815 Xmtr parts

Here's the collection of parts needed to build my transmitter (the power supply is separate). I'll mount the male 8 pin plug on the back of the chassis for power. The barrier strip is for metering points and for the key connection. I'll be able to connect in a plate modulator here also. In the final transmitter the two 25 watt pots will be replaced by wire wound resistors.

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Additional 815 xmtr Mod

Another feature missing from most pre-WWII transmitters is some sort of spotting capability. Two reasons drive the need for spotting capability today. First, with transceivers being the norm, operators rarely tune the band looking for a response to a CQ. If answering a CQ, you need to be close to on frequency. Operating in an open spot on the band is the second reason for a spotting switch. Choosing the right operating frequency/crystal requires spotting capability.

To add a spotting switch to the 815 transmitter I'll try a DPDT switch connected in the cathode circuits of both the oscillator and the final. In the "spot" position it will key the oscillator just as the key and also open the cathode-to-ground connection on the 815. The crystal oscillator should operate normally letting me locate myself in the band but I'll not have 50 watts of RF overloading my receiver.

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Another layout

Here's another thought on layout ... move C2, the final plate tuning cap, from on top of to under the chassis. This frees up space top side and there is still enough space below for the parts around the 815. C2, a major shock hazard, is now safely under the chassis. I also found ceramic plate cap connectors for the 815. L4 is still exposed with voltage on it but it is set back from the edge fairly well and it is opposite the crystal socket.

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815 Xmtr - Safety mods

Someone asked me about modifying a vintage design in the name of safety. Sounds like a good idea to me. I already modify vintage receiver projects to keep B+ out of the headphones ( see http://www.io.com/~nielw/onestep/onestep.htm ). I also try to keep transmitter B+ away from accidental touching. This design, unfortunately, has 500 VDC exposed on the final tank coil and C2. I plan a couple of things to to help. First a front panel. I'll probably use a piece of masonite painted wrinkle black. As a bonus this gives me room for a plate current meter should I decide to add one. The second modification is to move the crystal socket to the front panel. No need to be reaching around behind the front panel to change frequency. The rest needs to be handled by being careful. Obviously power down when changing bands and short the tank coil to ground before touching it. Not so obvious is the key. Cathode keying allows one side of the key to float to B+. Usually one side of a key is more protected than the other. Connect this side to R1 and don't diddle with the key contacts with power applied.

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815 Xmtr Parts Subbing

As I search for parts, the topic of subbing parts has come up, particularly for C1 and C2. Miniture 140pf dual section variables are called out for both of these. In my junque box I fine slightly larger (physically) parts and, for C2, only a 100pf dual section variable. I can probably work around the 100pf problem by adding two fixed 50pf caps to L4, a 80JVL plug-in coil. The physical size increase appears to be OK for C2 but C1 is going to cramp the wiring around the oscillator stage. Can I shift C1 to tune L2 rather than L3? That reduces me to needing only a single section variable. Unfortunately is also means that I need to insolate "C1" from ground. The mounting starts to take up any space that I saved by changing to a single section cap. Looks like I'll stick with the parts I've found and just carefully lay out the oscillator stage in the space I have.

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815 xmtr

I'm to the point of starting to layout my 815 transmitter. Here are a couple of figures from the 1943 ARRL handbook describing this transmitter along with a picture of my preliminary layout. My chassis is 3"x 8"x12" so I have a little more room. Good thing, since I didn't have the small size variable caps called out in the original design. I'm also allowing room for a VR150 (seen on the back left corner) in case the oscillator stage needs regulated B+ for stability. The original design used only 3.5MHz crytals to cover 80, 40 and 20. I'm going to use 3.5 MHz crystals on 80, 7MHz crystals on 40 and 7MHz crystals on 20. This leads to one change. I'm adding a switch across L1/C3. This will be used to short out the cathode coil/cap when I'm running the tri-tet oscillator straight through.

