The Good, the Bad, and The Ugly
There are basically two ways to build RF amplifiers: for show, and for ease of construction. If you like to build perfectly symmetrical, gorgeous amplifiers that you can proudly show off to visitors, the simple (but Ugly) construction method described herein is probably not your cup of tea.
The design came about because of a need for an easily constructed, compact, lightweight, 20db SSB amplifier. Such an amplifier was needed by me to thwart sociopaths who were attempting to jam communications in the daytime 40m "Combat Zone" (7250 - 7260 KHz). The sociopaths typically used manufactured 12db amplifiers to create interference. The "tetrode-with-handles" / Ugly amplifier afforded me a scant 8db advantage. Esthetic considerations were naught. The Ugly amplifier was about delivering 1200v-peak and low IMD. The reaction of most friends to the proposed design was disappointment that I was using Plywood. However, for strength, rigidity, ease of fabrication, and reasonable cost, Douglas fir plywood is a pretty good choice. Traditional, it's not.
An important component of the simplified - no special tools required - construction method is the self-crimping (captive-nut) fastener. Pennsylvania Engineering and Manufacturing is one source. PEM nuts distributors have a minimum order of $20. There are many PEM outlets across the U. S. Material choice is galvanized steel or stainless steel. PEM nuts come in various lengths for use with different thicknesses of aluminum. Metric and American thread sizes are available. I usually keep a stock of 4-40, 6-32, 8-32 and 10-32 thread sizes. Pressing a PEM nut into an aluminum panel requires a stout vise or a press.
Two joinable boxes are used: one for the power supplies and one for the RF deck. The boxes are fastened together with screws and PEM nuts. The lower box contains the screen and anode supplies plus the blower and electric-mains related items. The blower is directed up to pressurize the chassis. The rear of the box contains the air inlet and air filter. The lower box is constructed of screwed and glued 3/4" douglas-fir plywood. The result is an easy to make container of good strength. . The top and front of the box are 1/8" thick 6061-T6 aluminum. The interconnecting wiring is concealed when the units are fastened together. To disconnect or connect the interconnecting plugs, the top box is temporarily raised on wood blocks. The use of plywood led to the nickname 'The Plywood Box' for the tetrode-with-handles amplifier project.
The upper box houses the RF section, bias supply and filament transformer. It is slightly narrower and less deep than the lower box. The RF box is made from 1/8-inch thick 6061-T6 aluminum. The sides are fastened at the corners using 3/4" by 3/4", 1/8" aluminum right-angle stock. The angle stock holds numerous PEM nuts. Stainless-steel screws inserted from the outside fasten the box together. The side-panels can be removed for maintenance. Typical top box size for an 8171 amplifier is very roughly 18" high by 20" by 20". The chassis is made by ringing the inside of the box with 4 lengths of PEM-nutted angle-stock. A shelf of 1/8" aluminum is screwed to the angle-stock supports to form the chassis. The height of the chassis is dictated by the length of the vacuum-variable capacitor drive mechanisms below the chassis, including the Multronics-Cardwell right-angle drives. Typical chassis position is around 5 inches from the bottom RF box. Groth-type turns-counter dials work well for this application. (note: Even though a standard chassis may be used, it is too flexible for this application unless it is reinforced.
Virtually all sheet metal retailers have a digital shear for making precise cuts. This is The tool that is needed to make a precisely fitting box. From my experiences, measuring the dimensions of each plate immediately after it is cut is not a bad idea. Alas, Mr. Murphy was right.
Power Supply Layout
The blower discharge is aimed into the center of the socket above.
To afford cooling of the HV transformer, it is located near the air inlet. In my first amplifier, I used block rectifier units. I soon discovered that they require additional space because they need heat sinks. Henceforth, I began using individual rectifiers mounted on G-10 or G-11 perfboard. This takes less space, costs less, and affords better ability to dissipate heat. . // note: I recommend checking each diode for PIV on a high pot, and for forward volts with 1A flowing.
Sturdy handles mounted near the top of the sides are useful since typical total weight is around 230 pounds for an 8171 amplifier with a Hipersil anode-supply transformer. This figures out to around 70w output per pound (SSB). For comparison, a Heath SB-220 amplifier produces about 30 watts of SSB output per pound.
Most critical is the location of the Tune-capacitor. It should be placed close to the anode for optimal VHF stability.
For tetrodes only - another critical matter is placement of the grid termination resistor. For best VHF stability, it should be mounted as close as possible to the grid. The cold end of the resistor is preferably triple bypassed to the chassis to assure that there is a VHF path And a HF path to the chassis. // note - triple bypassing is three unequal capacitors connected in parallel to assure effective broadband bypassing. Typical values here are approx. 100pF in parallel with 470pF in parallel with 5000pF - all at 2000v or higher. // . The cold end of the grid-R also connects - through a 100 - 200 ohm resistor - to the grid-bias supply.
