Legal Limit The Easy Way
This simple to build, inexpensive amp is capable of running 1500 watts out without stopping to take a breath. And, although designed around a
tube that may be purchased quite cheaply on the used market, it's flexible enough to
accommodate a number of other power triodes and tetrodes as well. I have successfully built two versions of this
amp, and presently use it on a regular basis. What makes it work is the simplicity of design. Placing the grids directly to ground and keeping the input circuit as simple as possible is key to its
success. Additionally, the design does not employ an ALC circuit, which, in my opinion, is most often a source of needless grief. A superior resource that I use on a regular basis - my ham bible, or sorts - is listed at *Reference Materials. It provides, in simple, understandable terms, every detail any ham may need to both design and build a vast variety of equipment.
The circuit outlined here was originally designed around a pair of tetrodes that were utilized by Collins in their "big gun",
throughout the 50s'. Initial
tests proved astounding, near 1500 watts out, key down, with less than 60 watts of drive. Tuning, however, was critical and any mismatch required a dramatic reduction in power. To alleviate this
problem, heavier tubes were selected. When these were combined with small design modifications, not only did the amplifier perform as well as the original configuration, but became far easier to tune
and readily tolerated a moderate mismatch.
The power supply for this amplifier must be capable of delivering 2800 volts DC, at 2 amps, key down. Due to the potential size and heft of the
transformer and associated filter components, it is recommended that no attempt be made to incorporate it with the amplifier in a single chassis. Instead, the power supply should be built in a
separate chassis, and placed on four quality casters. The amplifier chassis may then either be placed on top of the power supply, or in the operating console, with the supply stowed underneath.
Click on for Full Page Schematic
The amplifier circuit, is quite simple. Tricky bells and whistles have been omitted in favor of a basic circuit capable of many hours of
maintenance free operation. The tetrodes in the circuit are in parallel, requiring approximately 85 watts to drive. There is a degree of mismatch in the input circuit on all bands. This does not
present a problem if the rig used to drive the amp uses tube finals in combination with a pi or pi-l network. If the finals are solid state, on the other hand, a matching network will be needed
between the driver and amplifier input in order to drive the tubes to full power.
The filaments are wired in parallel and use a common bi-filar choke to isolate the filament transformer secondary from RF.
In addition, generous use of bypass capacitors aid in avoiding feedback and self oscillation in the input circuit.
Switch S1 controls AC to the filament transformer and prevents applying B+ to the tubes before the filament has been lit.
S2, when open, disables the driving transceiver from switching K2 in and out. Relay, K2, externally switched by a spare set of contacts in the
driving transceiver, controls antenna change over and the plate current limiting resistor. When K2 is open, the antenna bypasses the amplifier, and places the plate current limiting resistor between
the center tap of T1 and the B- supply. This limits the idling plate current to 75-80 ma. When K2 is closed, the transceiver is connected to the amplifier input, the antenna is connected to the
amplifier output, and the plate current limiting resistor is shorted.
The meter circuit at M1, permits selective
monitoring of either high voltage, or plate current. Meter, M2, monitors grid current and
relative RF out. Grid current is monitored by metering a shunt between B- and ground. The
relative power indication is obtained through the use
of a sampling circuit on the antenna side of L2.
The pi-network functions extremely well in matching a limited load variation. However, harmonic attenuation suffers somewhat above 40 meters. To
avoid problems, care must be taken to ensure that the amplifier is not over-driven. Over driving the amplifier will increase the IMD products and spurious emissions. Of course, an ALC circuit would
be helpful. However, careful adjustment will accomplish the same goal without many of the problems that accompany ALC
circuitry.
Constructing the amplifier is relatively straightforward. Consideration must be given to adequate ventilation. The tubes require forced air
cooling. It is necessary that air be forced up through the tube sockets and around the base of the envelopes. In addition to running air through the bases, it is necessary to run air across and
around the glass envelopes as well. The use of a moderately forceful base blower and tube chimneys represents one workable approach. Another, would be to use a muffin fan to blow the bases, and
another to move air past the envelopes, such as that employed in the illustration above.
Considering the height of the tubes and the necessity to mount them upright, one the most efficient layouts involves a tiered frame, similar to
that above. The frame is constructed of angled, and 1X2 inch, aluminum tube-stock. A piece of rigid aluminum sheet, combined with two sections of tube stock, one on each side, forms the base plate.
Angle stock is cut, drilled and bolted to form a box frame, which is firmly attached to the base. Sheet aluminum is then used to form the sides, top, front and back of the
cabinet.
