2 x
3-500Z /
80-10 meters
(
NOTE: Please do not ask for schematics or a
how-to for
this amp. This article is meant to give you ideas for your own custom
design.
Every bit of documentation in existence is presented in this article. I
build them
as I design them and keep no notes!)
This was another "junque box" project and had several objectives,
some rather unique. I started thinking about this design when it became
obvious
that I needed to re-build my old trusty 2 x 4-400A amp. That amp deck
had never
been upgraded to cover the WARC bands and was a drive hog. With a 100
watt
exciter, it wasn't possible to achieve a full 1500 watts output (at the
available anode voltage).
So the first objective was to create a design that would produce a
minimum of
1500 watts output on all bands 80-10 with less than 100 watts of drive
power (and use the existing power supply).
The second objective was to wind up with an amp deck that integrated
into my
existing facilities with a minimum of effort and to be easy to use in
my
HF operating environment. This meant that the tubes selected had to be
"instant on" and precluded the use of the Russian triodes or 8877, etc.
Third was a self-imposed mandate to NOT buy anything to build this amp.
I have
been gathering and saving components for 50 years, time to use them up!
If some
of the parts in the photos look old and crusty, it’s because they are!
Included in the final design are parts from TV sets, salvaged military
and commercial radios, computers, and various other electronic devices.
Some date back at least 50 years!
Fourth was to make the thing "look better" than most of my projects.
I always start with good intentions but after all the "mods" and
"improvements", my projects usually look like the junk they were made
from.
The fifth (but not last) objective was to try out some circuit
"tricks" and "do-dads" that have been on my list of
improvements needed on some of my other amps.
The first step in any amp project is the selection of the major
components.
Using the above objectives as a guide, I rummaged around in the old
"junque box" looking for the right parts. I had a NIB pair of Amperex
3-500Z's so that part was easy. I also had a pair of Eimac air system
sockets
that I had picked up over thirty years ago so that was a match. Digging
deeper produced
a chassis, some coils and caps, a few switches and a bunch of smaller
parts. A long time was
spent
placing the possible parts on the chassis and moving them around for
the best
compromise layout. Several times it was necessary to toss a component
back in
the "junque box" and look for a better fitting part.
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Here’s
a photo of the top of the partially
assembled
chassis.
In
the above photo, you can see the tank
components, tube sockets, filament transformer, and fans. The black
object just
in front of the filament transformer is an air duct which directs the
air flow
from one of the two fans down into the chassis bottom. I was worried
about
cooling the tubes during RTTY operation and this was the solution I
came up
with. It provides ample air flow while meeting all the necessary
mechanical
requirements.
*************************************
In
this next photo, we are looking at the
back of the partially assembled chassis:
In
the rack where this amp will reside, there
is no clearance on the top of the cabinet. Air must be taken in from
one side
(or the back) and exhausted out the other side. The chassis base I had
“in
stock” was only 3 inches high so that prevented a fan from being
mounted below.
The duct was needed to allow the fan which cools the tube pins to be
placed above
the chassis where there was room to mount it.
On the
right you can
see the screened exhaust port. In operation the ducted fan pressurizes
the
bottom and forces air past the tube pins. The other fan moves a lot of
air
across the tube anode seals. Both air streams exhaust out the right
side. The
fans are speed controlled, more about that later.
The
sides, top and
bottom of the cabinet are made from salvaged aluminum sheet cut to
size. The
chassis base is a commercial Bud product that I had “in stock” as well
as the
19 inch rack panel. The pieces are screwed to ½ inch aluminum
angle stock.
*************************************
Here’s a
shot from the
side:
In this shot, you can see the feed-through caps which the cathode
network relay control lines pass through. The small coax is
RG-142/U and connects the PI network to the output connector located
below the chassis. Both this coax and the HV lead (seen between the
tube sockets) caused me some grief. I wanted to drop them straight down
but there was no easy way to do that due to components below the
chassis. They had to be routed across the top to a place where I
could pass through without mechanical interference.
*************************************
This
view shows the top of the chassis with the tubes installed. Except for
a few minor components, it is complete:
The silver plated inductor on left is the
10 meter tank coil. It actually has one turn too many for optimum "Q"
but since it worked out OK, I just left it as was. The edge wound
inductor used for the remaining bands actually has one or two turns too
few for optimum 80 meter tank "Q", but it too worked out OK so I left
it alone as well. The edge wound inductor actually has a lower measured
Q than a same size same inductance coil wound from heavy wire or tube.
It was used it because it was on hand and it was very easy to install
the band taps with the matching clips.
*************************************
Here's a close up of the tubes. The parasitic suppressors
are copied from the Eimac 3-500Z data sheet and had to be replaced
after the first time 10 meter operation was attempted. The inductance
was way too high. I wound new inductors with approximately half the
inductance and replaced the metal film resistors with Ohmite OY's. The
small inductor you see just above the left hand tube is used for the
"L" network for 10 meters. This compensates for the high output
capacitance of the tubes and layout. Without the "L" network
compensation, the tank "Q" on 10 meters is too high.
