Entries from June 2013 ↓

Selecting Lawn Grass

Take a walk through your neighborhood. Look for cracks in the sidewalks. Do you see grass growing in the sidewalk cracks? That is the kind of grass you want for your lawn.
In my area that happens to be Bermuda.

Kentucky blue grass is fine if you are in Kentucky. I live in Texas and I think grass should be green.

St. Augustine grass does not like the sun, or the heat, or lack of water. It does not like the area where I live so I don’t use it. Grass abuse is a terrible thing.

When you decide on the grass you want, buy it in seed form. If you can’t grow it from seed, (assuming it propogates from seed), it will probably not survive anyway. Grass that propogates from seed is particularly low maintenance. Just let it go to seed a few times a season to replenish any weak spots in the lawn.

Sod is expensive to buy and install. Sod guarantees a finished lawn almost immediately and is the best way of establishing non-native grasses quickly. That might last a year or two but eventually you will find out why those grasses are not native.

If you are looking for Bermuda seed, then buy Bermuda seed. The bad should read Bermuda Seed on its face. Grass Mix, Sun and Shade, Quick Grow Grass Patch, these are all examples of products to avoid. If it does not say Bermuda, it is not Bermuda. In fact , generic platitudes are an indication the vendor does not want you to know what is in the bag because if you knew, you probably would not buy it.

Just because it is not approved for use in parts of California does not mean it is bad grass. California is run by people of low intellect and low morals. Being banned there may be a good thing.

Expect to pay at least $3 a pound for real bermuda grass seed.

Scotts products are usually overpriced and underperforming.

4X811HB

The HB stands for Home Brew.

newampon

In its current configuration this amp is equipped with four 811As and puts out 550 watts on 75 meters.

The total investment was about $250 and four weeks.

Purchased parts include:
Plate tuning cap
Antenna load cap
Misc relays
Two meters
High voltage diodes
HV filter capacitors
HV filter equalizing resistors
Parasitic suppressor kit
Vernier drives
Band switch.
Tube sockets
Bypass caps.

All other parts were scrounged or donated. If all parts were purchased at current (2013) rates cost would exceed $700 and the project would not be worth doing.

It is built into a much modified tower computer case. An old computer power supply serves to exhaust air form the case and act as a source of 12 vdc at 15 amps and 5 vdc at 20 amps. The 12 vdc is used for powering relays but it can also be accessed off the rear panel to power other equipment. Like a transceiver to drive this amp.

A second computer power supply case has been gutted of all parts except the fan. It is used to house the filter capacitors and rectifiers for the Plate supply. The plate supply uses the plate transformer out of a Heathkit SB-220 and can be configured as a doubler or a bridge. For the four 811As we only need 1500 volts so the configuration is as a bridge.

The tubes used in this amp are mounted to a removable plate. Additional plates are available for other tubes as they become available. This design can use 811As, 572s, one or two 3-500s, 4-125s and even 4-400s. The basic circuit was taken from: How To Build The DX-811A All-Band Linear Amplifier. CQ Magazine September 1982, BY F.T. MARCELLINO, W3BYM.

A very simple soft start circuit using 120 vac relays is employed. One for the filament supply and one for the plate supply.

Controls include Plate tuning, Antenna loading, Band switch, ans five toggle switches with functions as follows:

Right to left

1. SSB/CW engages primary tap on plate supply transformer. Transformer secondary delivers 1000 vac in SSB and 1200 vac in CW.

2. Turns on ATX PC supply. Provides 12 vdc at 15 anps and 5 vdc at 20 amps. Also powers primary supply relays and three cooling fans. Nothing works until this power supply is turned on/ You can tell iy is on if you hear fan noise.

3. Filament supply ON/OFF

4. Plate supply ON/OFF. Plate supply will not come on unless filament supply has been turned on.

5. Transmit/Standby Interrupts amp keying line to transceiver. Keys at 12 vdc.

Transmit ready light comes on when all conditions for transmit are met.

Tuning controls are equipped with 3:1 velvet vernier drives.

Band switch has 6 positions. Correlation to frequency through tuning chart. Covers 3.7 mhz to 30.5 mhz. Frequency coverage below 4.2 mhz requires addition of a fixed 120pf capacitance to plate tuning capacitor.

Meter on the left is for grid current, measures 0 to 240 ma. Drive should be adjusted to 120 ma which correlates to .4 on the meter scale.

Meter on the right is for plate current. Reads directly from 0 to 1 amp. Typically runs at 550 ma when amp is running properly.