Brackets are attached at mid-frame to support a tier, or top rack, upon which mount SW3, L1, L2, C13, K2, and the RF sampling circuit for the
M2. Shaft extensions carry control of SW3 and C13, from their positions on the top rack, to the front panel. Across the rear third of the top rack is a shield that runs to the top of the cabinet,
formed from sheet aluminum. This isolates the relay and sampling circuit from the strong RF present in and around the tank circuit.
On the base plate, mounts a shield formed from sheet aluminum, that extends from the base plate to the bottom side of the top rack. The effect
of this shield is to protect the filament transformer and associated control circuitry from RF present around the tubes.
The tube sockets are mounted on a 5 wide by 1.5 inch high channel that spans the width of the amplifier, and rests on the base plate. The
channel may be formed from a piece of aluminum. Inside the channel, is placed the bi-filar choke and input capacitors. The base plate immediately beneath the tube sockets is cut and drilled to
accommodate a muffin fan, which is then mounted on the bottom side of the base plate. The effect is to blow air from beneath the chassis, up through the tube sockets and around the base of the tubes.
It also serves to reduce any heating that may occur in and around the bi-filar choke.
The front panel has a
cut out that serves two purposes; it provides a view of the
tubes in operation, and permits air drawn out the back of the
chassis to flow around the envelopes. Grounded hardware cloth
or other metallic grill material should be used to retain
proper shielding. To avoid a pressure lock that would restrict
the free flow of air, the fan used on the back panel must have
a greater capacity than the fan used under the tube sockets,
and must be positioned to draw air from the chassis.
As mentioned, this is
merely intended to provide an idea of how a chassis may be
built, quite inexpensively and is a design that I have used
successfully. However, as long as careful attention is given
the placement of parts and steps are taken to isolate the
input from the intense RF present at the output of the
amplifier, a wide variety of layouts are possible. Regardless
of the layout, however, the following caveats may help avoid
problems.
- The input and output
circuits must be sufficiently isolated from one another.
- Tubes similar to
those employed in the designs illustrated on this page must
be mounted vertically. Attempting to mount them horizontally
may cause the filaments to sag and short.
- Filament voltage
must be +/- .2 volts. Filament leads must be as short as
possible, and of a conductor capable of handling the current
required.
- The ventilation
holes in the tube sockets must provide for the unrestricted
movement of air to the tube base. Parts placement in and
around these holes is crucial.
- Lead lengths from
the tube plate caps to RFC1 must be kept as short as
possible.
- Do not place an
inductor in a position that will permit its theoretical
magnetic lines of force to cut a tuning capacitor (such as
would occur if they were mounted parallel to one another),
or potentially interact with a tube plate.
- When running high
voltage leads, care must be taken to shield them from the
intense RF that surrounds the tank circuit. When exposure
may not be avoided, the liberal use of bypass capacitors
should be considered.
This amplifier is
extremely versatile. The design is flexible enough to permit a
variety of layouts without sacrificing performance. The tubes
specified in the parts list may be replaced with a number of
"little sisters", including the 4-250 and 4-125. Subbing these
tubes will, however, reduce performance. In addition, the taps
on L1 and L2 will need to be adjusted to suit the load
requirement of these tubes.
Providing
sufficient air circulation must be a prerequisite to any
layout that may be considered. Other than noise, there is no
penalty for moving too much air. However, there is for not
moving enough.
It is anticipated that the amplifier unit and power supply
will be housed separately. A good quality coaxial cable (RG8
- solid) for each supply lead eliminates much of the anxiety
associated with exposed high voltage leads. However, effort
should be made to keep the leads as short as practically
possible.
The meters are both 0-100 uA panel mount units. M1 measures
Plate Voltage (0-10 KVDC) and Plate Current (0-900 MA). M2
measures relative output and Grid Current. The necessity to
monitor grid current, per this design, is minimal at best.
As with any amplifier, the keys to avoiding problems are 1)
keep all unshielded RF leads as short as possible, and 2)
keep the power supply properly isolated and all leads
thoroughly bypassed.
L1 and L2 should be mounted at right angles to one another.
When wiring SW3, the 80 meter position should permit the
Pi-Net to use all of both L1 and L2. In the 40 meter
position, SW3 should short out the 10 turns of L2, on the
output side of that inductor. In the 20 meter position, SW3
should short 15 turns of L2 on the output side of that
inductor, leaving L1 plus the 4.5 turns of L2 that are
closest to it. In the 15 meter position, SW3 should short
out 17 turns of L2, leaving L1 plus the closest two and a
half turns of L2. The taps, as described, are estimates.