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815 transmitter

According to the 1942 ARRL Handbook the 815 requires less then 2/10 of a watt of drive. A 6L6 is rated for much more than that. The 6V6 has the same base pinout but lower rating. I'll switch over to a 6V6 for the oscillator stage. My power supply will deliver about 500 VDC for the 815 but I don't want to run the 6V6 at 500 volts. I'll need to somehow drop 500V down to about 150 VDC for the oscillator stage. Depending on current requirements a VR150 and a dropping resistor might work. I don't want to use just a dropping resistor since then the 6V6 plate voltage could go as high as 500VDC with the key up.

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6L6 - 815 three band xmtr

I've always been interested in the 815 based 50 watt transmetter that showed up in QST and the ARRL handbooks just before WWII. The 815 is a dual "beam" tube designed for UHF (150 MHz in 1940). I'm starting to plan a tramsmitter using an 815 as a push-pull final (as shown in Feb '41 QST) driven by a 6L6 tri-tet oscillator such as used by Millen in his 6L6-807 rack moount transmitter.

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Welcome

Welcome to my blog about my ham radio activities. Here's where I plan to "talk" about what I'm doing in the shack. Feel free to comment and/or email me if you see something of interest. 73, Niel

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Grinding Transmitting Crystals

As I played with my 6J5/6L6 transmitter I saw that I had some holes in my 80 mtr crystal selection. I had several crystals at 3550 but practically none between 3550 and 3570. Before stable and well calibrated VFOs were plentiful, grinding crystals was a common activity. Military surplus crystals were obtained cheap and ground to the frequency of interest. I decided to grind some of my 3550 KHz crystals up a few KHz to fill in the gaps in my crystal selection.

I used 400 grit wet sandpaper face up on a flat surface as my grinding "station". I removed the quartz crystal plate from the holder and, pressing on two opposite corners, ground in a figure 8 pattern. I'd typically grind for 5 to 10 passes and then rotate 90 degrees, repeating four times. I then cleaned and dried the crystal, reassembled it and checked the frequency. If I hadn't shifted it far enough I repeated the whole process. To insure that I ground only one side of the crystal I marked a side with a dot of ink.

I found that my Millen grid dip oscillator and a frequency counter made a handy crystal checker. I plugged the crystal in place of the GDO coil and capacitively coupled one pin to my frequency counter. The counter then read the crystal frequency and the grid current indicated crystal activity.

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Alternator whine and tips on clean copy in your mobile.

