|

|
|
AM-6155 / AM-6154 Conversion to 432 MHz
By:
Bertrand
Zauhar, VE2ZAZ
Page last
updated: 12/12/2008
This
page
presents my experience in converting an
AM-6155 (AM-6154) FAA
amplifier to the UHF Amateur Radio 70cm band.
My intent is not to
repeat everything that is well documented on
other websites, but
instead to supplement whatever others have
done with my own findings.
By
following the links I provide and by executing
the additional steps I
describe, the user will have a very good and
reliable linear amplifier
capable of producing 350-400W of clean RF
output on 432 MHz.
Please
report any
broken links to me. Thanks!
The
FAA used the
AM-6155 in the 1970's and 1980's with
ground-to-air transmitters.
They were designed to cover 225-400 MHz at
50 watts AM continuous
output. With only a few hours of work, and a
few extra parts, they
are capable of 400 watts RF output on 144
MHz, 222 MHz or 432 MHz
in SSB/CW at Amateur Radio
duty cycles. This amplifier used either an
Eimac 8930 or an Amperex
DX-393 tetrode tube. There are tons of these
amplifiers currently in
circulation mainly in North America.
Web links to the original ITT (the
manufacturer) AM-6155/6154 manual
and circuit schematics can be found below:
Basic
Chassis conversion
The main chassis (power supply) conversion
is described in numerous web
locations. Since there were several
iterations of the conversion
developed over the years, reading the
various websites becomes
confusing and even contradictory in
some cases. Luckily, I found a web link by
W3RJW that simplifies
everything. As a main reference, use his
chassis
conversion webpage.
But apply the
following modifications to his instructions
provided in the link:
- In step II-A-2, use 2 x 20
ohm 2W
resistors instead ox 2 x 10 ohm. This
provides a x10 meter reading
factor on the plate current instead of
x20. In other words, a reading
of 34 would indicate a plate current of
340 mA, much easier to read
than
a reading of 17.
- Do not perform steps IV-C
and IV-D.
Instead, use the re-designed grid bias
circuit described below. With
the new grid bias circuit, these steps
should not be performed.
Improved
Grid bias regulation
The
first
tests I performed with my AM-6155 on 432 MHz
revealed a major
non-linear output response. The output RF
power was not tracking the
input RF power. The unit picked up gain as
the input level increased. I
noticed a major sag of the grid bias voltage
when keying down at full
power. This voltage would go from -78V to
-50V in a matter of a couple
of seconds. This is due to inadequate grid
bias regulation. Apparently,
this is worse at 432 MHz due to decreased
tube efficiency. If you intend
to operate this amplifier in
a linear mode (AM, SSB), you MUST perform a
grid bias modification. I still
definitely recommend that you perform the
mod in all cases and for all
amateur bands.
A
glimpse at the original circuit schematics
revealed a resistive divider
and a potentiometer as the way to set the
bias. Looking around on the
web, I found K4HV's bias
circuit
proposal. That
circuit improves the regulation somewhat,
but requires numerous
resistor values and a 2W pot which is
rather expensive to acquire. So I
decided to design my own. With my improved
grid bias supply, the
voltage is now rock-solid and RF response
is linear!
The mod consists of
disconnecting the existing grid
bias circuit and connecting a new
regulator circuit built on a small
piece of prototype PCB. The new
grid
bias
regulator is based on an LM337T
negative linear voltage regulator.
The
circuit schematic is shown on the
right. This circuit allows to
adjust
the grid bias voltage to between
-69V and -95V DC. This is
appropriate
for setting the idle plate current
on both the Eimac 8930 and the
Amperex
DX-393 tetrode tubes. For the
4CX400A, a different bias voltage
range
is required. See the section below.
This grid bias circuit is quite
straightforward to put together.
All
components can be found at major
component suppliers such as
Digikey,
Arrow and Mouser. Use a small
heatsink on the LM337T regulator.
A
simple, common 1/2W trimming
potentiometer (single turn or
multi-turn)
is used.
I
have
decided not to locate the pot on
the rear panel. I find the adjustment
pretty stable for a "set once and
forget" approach. Besides, long
leads
on the potentiometer might have
promoted instability on the
voltage
regulator. Of course, this
requires that you make the idle
plate
current adjustment with
the amplifier top cover removed
and the interlock switch depressed
or
bypassed.
Be careful! Lethal voltages
everywhere
in there...
