parallel came to be. I had plenty of broadcast "pulls"
and three spare sockets on hand. The biggest obstacles were filament
supply and cooling. With a total of 45 amps current required to light
all those tubes, both the transformer and the filament choke were cause
for concern. I found a transformer capable of that current in my "Junqe
Box" but when connected to three tubes through a typical filament
choke, the voltage across the tube pins was less than the minimum
required by the tube manufacturer. Eventually, a choke was wound with
10 gauge wire wrapped with teflon tape (for heat resistance) that was
sufficiently low resistance to minimize the voltage drop.
Cooling was assured by having two muffin fans blowing across the tubes
and tube sockets. The sockets are mounted on a sub chassis which is
mounted to the main chassis with threaded rod standoffs. Here's what
the underside of the sub chassis looks like:
Notice the coax for the cathode RF drive and the ugly filament choke!
Control and screen grids are connected directly to the chassis with
copper strap. Many years ago I gave up on the idea of monitoring grid
current in this type of grounded grid amplifier. I always ran what ever
drive I needed to get the output power I wanted regardless of what the
grids were doing, so why bother? You can't hurt the tubes in normal
operation and as long as you stay out of compression, you can't
All of my HF amps are installed in a rack which provides selected RF
and HV lines and also provides what metering is required. This makes it
easier to build each individual amp. The HF amps are all built behind
rack panels. The entire chassis for this amp was fabricated from
salvaged aluminum sheet. All of the components were also acquired from
swap meets or salvaged equipment. Some of the parts date back to old TV
sets I salvaged out in the early 60's! A few parts are from old WW II
military units acquired and salvaged out through the years.
Monobanders are really simple to build. Here's a photo looking from top
In the upper right is a small filament transformer which provides the
DC center tap for the cathode return. The 0.25KVA filament transformer
is just below it. The diode string provides a small amount of cathode
bias. The two muffin fans pull air in from the right side of the
chassis and exhaust it out the left side. I wanted to use two larger
size fans but I just couldn't squeeze them in the space I had
available. In any case, they seem to be able to cool things adequately.
Top center and top left are the resistors and relay for the filament
in-rush current limiter circuit. Left center at the coax connector is
the pi-network input matching circuit. The anode RF choke is salvaged
magnet wire wound on a piece of teflon rod. Notice the parasitic
suppressors. I had hopped that I could build this thing in a way which
would not require them. The last three amps I built didn't need them
and I thought I was on a roll. Oh well! It turned out this thing made a
great 190 MHz oscillator. No science was involved in the suppressor
design. Since no 10 meter operation was contemplated, I didn't have to
worry about too much inductance so I just made them "big enough"!
Looking at it from top right:
Air variable caps are used for both output tune and load functions. No
need for an expensive vacuum variable in this application. A couple 100
pF doorknob caps are used to increase the load capacity. No
screens are on the fans in this photo. I didn't have the right kind of
hardware cloth handy and didn't want to hold up the project. If an when
I ever get some, I might put it on. Probably never happen!
Looking from top rear:
Along the rear apron left to right RF input, AC mains input, HV
input, and RF output. Most of my recent amp projects have an AC
connector salvaged from old computer power supplies. I used to always
step on the dangling line cord, now the line cord stays in the rack.
The tank coil is wound from #10 bare copper wire. The first coil
I made was from 1/4 inch copper tube. It was a thing of beauty but
didn't fit well in the space allocated. The 10 gauge is good enough for
40 meters and is somewhat easier to work with. The rear chassis cover
is made in two parts. The bottom piece stays put and has all the
connectors on it. The top piece is easily removed for modifications.
Notice I said "modifications" instead of maintenance? I seldom do
maintenance, because by the time the thing breaks, I'm tired of it and
want to modify it into something else! The input network and the output
coil can be changed in a matter of minutes if need be and I can have
this on 80 or 20, whatever, quite easily.
Here the amp chassis is placed in the rack and powered up for some
testing prior to final assembly. This is at full power output running a
string of fast dots. The bright orange spot on the left most tube is an
artifact of the photograph, they all run the same color in reality.
As you can see here, the left side of the chassis is covered with a
sheet of perforated aluminum. All other surfaces are sealed so that the
air flow is from right to left across the tubes and other components
that require cooling. I was worried about hot spots, but it seems to
In the rack:
The finished 40 meter monobander is placed in the rack right above the
4 x 813 amp deck. Some day I'll have to finish the detail work on that
one! But that's another story!
Like all my amplifiers and most everything else I build, I have no
documentation! Don't ask, you won't get a schematic because I don't
have one. All the RF stuff is right out of the text books and uses
commonly available computer software to do the calculations. I just
wire it up as I go and try to make it look like an amplifier. I do this
for fun, not to prove I'm a world class RF engineer.