*************************************
Moving on to the bottom of the chassis:
This photo was taken before the
cathode matching network board was installed. Top left is the
adjustable power resistor used for the filament inrush current
limiting. Below that is a 6.3 volt filament transformer connected as a
line boost. The filament voltage at the 3-500Z pins was right at the
low end of the spec, so I put in the boost transformer to raise it to
the middle of the spec, allowing for minor live voltage variations.
Below the boost transformer, you can see the hole and air duct for the
top mounted fan.
If you look closely at the tube sockets, you can see that the grid pins
do not run directly to the chassis. They are bypassed to RF ground with
three different sized capacitors on each pin. There is a 250 pF
Unelco mica UHF capacitor connected between the pin and a copper strap
bolted to the chassis. These are the same capacitors that are used for
solid state UHF amplifiers and have extremely low stray inductance and
a series resonance in the microwave region. In parallel with each of
the
Unelco caps is a 1500 pF disc ceramic cap and a 0.01 uF disc ceramic
cap. This configuration eliminates all the usual problems associated
with bypassed grids and is RF-wise equivalent to a direct ground
connection. Having the grids DC isolated from ground makes bias and
metering much easier.
At the lower edge of the image, there is a 28 volt transformer which
provides power for the fans, the control board, and the cathode network
relays. This transformer has AC power applied at the same time as the
filament transformer.
Unlike all the rest of my amp projects, this control board has multiple
functions. Usually I make separate modules for the filament inrush
limiter, bias, etc. This time I thought I had my act together enough to
put them all on one board. This proved to be a mistake and in the
future I will put each function in it's own module.
Filament inrush current limiting is accomplished with a power resistor
in series with the line side of the filament transformer. A set of
relay contacts shorts out the resistor after a preset time delay. The
value of the resistor was adjusted to have the initial inrush current
be the same as the secondary inrush current. Next time I will use two
stages to get better control of the inrush current.
The relay coil is supplied from the 28 volt supply through a series
resistor and a parallel capacitor. The relay is a "TV power" type and
has a 9 VDC coil. The capacitor value was selected to allow enough time
delay for the filaments to reach equilibrium before the realy energizes
shorting out the current
limiting resistor.
The 28 VDC supply is a positive ground supply. I did that to make the
grid bias circuit easier. With the grids above DC ground it made sense
to have a grid bias system instead of the usual cathode bias
arrangement. The bias regulator consists of a common adjustable three
terminal regulator and a darlington pass transistor configured as a
shunt regulator. It has sufficient voltage adjustment to allow the
Eimac or the Amperex tubes to be used. It will handle several amps of
grid current. The grids are routed to the bias supply through a
1.0 ohm resistor. Leads from that resistor are brought out to jacks on
the rear panel allowing grid voltage and current to be measured. I
do not normally
monitor
grid current during operation.
The two fans are 28 VDC muffin fans. Each one will provide sufficient
air through the two air system sockets to meet Eimac's cooling
requirement for 500 watts dissapation for each tube. With two of them
running the noise level is pretty high. It turns out that with only the
filaments running on the tubes a lot less air is required for cooling
so I decide to slow the fans down when max air flow was not required.
The DC type fans allow the speed to be easily reduced by simply
reducing the applied voltage. A series resistor is placed in the 28 VDC
line and switched out with an opto-coupled relay. The tube cathodes are
returned to ground through three 3 amp diodes. Not only do these povide
some measure of safety bias in the event of a bias supply catastrophy,
the small voltage across them triggers the opto-coupled relay. The fans
go to full speed anytime current flows through the tubes and drops back
to slow speed when the tubes are not conducting.
*************************************
The next shot shows the cathode matching network board in place:
A
printed circuit board was contemplated for this module
and
rejected due to the fact that it would be a one of a kind board. I
could not
justify the time and expense required for that. A piece of double clad
copper
board was cut to size and the relays glued to it dead bug style. Not
only is dead bug cheaper, it allows shorter leads between the coils and
caps and has lots of ground plane to connect to. The relays are SPST 9
volt coil jobs that I had procured some time ago for another project
and didn't use. Each pair of band related relay coils are connected in
series and routed to the proper band switch position. The -28 VDC
source supplies power through an appropriate dropping resistor for 18
volts across the series coils.
The PI matching networks are designed for a Q of 2 and are constructed
with small torroid cores and small silver mica capacitors. Several
small value caps are put in parallel to give the correct capacitance.
Load resistors were temporarily connected from the tube cathodes to
ground and the individual matching networks are adjusted for best
return loss (SWR) by squeezing/spreading the turns on the torroids
and/or altering the
value of the capacitor on the tube side ot the network. Because the
correct torroid material and silver mica caps are used, the network
efficiency is very high. If a builder tries to shortcut by using slug
tuned coils and/or ceramic caps, the input efficiency will suffer and
more drive will be required. An input SWR of 1.1:1 or less is obtained
across all 8 bands except for the top end of 80/75. That band is just
too wide!
*************************************
The next photo shows the completed amp deck installed in the rack:
*************************************
And the full rack with
some of my other HF amp decks. Top one is the 3x4-400A deck for 40
meters. Next
is the 4x813 deck used on 160. Below the meter panel is the 2 x 3-500Z
deck and bottom is the YC-156 deck. At the very bottom is a HV supply
(for the 6 meter GS-35B amp not shown here).