For
mounting
the new PCB, I used metal tabs
covered with plastic tubing,
those commonly used in consumer
electronics to keep the wiring
in nice
bundles. I attached the tab
eyelets to the PCB corners using
screws and
nuts. I then wrapped the tabs
around one of the A3 assembly
horizontal
standoffs as shown on the
pictures on the right. Use your
imagination
and mount
the PCB on the A3 assembly your
way!
Once the PCB is assembled,
follow these wiring steps to
install the new
grid bias circuit:
|
Click
on
the figure to
enlarge it.

Click
on
the figure to
enlarge it.

Click
on
the figure to
enlarge it.
|
1-
On the existing A3
assembly printed circuit board, cut one of
the two leads of the
following resistors and lift up their end so
that the cut off lead no
longer makes contact:
- R14, 1.5K, 2W
- R16, 22K, 2W
2-
Optionally, on the
existing A3 assembly printed
circuit board, replace VR1,
the 100V zener diode, with a
new 100V 5W
one such as a 1N5378B. Since this is an
old
amplifier and we are revising it, I figured
it would not hurt to
replace that one. Up to you.
3-
Solder the new grid bias circuit input wire to the TP4 post on
the A3
assembly printed circuit board.
4-
Solder the new
grid
bias circuit output wire to the E10 post on
the A3
assembly printed circuit board. Do not
disconnect the existing orange
wire form the E10 post.
5-
Run the T/R
Control
wire to the back panel as suggested in Step
IV-D of W3RJW's
chassis
conversion webpage.
Grounding this wire makes the
amplifier switch from an RX idle to Tx idle
mode, which allows the idle
plate current to flow.
The grid bias adjustment is made with the
amplifier powered up and
keyed in Tx mode (no RF at the input).
Adjust the trimming
potentiometer for an idle plate current of
between 60 and 90 mA. It
should normally represent ~ 20% of the
maximum plate current. I set
mine to 80mA.
That's it. Solid output and good linearity.
Chassis
Re-Wiring to 240V
AC
I
suggest re-assigning the amplifier so that
it runs on an AC mains input
to 240V. Considering that we run the
amplifier beyond its specified
limits, this will give some relief to the
transformer primary winding.
The switch to 240V is done by re-positioning
jumpers on the two TB1
terminal blocks. There is one terminal block
on top of the A3
Assembly (hidden under a removable "Caution
High Voltage" plate) and
one block next to the power transformer
under the long high voltage
hinged cover. Jumper assignment for 240V is
the same for both blocks:
- Jumper linking positions
5-6
- Jumper linking positions 7-8
For
the RF
plugin conversion, make sure you use the
circuit schematic that
corresponds to the right type of plugin you
own. There exists a VHF
(AM-6154) plugin and a UHF (AM-6155) plugin
out there. Select the
proper schematic page, otherwise it will be
confusing when doing the
grid compartment component removal process.
RF Input
Compartment Conversion
For Input
(Grid)
circuit conversion, I basically followed W3RJW's
modification
page,
steps 1 to
10, with the following adaptations:
- In step 6, I
failed in trying
to
remove the input pedestal
inductor shaft from the input
cavity. The set
screws were frozen hard.
Instead, I unscrewed the
pedestal from inside
the grid compartment. I then cut
off the threaded shaft as far as
I
could using a Dremmel router
tool with a cutting disk. I then
covered the input cavity entry
inside
the grid side compartment with a
piece of adhesive aluminum tape,
the
type used to seal ventilation
ducting.
- In step 7, I
used a
Dremmel tool to cut off half of
the
Input tuning capacitor. This
gave a neat result with no bent
or damaged
plates. Use a bench vise to keep
the capacitor firmly in place
while
cutting.
|
Click
on
the
figure to
enlarge it
|
- I used 240pF silver-mica
capacitors
instead of the suggested 200pf values.
This is what I had on hand and
it worked fine.
- I adjusted the position of
RF input
injection on the brass grid line to get a
good input VSWR. The sweet
spot where to connect the input capacitor
ended up much closer to the
end of the grid line than the 5/8"
suggested. I now get a VSWR of less
than 1.5:1.
RF Output
Compartment Conversion
- As mentioned by
K4HV in
his Loading
Capacitor
Modifications section, do not forget
to take
the black plastic washer out
from the plate coupling
gearbox.
This allows
to increase the travel of the
plate disk away from the plate
ring.