The hopefully-short wire from the grid's DC blocking capacitor to the trifilar 1:9, 50/450 ohm Z-matching transformer should be 1 to 2 inches above the chassis and not near other wires.
One end of the grid roller-coil needs to be close to the grid - and the other end should be close to the feed-through insulator that connects to the neutralization bridge's 130 to 1 capacitive voltage-divider. Stray inductance in the neutralization bridge should be held to a minimum unless you are trying to build a VFO. Remember that a badly out of balance neutralization bridge can cause oscillation. An in-balance bridge causes stabilization at the operating frequency,. Neutralization does Not affect VHF stability.
For the 8171 amplifier, the location for the hot air discharge is directly above the anode cooler. An inside-mounted screen keeps fingers and paws out. Four nickels, a square of cardboard and some glue will make a matching Ugly-flap that opens when the blower is running. Glue the nickels along one edge the cardboard. // note -- The 8169 amplifier shown in the photos uses a side discharge for hot air because of the need for sound-absorbing baffling (due to the 8169's noisy anode-cooler design).
No connection is made to the "plate cap" on the top of the tube. Using this connection would add inductance to the VHF resonant anode-circuit. The anode connection is made with a 1.2" length of finger-stock fastened to the chimney just above the neutralization bridge's capacitive V divider. Presumably, the "plate cap" is intended for audio applications.
Manufacturers' pressure and volume ratings of blowers seem to be somewhat less than truthful. To find out what's what with a particular blower, one needs a manometer, a cardboard box, the socket, the blower, and the tube. Cut a hole in the cardboard box so that it will provide a tight fit for the socket. Cut a hole in the cardboard box that will fit over the blower outlet. Connect the manometer to the cardboard box. Insert the amplifier tube and operate the blower. Measure the pressure in inches of water with the manometer. Consult the Eimac spec sheet for CFM versus pressure.. For laughs, compare your measurements with the blower manufacturer's ratings.
Tank Inductor Cooling.
RF amperes are different than DC amperes. Few people seem to realize that 28MHz current causes roughly five-times more heating in a round, solid conductor than does the same amount of DC. One of the less-well-understood design features of the venerable Heath SB-220 is the use of a plenitude of cooling air on the tank. The AL-1200 is an example of an amateur amplifier that has no cooling air on its tank. The AL-1200 uses clearly undersized conductors in the tank inductors for the high bands. Reports of tank melting on the higher bands, as well as decreased output on the higher bands, are fairly common. // note: The "recognized expert" (p.72, 9-94 *QST*) who designed the AL-1200 ridiculed the SB-220's design.
In the Top View of the 8169 Ugly amplifier, it can be seen that the 22uH tank (variable) inductor is in front of the air outlet screen, so c. 100CFM of passing air helps to cool the tank inductor. In this amplifier, the tank inductor carries roughly 2 (anode-amperes) multiplied by 10 (Q) = 20 RF amperes. At 28MHz, 20A typically has the heating equivalent of roughly 100A DC.
To help cool the 8171 Ugly amplifier's tank (which has about 40A of circulating current) I drilled a grid of small holes in the chassis directly below the E. F. Johnson, 222-type, 1/2" edgewound tank inductor. The net result was brisk movement of air through the inductor.
I used a BNC-type bayonet lock connector for the RF input of the Ugly amplifiers. I used a type C connector for the output due to it's superior voltage handling ability, bayonet lock and excellent moisture seal. For the power connector, I used a #6 Cu electric-oven 6' cord-set. .
The 8169 offers exceptional linearity, instant-on, and ease-of-drive on 10m. The disadvantages are a noisy anode-cooler, the requirement for a high-pressure noisy, costly blower, an expensive socket, and no handles. .
The 8171: is as linear as the best transceiver, has a quiet cooler design, is widely available, is instant-on, and it uses a tube socket that does not cost a lot. The 8171 has plenty of emission. However, it requires a box with a bit more height in order to accommodate the larger anode cooler.
The 8169 gets one thumb up. The 8171 gets two thumbs up. The 8281 gets two thumbs up.
A Strange One: During the time that I used The Plywood Box on the air, I received numerous reports of an increase in signal strength of more than 20db. One witness said that when The Plywood Box was switched on it was like Godzilla emerging from the ocean. Someone commented that The Plywood Box was like a Transmitting Noise-Blanker.
I was skeptical of the more than 20db observation because I had measured a voltage-gain of 10.0 with my calibrated oscilloscope and x100 HV probe. Thus, the gain was 20.db. I eventually sold The Plywood Box to a friend who frequented the "Combat Zone". One day he showed up on 7258KHz and asked me to watch his signal strength. What I saw on my calibrated S-meter was seemingly impossible - an increase of 23db.