Entries from May 2013 ↓

Meter Shunts

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There is an easy way and a hard way. First the hard way otherwise you would not appreciate the easier way.

The hard way requires lots of arithmetic and knowing the internal resistance of the meter.

Secure a voltage source. A regulated supply is fine or get a battery. Lets say you found a 9 volt battery and it is actually 9 volts. Now lets assume our meter is a 1 ma current meter. Actually all meters are current meters. Volt meters are current meters with series resistances.

Now, using ohms law E=IR we find we need a 9k ohm resistor in series with the 9 volt battery to make the 1 ma meter read full scale. So hook it up and verify that the meter reads full scale.

Now, select a potentiometer. A 10k would be a good choice. Wire the wiper of the pot to one of the fixed terminals and connect the pot across the meter. Adjust the pot until the meter reads half scale. Remove the pot without disturbing its setting and measure its resistance. That resistance reading is equal to the internal resistance of the meter coil. Why? Because it shares the current equally with the meter. Two equal and parallel resistances take an equal and half share of the current previously conducted by only one of the resistances.

Now you can use that value to calculate the shunt you need.

Now for the easy way. An example will be used to explain the process. I had two meters. They were both 1 ma meters. One needed a shunt allowing it to read grid current as high as 200 ma. The other needed to read plate current as high as 1 amp.

These meters were to be used to measure plate and grid current in a linear amplifier I had built around four 811As. This amp’s relay supply was being provided by an old PC supply that was delivering +5and +12 volts. The PC supply was being used in normal fashion and was at home in the computer case that was being used to house the new amp.

I had just finished setting the band switch taps on the tank coil and had removed the 50 ohm resistor across the output of the pi network. This 50 ohm resistor was now employed in series with the regulated 5 volt supply to ground to generated a current flow of 100 ma.

I had previously purchased a bag of 50 1.8 ohm metal film resistors at a very low price and decided to use these to build the meter shunts I needed. I verified the 100 ma current flow with my $3 Harbor Freight multimeter. Then I placed one of the 1.8 ohm resistors in series with the 50 ohm current limiting resistor. I placed the meter across the 1.8 ohm resistor and noted the meter deflection. Too high. So I added a second 1.8 ohm resistor in parallel with the first. The deflection was now 17 divisions on the meter scale. 100/17= 5.88. Each division was was now indicating 5.88 ma for a full scale of 294 ma for the 50 divisions. My goal was to measure 120 ma which would be about 20 divisions or 0.4 on the meter scale. Close enough. The additional range would come in handy for use with other finals. I had purposely set this amp up to use other tubes as they became available.

For the plate shunt I kept adding 1.8 ohm resistors until the 100 ma current source deflected the meter to 5 divisions. 100 ma X 10 = 1000 ma or 1 amp. 50 divisions full scale divided by 5 = 10. Thus full scale translates to 1 amp on the 1 ma meter.

Done. Accuracy so – so. Ten percent would be fine.

Installing meter parts to tiny single hole solder lugs is an unpleasant task. That is why I mount the parts to circuit boards that can be installed to the meter terminals with two nuts. Wires soldered directly to the printed circuit board pads.

Select a fairly thick cicuit board scrap and drill two holes to fit the meter studs. Use a dremel tool to cut away a strip of copper on both sides creating isolated pads. Now solder, surface mount style, all the required parts across the isolating gap.

Once done, solder the meter leads to the pads and mount the assembly to the meter.

Using Obsolete Computer Hardware for Ham Equipment

I am suggesting the use of old computer cases to house communications equipment. Primarily for the housing of linear amplifiers. In a recent project I used a mid sized tower case to house the parts of a four tube 811a amp.

A small tower case measures 7 x 15 x 13 inches. The case I found measured 9 x 15 x 17 inches and was just barely large enough to house the required parts. To prepare the case I cut the cover into three parts using a metal cutting blade on a table saw. The sides of the original cover were used for the top and bottom plates of the resulting cabinet. What used to be the top of the case became the front of the amp. The bottom of the case became the rear wall of the amp. What used to be the front of the computer case became the right side of the amp after the plastic computer front was removed.