Otherwise
plate coupling will be excessive
and you will not get the full
output
power. See the figure to the
right.
|
Click
on
the
figure to
enlarge it
|
Output
Modules Removal
Remember
to
disconnect and remove the low pass filter
module and the directional
coupler
module
from the output coaxial line. Replace them
with N-Female/N-Female
adaptors.
4CX400A
TETRODE
SUBSTITUTION
(OPTIONAL)
|
I have
also
implemented and tested an improved grid bias
circuit to support the
Svetlana
4CX400A (GS-36B) tube requirements. Since the
original Eimac and
Amperex tubes are becoming scarce, this tube
is a good substitute. I
purchased a pair of New Old Stock (NOS) GS-36B
(4CX400A) on eBay for
around $100 per tube, delivered. My initial
thoughts were that I would
get additional output power over the Eimac
8930 just by comparing
the specs.
Before we go any further, let me warn you that
you MUST CONDITION any
new transmitter tube before applying
high plate voltage to it,
otherwise the tube will arc internally and
this will damage the tube
and your amplifier. I learned this the hard
way! The conditioning
procedure consists of running the tube with
the amplifier powered on (filament on,
fan running, no
plate voltage, no RF at the input) for a few
days
(I suggest 4 days). This allows the getter to
capture any remaining gas
molecules inside the tube. Feel free to search
the web for more
information on power tube conditioning and the
getter electrode.
Anode
Ring Modification
The anode heat dissipator on
the 4CX400A is
slightly smaller than on the
original tubes. To accomodate this
smaller
diameter, you must
bend the amplifier anode ring fingerstock towards the
center by about 0.5mm, one
finger at a time. In order to
accomplish this right, I suggest
you take
the anode ring out of the
cavity. Only two screws need to
be removed to
do
this.
Improved
Grid
bias
regulation
For
the
4CX400A, the grid bias voltage is
higher (less negative) than for
the original tubes. The circuit
shown to the right does the trick
for
the '400A tube. Follow the procedure described above
to
modify the AM-6155
chassis to the improved grid bias
circuit, but instead, build the improved grid bias
circuit shown
to
the right.
|
Click
on
the figure to
enlarge it.
|
Chimney
and Cover Re-Assembly
One thing I
noticed while re-installing the chimney and
cavity cover is that the
chimney protrudes outside of the cavity by a
millimeter or so. Once
installed, the 4CX400A tube stands probably
slightly taller than the
original tube. This causes the cover to not
quite sit on the cavity
flange. When tightening up the four cover
screws, go gradually on the
tightening, working diagonally and trying to
maintain equal tension on
the four screws. Do not overtighten,
as this force gets
transferred to
the tube itself. Once complete, there will
still be a small gap between
the cavity and its cover, but certainly not
enough to disturb cooling.
If
RF leakage is a concern to you, you may want
to use an RF gasket to
fill the crack, or simply use aluninum tape
all around to cover the
gap. I did not care since the RF plug-in
sits inside the chassis.
Results
Well, the bottom line is to not
expect a big
improvement over
the original tubes with the 4CX400A. From
what I could see, you can get
maybe 25-50W more, so 350-400W but the plate
current is significantly
higher too. This shows an even less
efficient configuration compared to
the original tubes, which is not high to
start with. The power supply
is the main limitation in trying to push
more output. Cooling is also
limited with the stock blower.
While testing, I pushed the input power a
bit too far (more than 15W),
and blew up the three panel-mounted zener
diodes ($10 each) that set
the screen bias and a few other resistors
blew up as well. Simply put,
the AM-6155
chassis is not designed for such stress. I
suggest you settle for
300-350W output in CW.
OPERATION AND
PERFORMANCE
|
With
the
above conversions performed, I get the
following performance on 432MHz:
Original
Eimac or Amperex
Tube
CW/SSB: 15W
input yields 325W output @ 300mA of DC plate
current. I can push to 375W output
for short
bursts.
JT-65: 250W output @
250mA of DC plate current is the maximum I can
run in 50-second
continuous
transmissions without the overheat protection
tripping.
Svetlana
4CX400A (GS-36B) Tube
CW/SSB:
15W
input yields 350W output @ 350mA of DC plate
current. I can push to 400W output
for short
bursts.
JT-65:
Like
with the original tubes, reduced output power
by at least 100W in
order no to trip the
overheat
protection.
AC Mains
Current
In the 3A to 4A range @ 240VAC.
|