Stand Alone Soft Start

This would be neat if it could be made to work. The trouble is that in order for this to work, an on/off switch has to be located at the soft start relay and the on/off switch in the equipment has to be on and configured in a way that allows the impedance condition of the primary of the power transformer be reflected to the soft start circuit. Otherwise the soft start relay will engage immediately taking the current limiting resistor out of the circuit before the current surge has abated.

The short version of this story is ‘stand alone soft start does not work’.

If we can agree that a soft start feature is good, we might also agree that an external soft start might also have merit.

All soft start devices I have seen are added internally to the target equipment. This requires digging into the device, finding room for the circuitry, coming up with an acceptable mechanical installation and deciding if two soft start mechanisms are needed, one for filament and one for high voltage.

All of these concerns disappear when we build an external soft start device. The external device is built into a cast aluminum enclosure. The proper power socket is mounted onto the enclosure and the current limiting resistor and soft start relay is mounted inside the box. A line cord and strain relief exits the box. The line cord is plugged into the power connection on the wall. The equipment is plugged into the the soft start box.

For it to operate properly, the equipment needs to be left turned on. It needs to be on such that the turn-on load will be reflected back to the soft start box. This means there can be no relay switching of power inside the equipment. The transformer primaries must be a continuous circuit to the power connector of the equipment. Otherwise the soft start relays will engage immediately bypassing the current limiting resistors and the equipment will see the full startup current surge.
This also requires that the equipment to be protected be energized by a switch mounted in the startup box. One switch per circuit. These switches need to be rated at the current limit level. For instance, if the current limiting resistors are 100 ohms and the line is 120 vac, the startup current would be limited to 1.2 amps and a 2 amp toggle switch would be adequate as an on/off switch.

These considerations may not be realisable in all aplications but where they can be used we have a portable and universal soft start feature.

Generating RF Power

You can work the world with 50 watts and a dipole. Yes, you can also hike from San Fransisco to New York. You have to be insane to do things because you can. Look what I can do, look what I can do, those are the ravings of a two year old. Adults set relisable goals and use adequate resources to get there.

Work the world is not a realistic goal, how about lets talk to folks in 100 countries. We don’t want to spend the rest of our lives on this one goal so put a one to two month time limit on the effort. Get at least a three element beam so you can hear the DX. Run at least 500 watts to the antenna so the DX can hear you.

That brings us to the subject, where do we get 500 watts? The most you are going to get out of a transceiver or transmitter is 200 watts. We need an amplifier to get to 500 watts.

Note that the specification for 500 watts is the reading we get on a watt meter two minutes after we go key down, constant carrier, in CW mode, driving a load with an SWR of less that 1.5:1. We don’t want to play games with PEP input or PEP output and have no respect for those who do.

This is 2013 not 1960 so we might want to see what sort of solid state solutions are available. Indeed, a 500 watt solid state amp is available at a price of $3000. It uses 8- MPF-150 transistors. These transistors are rated at 150 watts each and cost $90 each. There is nothing particularly difficult about building your own but the transistors to do that are going to cost over $700 all by themselves. It appears that the solid state solution is more of a problem than solution.

If I had $700 I would buy a used tube amp. A used tube amp in that price range will get you 1000 watt OUTPUT!. Unless you have a mobile application and are wealthy we might want to forget solid state amps for now.

Going back to 1960 we see that we can buy 811 tubes for $5 each. Four of those will give us 500 watts output. If we drive them to 150 percent maximum ratings, like Collins does with the 30L1, we can get 800 watts output. It will shorten the life of the tubes and only add about 2db over the 500 watt situation, but the tubes were cheap at $5 each.

Today, 811s are still being produced but the price has increased to $20 each. Well, everything has increased in price since 1960, some things have increased by more than 5x. The apparent increase in tube prices merely reflect the loss of buying power of the dollar.

Is there a better deal for our money? Yes, the 572 at $40 is a better deal than two 811s at the same price. Looking at plate dissipation alone, two 811s are rated at 120 watts for the pair. At 65 percent efficiency for linear operation we find that we can get 184 watts out of two 811s without exceeding maximum plate dissipation.

A single 572 is rated at 160 watts maximum plate dissipation an can deliver 240 watts under the same conditions. We can get 64 watts more out of a 572 and only have to light up one filament It is interesting to note that 4 x 240 is a lot closer to 1000 than 4 x 184. Makes you wonder why the 30L1 was not designed around 572s because they seem far better suited to their evident goal, no need to abuse a set of 811s.