Depending upon the placement of L1 and L2, and the proximity
of other parts, the taps may need to be adjusted as much 3/4
of a turn, one way or another. Adjustment to L1 should be
made by slightly compressing or expanding its length.
An excellent way to go about determining the precise tap
location is to lightly solder a small clip to the end of the
switch lead. It may then be relocated between adjustment
cycles to determine the location that properly loads the
tube. When working with the amplifier to establish the
proper tap location, be certain that the high voltage is off
and sufficient time has elapsed to permit the filter
capacitors to drain. Additionally, use only the minimum
amount of power necessary, and key-down for only short
periods of time (no longer than 10 seconds). Once the proper
tap position is located increase the power and go through
the loading procedure. As a reference, with 100 watts of
drive on 40 Meters, at 2800 volts (key down), into a viable
load, with less than a 2:1 VSWR, plate current should dip to
around 650 MA, with only a slight blush on the plates of the
tubes.
When tuning this amplifier, it is suggested that the builder
begin with a small amount of drive and increase it in steps.
Use the plate current indicator to find the initial dip, and
then rely primarily on the relative output meter to
establish peak power settings. This approach will reduce the
possibility of VHF/UHF parasitic damage to the grids and
virtually eliminates the necessity to monitor grid current
during tune-up.
Plan change: Due to the grid voltage generated across R2, R1
and its corresponding relay connection should be eliminated,
leaving the center tap of T1 connected directly to the
junction of R2 and R9. The result will be an idling current
of approximately 80MA, and better overall voltage
regulation.
Component values are as follows:
- C1 - C3 =
.01 uF Mica @ 1KV
C4 - C11 = .01 uF Disk Ceramic @ 1 KV
C12 = 150 pF Transmitting Variable @ .07" Plate Spacing
C13 = 1100 pF Variable (3 section broadcast type will work
if in good condition)
C14, C16 = 350 uF Electrolytic @ 250 VDC
C15 = .001 uF Disk Ceramic @ 6 KV
C17 - C18 = 500 pF "Door Knob" Transmitting Cap @ 10 KV
D1 = 1N34A or Equiv - Small signal diode
D3 = 1N4005 or Equiv - 600 PIV @ 1Amp
J1 - J2 = SO 239 Chassis Connectors (may be part of coax
relay)
K1 = 100 VAC SPDT - Contacts rated at 5 Amps
K2 = 12 VDC DPDT - Contacts rated at 1 Amp
K3 = 12 VDC Coaxial Relay -
A DPDT Ceramic Relay may be subbed if input and output can
be suitably isolated
L1 = 10 Turns, .25" OD Copper Tubing, 2 TPI on a 1.5" Form,
5" Long
L2 = 19.5 Turns, 1/8" OD Copper Tube, 4 TPI on a 1.5" Form,
4.75" Long
L3 = 3 Turns - No.12 Solid Copper, spaced 1/8 th above and
around R9.
M1 - M2 = 0-100 uA Panel Meter - See Parts is Parts on meter
rescaling.
R1 = 10K @ 10 Watts
R2 = 25 Ohm @ 10 Watts
R3 = 1500 Ohm @ 2 Watts
R4 - R5 = 47 Ohm Carbon @ 5 Watts
R6 = 25 K Carbon @ 1 Watts
R7 = 1.5 K @ .5 Watts
R8 = 12 K @ .5 Watts
R9 = .24 Ohm Shunt Resistor
R10 = 5 Ohm 10 Watts
R11 - R15 = 2 Meg @ 2 Watts
RFC1 = 87 turns of No.18 enameled wire, space wound 4 inches
long, on a 3/4 by 6 inch rigid Teflon or ceramic form.
RFC2 = 30 Amp Bifilar wound Filament Choke - See Parts is
Parts for complete instructions on winding this choke.
RFC3 = 2.5 mH RF Choke
SW1 - SW2 = SPST Switch - Rated 110 VAC @ 1 Amp
SW3 = 5 Position Single Pole Heavy Duty Ceramic Rotary
Switch.
SW4 - SW5 = SPDT Switch - Rated 110 VAC @ 1 Amp
T1 = Primary 110VAC - Secondary 5 VAC, CT, @ 30 Amps
T2 = Primary 110 VAC - Secondary 12 VAC @ 1 Amp
V1 - V2 = 4-400C (may also use 4-400A)
NOW
FOR THE HV SUPPLY
Click on for Full Page Schematic
Above is the schematic for
the standard HV linear supply.