A shelf used originally to support the mother board was mounted to the bottom of the five inch hard drive enclosure and supported about two inches above what ended up being the new bottom of the case. This shelf was cut out to receive the upper half of a filament transformer which further stabilized the shelf. The plate transformer was a spare for a Heathkit SB-220 and fit nicely into the five inch drive enclosure.

This particular tower case had provision for two standard computer power supplies. I think it was an either or sort of deal instead of providing for two supplies at the same time. In my project I used both locations. One to house an old Dell supply to provide relay power, 5 volts dc, and 12 volts at up to 15 amps for powering an exciter or other lower power equipment. The second supply location was used to house an empty PC power supply case that was used to house the rectifiers and the filters for the HV supply. Both PC power supply cases have fans to exhaust air from the case. A third fan blows air into the case over the tubes.

I am not too hopeful that all the parts could be fit into a more normal sized case. The more normal dimensions are 7 x 15 x 13. In my case the 7 inch dimension would become the height. Seven inches is not enough to accomodate an 811 standing up. Of course you don’t have to lay the case down on its side. There should be no reason not to use it in its original ‘tower’ c0nfiguration.

Homebrew

Homebrew, as in building your own equipment. Why homebrew? Some will tell you it is because they can build better performing equipment than they can buy. That is going to be my reason too but it is not because I am particularly advanced in the art of building communications equipment. It is because I can’t afford to buy anything that is worth a flip because I don’t have the money. Well, that is not entirely true. I have money, I just don’t want to spend it on communications equipment.

Unless you are an absolute failure when it comes to scrounging, you can build something useful for far less than it would cost to buy. I once had a school friend who was a failure at scrounging but he was very good at making money off his paper route. One fine day I accompanied him to the local electronics supply house. He proceeded to purchase ALL the parts required for a two band 6146 oscillator transmitter he found in the handbook. He paid more for those parts than he would have spent on a Heathkit transmitter but he wanted to build his own from scratch.

That is not the sort of homebrew I do. Although I enjoy building, I take even more enjoyment from building on the cheap.

For instance, I recently built an HF linear amplifier at a cost of under $200. Performance wise it will do everything a Collins 30L1 will do except sell for $1000.

That is not entirely true. I recently rebuilt a 30L1. New rectifiers, new filter capacitors, and new tubes. Constant carrier key down power output was 800 watts with 100 watts input and 1800 volts on the plates. I imagine that 60 watts of the 800 were feedthru from the 100 watts of drive. Still, that means the four 811s were delivering 740 watts output. That is pushing them a little harder than their maximum specs allow. Maximum ratings are 1500 volts on the plates, 160 mils plate current and 60 watts dissipation per tube. No wonder the 30L1 runs so very hot! My amp does 550 watts out with 80 watts drive on 75 meters running a plate voltage of 1540 volts.

Back when the 30L1 came out you could buy a set of four RCA 811As for $20 on the surplus market. So what if you had to buy a new set every year. Today they are $20 each. A set of 4 will run you right at $100 with tax and shipping. Today you would be better off buying four 572s at $200. Get them now before they go up in price. The 572 can dissipate 160 watts compared to the 811A at 60 watts. Imagine that! Two 572s can replace four 811As! Well, what do you know, the two 572s in my old Yeasu 2000 do put out about 500 watts, exactly what I get from the four 811As. There is one major difference. Those 572s are the original tubes that came with the 40 year old amp. I seriously doubt that the 811As would last that long. Certainly not in a 30L1. Here is another interesting factoid, two 572s will cost the same as four 811As. All this time I thought the 811As were the better deal. I guess I was wrong!

Here we have the most serious downside to homebrew, regardless of how well your creation works, no one will be willing to pay you what it is worth. Why? Well what would you do with $1000, buy the 30L1 or give me the money for my homebrew 30L1?