Alternator Whine
I want to revisit this problem, because there seems to be a lot of poor advice floating around on these pages. Let's start out with a few basic facts, but keep in mind this is NOT an alternator primer. If you need or want more data, the internet is your best friend.
The average alternator's output is between 13.9 and 14.2 VDC. It might be less if there is a problem with the alternator. In some cases it may be a little higher, but voltages over 14.6 VDC should be considered abnormal.
Continuous output and peak current ratings vary quite a bit. The requisite amperage ratings selected by OEMs are largely based on content. That is to say, how many features like rear window defrosters, premium sound systems, electric windows, and heated seats any given vehicle is equipped with. Heavy duty and high-end vehicles usually have larger ones as do those with extra-cost trailer towing packages.
Nowadays, the smallest OEM ones are rated about 90 amps peak, and the larger OEM ones about 150 amps peak. There are a few exceptions, but the highest rated OEM units are about 225 amps peak. The reason I use the term peak is this; very few OEM alternators will deliver their rated output continuously, and contrary to popular belief, there isn't any standard rule for peak versus average.
Almost all alternator stators (the non-rotating part) are wired in a wye configuration (as shown), and the rest are wired in a delta configuration (primarily Ford products). Rotating within the stator is the field. The field current and/or voltage is varied by the regulator so the output voltage is constant, regardless of the load, up to their peak amperage rating. There are several different regulation strategies employed. Some simply use a pass transistor, others use pulse width modulation, and some almost defy definition.
Depending on the engine type (diesel or gas), alternators are driven from two to five times engine speed, up to a maximum of about 16,000 rpm. As a general rule, the output frequency of an OEM alternator is equal to the engine rpm. That is to say, 1,000 rpm equals 1KHz. Their efficiency is about 90%. Thus, an alternator rated at 130 amps, with an output of 14 vdc, will have an input of around 2 KW, and will require about 3 HP to drive.
In a never-ending quest to reduce weight, and improve efficiency, most new-generation OEM alternators are double wound, and use twelve diodes instead of six. This not only reduces size and weight, the lower mass of the rotating field allows the alternator to be driven faster, which improves low rpm power output. It also doubles the ripple frequency.
As long as the diodes are doing their job, the output ripple is nearly nonexistent, as the battery is acting like a very large capacitor. When they don't do their job, the result is what we commonly call alternator whine. To be sure, there are other causes which will be discussed later.
While alternator whine can be a bane for us amateurs, as long as the alternator delivers its rated output, dealers don't care, and typically will not replace noisy ones under warranty. So this leads those who are plagued to seek other avenues of relief. For example, using RG8 as a power cord, or twisting the factory power cords of their transceivers. Doing so is junk science. Let's visit this in more depth.
First, any technique we use to shunt alternator whine to ground must present a low impedance at the frequency we're trying to suppress (less than 8 kilohertz typically). Further, it must be of lower impedance than the circuit it is attached to. In the case of vehicle DC wiring, that's seldom higher than a few tenths of an ohm.
An average power cord is ten foot long. A ten foot piece of RG8 has 250 pF of capacitance. At 8 kHz, 250 pF has a reactance of about 1,500 ohms. In terms of suppression, this amount is insignificant.
Twisted or not, a 10 foot power cord made from two number 10 conductors will have about 2 pF of capacitance per foot. Ten feet of it is an insignificant reactance even at 80 kHz! What's more, those who support twisting the power cord as a fix for alternator whine, and a host of other maladies, ignore some basic facts. Twisting works to reduce noise pickup only if both inputs and outputs are balanced, and neither end is grounded. That's not the case here.
Brute force filters offer some help, but there is a big downside too, and that's voltage drop. Radio Shack used to sell one that was rated at 20 amps. Inside its tubular construction is 20 feet of what appears to be number 16 Thermalese wire wound around a laminated steel core about 3/8 of an inch square, and and 2 inches long. A 1 uF coaxial capacitor completes the package. The input and output are size 10. The voltage drop at 20 amps is almost 2 volts. At 8 kHz, the suppression is less than 2 dB.
In some cases, a 1 Farad cap, like those used in mobile sound systems will suppress alternator whine if they're placed near the radio end of the power cord. However, they have a lot of drawbacks, not the least of which is their propensity to explode if dead shorted.
The best place to cure alternator whine is at the source. If you think it is a leaky diode causing your problem, use an O scope to look at the alternator output directly at the output terminal. If it is a diode, you'll easily see it. The fix is obvious.
As alluded to above, there are another situations which can cause what ripple there is to invade the circuitry of your transceiver. One of those is a ground loop. Ground loops occur when there is a differential in current flow between the positive and negative power leads feeding the radio. This is typically caused by incorrect wiring techniques. Poor bonding of body on frame vehicles, and poor coax connections can also cause the problem.
Another problem altogether, which is often incorrectly identified as alternator whine, is the switching transients from the alternator's regulator. While diode induced whine directly varies with engine speed, regulator whine normally does not. It will appear louder at low rpms, and when there is a high amperage load. Since it is radiated RF energy, removing the antenna will cause it to go away. The only fix is to replace the regulator.
Distractors will surely point out that they fixed their alternator whine with one of the aforementioned anecdotal remedies. If that is indeed the case, then the original wiring was amiss.
Alan, KØBG
http://www.k0bg.com/

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