The diode stack is a typical bridge, utilizing equalizing
resistors and capacitors. There is a school of thought, that
although I agree with, I have never employed. It is simply,
if care is taken in the purchase of the diodes, all from a
single lot, with identical specifications, the use of the
equalizing resistors and capacitors will not be necessary.
Additionally, in the alternative, a single HV diode could be
employed for each leg.
Capacitors C1-18 are
filter caps, that may be purchased inexpensively and
combined to provide at least 20 MFD, for SSB operation.
Their values are specified on the parts list, available
below.
Two switches and
accompanying relays are utilized to power up the supply. The
first switch, SW1, applies voltage through the surge
resistor, and may be followed up with SW2, 5-10 seconds
later. These permit the filter capacitors to partially fill
at a lower current rate, extending the life of the power
transformer. Of course a timed circuit could be developed to
eliminate the necessity to engage the second switch, at a
slight premium in cost.
Resistor, R2, the bleeder
resistor needs to be fairly stout, 50-100 Watts. Units in
the 50 watt range may be used with a small fan,
strategically located in the power supply chassis.
Except for the diode, and
filter capacitor stacks, wiring will be point to point. The
chassis for the power supply will need to be shielded, and
the transformer core securely grounded. The unit will be
quite heavy. A cheap, easy and relatively attractive means
of housing the unit is to obtain the metal outer cover from
an expired air-conditioner or similar large appliance, build
a fortified plywood base to fit it; secure a front/back
panel to the plywood base, and attach a set of four or five
casters on the bottom. Staple copper hardware cloth,
available at most hardware stores, on the plywood base, and
make several connections to ground. Switches and indicator
lamps may be mounted on an aluminum plate, and the plate
fastened over a cut-out in the box. Paint the cabinet with a
color of your choice and it should look quite attractive.
Lead outs may employ UHF
connectors (Teflon dielectric only), together with RG8U
coax, of the solid dielectric variety. In light of its lower
break-down voltage, foam coax should not be used. Paint the
exterior of the B+ connectors red, to avoid any confusion.
The
corresponding supply circuit is designed to power a grounded
grid linear amplifier running the legal limit, but may be
used for a variety of others as well. The design is
relatively simple and virtually all the parts should be
obtainable at the local level.
When building the supply, extreme care must be taken to
properly insulate all components on the secondary side of
the transformer. This includes the electrolytic capacitors,
which may have the negative lead attached to the outside
"can" of each. I cannot stress this enough - 2500-3000 volts
can produce incredibly dangerous and destructive arcs
without exceeding the current rating of breakers and fuses.
For example, an arc of moderate intensity, for a relatively
short duration will look very similar to AC and will likely
destroy most components on the dc side of the supply, to say
nothing of the fireworks that will ensue. I can attest to
the destruction - been there, done that!
Remember also that death is the likely outcome if you come
in contact with the voltage produced on the secondary side
of the transformer - whether it is on or off! Before
attempting to work on the supply, always make sure that the
capacitor bank has been properly discharged.
The supply components are as follows:
C1 - C18
= Filter block capable of providing a minimum of 20uF and
preferably at least 45uF @ 1.15 times the secondary
voltage of T1. For example, 6 - 350 uF electrolytics at
450VDC each. Note that each capacitor requires a 50 K 10 W
swamping resistor (not indicated in schematic) across its
terminals.
C19 - C39 = .01 uF Disk Ceramic @ 1KV
D1 - D20 = 1000 PIV @ 2 Amp
K1 - K2 = 120 VAC SPDT Relay - 5 Amp Contacts
PL1 = 110 VAC Pilot Light W/Socket
R1 = 50 Ohm wirewound @ 25 Watts
R2 = 75K - 80K @ 100 Watts (may be 50 watts if forced air
cooled)
R3 - R22 = 470 K Carbon @ 1 Watts
T1 = Primary 110 VAC - Secondary 2400 VAC @ 300ma. (Any
unpotted power transformer producing these voltages that
weighs in the neighborhood of 25 pounds up.)
SW1 - SW2 = 120 VAC SPST @ 1 Amp
Operation of the completed unit is straightforward. The
switches, however, should be placarded to ensure that both
are in the off position to begin with; that SW1 is engaged
first, and that a period of approximately 3 seconds
transpires before SW2 is switched to the "ON" position. A
good upgrade would be to automate the power-up sequence
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