I am pretty sure I can get my $200 out of it, but I would be a stupid fool to sell it for that. Prospective buyers would claim they would be stupid fools to pay more than that. So, to keep everyone from becoming stupid and foolish, I will keep my amp and use it for myself and avoid paying $1000 for a 30L1. After all, the fellows I talk to on the air will never know the difference from the strength and quality of my signal.

There is another way to view this. The $200 I spent and the effort to build the amp allowed me to avoid spending an additional $800 to buy a ready made amp. In both cases I can recoup my dollar investment. The only resource at risk is the time I spent building… maybe. Actually, the building effort allowed me to avoid spending an additional $800. When viewed that way, there was nothing at risk.

That is not entirely true either. Anytime you build something you risk the money spent on parts, tools, and money lost in the time it takes to build it when that time could be used to earn money elsewhere. Then if your project turns out to be a failure and does not work, you have lost all those resources.

Last Minute New Amp

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Need to connect grounds for filament and plate relay switches. – DONE

Need to figure out bias and bias/bypass elay setup. – DONE

Antenna change over and two coax connectors.

mount tank coil – DONE

Wire load cap -DONE

wire plate cap – DONE

Install bandswitch and taps. – PARTIALLY DONE TAPS NEED WORK

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install coax connectors

build and install plate supressors – DONE

meter shunts for 1 amp and 0.2 amps

meter shield

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input tuned circuits/sense band switch/enclose in sheild

toggle switches right to left:
1. hi/lo power
2. PC power supply on
3. filament on
4. hv on
5. amp bypass

The plate tuning cap has a problem. I thought it was about 300 pf. Seems it is less than half that value. It will not tune below 6 mhz with the existing tank coil. It IS perfect for 40 meters and up and has wide plate spacing. I want to keep it and the tank coil because they will be ideal for when I upgrade the amp to higher power.

I have a fixed value vacuum cap. Adding the vacuum cap allows tuning across the 80 meter band by adding six turns to the tank coil. Will try that. Will switch that in and out internally with a strap. Not part of the band switch.

Turns out that the vacuum cap is only 110 pf. The lower value capacitance requires the addition of windings to the coil which I did not want to do. Ended up using five 850 style caps in parallel for a total of 120 pf which works just fine. The vacuum cap was too large anyway.

New Amp What is Left

Not much left to do. Most parts are available, their locations identified and mounting holes drilled and de-bured.

Things needing attention as follows:

Antenna change over relay. Intend to use two SPDT 12 volt relays. One on the RF deck, the other at the input connector. All connections done with coax.

Band switching input circuits. Need to ensure the input is a match to 50 ohms on all bands. Don’t want a second band switch or input tuning control. Need to sense band selected by main band switch position and automatically select the proper input circuit. Needs to be designed around relays and optical sensors.

Install band switch and locate coil taps.

Build and install plate supressor chokes. One choke per tube. Don’t do what Collins did on the 30L1.

Install input and output connectors.

Install shields between RF deck and power supply sections. Part of shield needs to be perforated to allow exhaust air flow. DONE

Install and wire up control switches and pilot lights. DONE

Controls and lights as follows:

Toggle switch for starting PC supply
Toggle switch for HI/LO power
Toggle switch for filament
Toggle switch for HV
Toggle switch for amp bypass

Indicator for PC supply (12v)
Indicator for Filament NO
Indicator for HV NO
Indicator for amp active

Rear panel:

Yellow LED for +12v
Red LED for +5v
Fuse Holder for +12v external
Fuse Holder for +5v external

Universal Linear Amplifier Design

Primary Features

The grids are directly grounded.

Uses cathode bias generated by a series string of forward biased diodes

Fused cathode circuit. Fuse and diodes paralleled with high value resistor to provide cutoff bias should the fuse fail.

Both primary power leads are fused.

Soft start limits inrush to 5 amps for filament and plate supplies. Inrush can be limited further by increasing current limiting resistor values.

Makes use of computer power supply to provide 5 and 12 volt relay control voltages and auxiliary power for exciter. The computer power supply fan exhausts hot air from the cabinet.

A computer power supply empty shell is used for additional fan cooling and safe housing of the high voltage rectifiers and filters.

Filament transformer is rated at 400 watts.

Plate transformer is rated at 2kw and selectable 1000 or 1200 vac secondary via primary taps.

All band capability.

Equivalent to the 30L1 when using 811 tubes.
(Plate supply is robust enough to properly run a quad of 572s in a 30L1 configuration).

Equivalent to the SB-220 when using a pair of 3-500 tubes.

Equivalent to the Loudenboomer when using a single 3-500 tube.

Desktop unit.

Grid, plate, and output metering.

Input tuned circuits allow use of solid state exciters at full power.

Plate tune and antenna load controls use 3:1 velvet vernier dial mechanisms.

Plate current meter switchable from plate current to RF output.

Wired for 120 vac operation. Can be wired for 220 vac operation.

PTT only. No QSK for CW.

Keying line at 12 volts 100ma.

Universal High Voltage Power Supply

This power supply design is intended to support the Universal Linear Amplifier. The entire system is designed to provide from 500 to 1500 watts OUTPUT ( that is power available at the feedline to the antenna) using a variety of transmitting tubes. The only solid state devices are the silicone rectifiers used in the power supply, bias circuitry, and relay inductive spike suppression.

Tubes under consideration include the 811, 813, 572, 3-400, and 3-500. These tubes are listed in an ascending order of cost with the 811 being the most economical.

The universal power supply uses a circuit that can be configure as a bridge or full wave voltage doubler. The plate transformer for this supply is intended to supply 800, 1000, and 1200 vac at up to 1amp. Output voltages from this design are 1120, 1400, 1680, 2240, 2800, and 3360 vdc.

The ac plate supply voltage is adjusted by using a tap on the primary of the plate transformer. The plate transformer is selected for operation from 120 vac or 240 vac.

The amplifier is designed with a removable tube plate. A number of identical plates can be fabricated to allow the use of any of a number of the tubes listed previously.

The most economical and well supported solution is the 4×811 amp operated at a plate voltage of 1680 volts. 1680 volts at 1 amp is 1680 watts input which at 60 percent efficiency results in 1000 watts output to the antenna.

Filament power is supplied by a modified 400 watt transformer. The normal secondaries were removed from this transformer and replaced with one layer of stranded #12 insulated wire with center tap to provide 6.3 vac under the load of four 811 tubes. It is estimated that should that load current be increased to 24 amps (as requored by two 3-500s) the voltage would drop to around 5 volts. If the drop is less than that the voltage could be dropped further to ensure 5 volts at the tube pins by adding a sufficient length of small gauge wire. Note that at 400 watts the transformer is capable of supplying up to 80 amps at 5 volts. The secondary voltage of the filament transformer increases by 0.5 volts per secondary turn. Thus it takes about 12 turns to generate 6 volts.

G5RV

Some believe that the G5RV is an all band antenna. They claim that the G5RV can be used on all bands when driven by open wire line and an antenna tuner. While that is true of the G5RV it is also true of ANY antenna. Any and all antennas can be made to work on all bands when driven by open wire line and an antenna tuner.

The ONLY special consideration specifically associated with the G5RV is its use on 20 meters. The G5RV was designed to be used on 20 meters. It is a 1.5 wavelength dipole on 20 with a quarter wave matching transformer to allow it to be fed with 50 ohm coax. Twenty meters is the only band where the unmodified G5RV will deliver decent performance. While its performance on 20 meters might be decent, it is no match for even a two element beam.

In summary, The G5RV is a good 20 meter antenna solution for the ham on a shoestring. If that shoestring also has you using QRP style equipment you may find yourself doubly disadvantaged.