815 Transmitter Coil/L1

L1 of my transmitter calls for ten turns on a 1" coil form spaced over 7/8". I had the form and the wire I needed. Winding spaced turns, though, takes a trick. I first wound the ten turn coil on the form, tightening the wire as much as possible. The turns were not even. It was not pretty. I then wound another "coil" (the red wire) forcing evenything to space out evenly over the 7/8". This I heavily coated with clear fingernail polish. After the fingernail polish had dryed I removed the red spacer coil and recoated with fingernail polish.

Picnic Table QRPing in Decorah, IA

We spent the weekend in Decorah, IA and on Saturday and Sunday I had some spare time for hamming. I used my K1 with a 67' endfed wire up in the tree above the campsite. With the K1's built-in antenna tuner it loaded up on 40, 30 and 20. Sunday afternoon at 3:00 I did finally did have a QSO with Ken, WA8REI/4. He was on the Appalachian Trail at Newfound Gap, NC near Gatlinburg, TN. We were on 14.060, both running battery/5 watt power into end fed wire antennas. With some amount of QSB, signals were around 579. For the bottom of the sunspot cycle I was pretty happy.

See more examples of my QRP operating at http://www.io.com/~nielw/qrp/QRP_Stn.htm

Final 815 Transmitter Layout

I've finally committed to a layout and mounted the major parts. From left to right along the front is the xtal (with B directly above it) C1 and C2. Left to right on top is 6V6, L2/L3, 815 and L4. RFC2 is mounted upright behind the 815 and L4. I elevated L2/L3 so that it could be mounted directly above C1. The toggle switch on top of the chassis in the right front corner is to short out L1 when not multipling in the tri-tet oscillator. The extra space along the back is reserved for whatever is needed to get the screen and oscillator voltages right. The spotting switch will probably go in the space between C1 and C2 with a cathode current meter mounted directly above it (if I add a front panel).

Final Layout / Drilling Guide

After "playing checkers" for several days tweaking the layout, I'm ready to drill. I usually mark major component locations and holes using masking tape. After I'm done the tape can be removed leaving no markings on the chassis itself.

Another Power Supply Option

Bryan, WA7PRC, pointed out to me that an "economy" power supply would be a more elegant way of getting the lower B+ needed for the oscillator stage then wasting power in multiple dropping resistors.

From Bryan:

"It's an easy way to obtain high- and low-B+ voltage sources.  In fact, there's more than one way to skin a cat... center tapped or non-center tapped transformer using a fullwave doubler.  In both cases, the output voltages differ by a factor of 2. The doubler, aside from needing a transformer with only a single secondary winding, has the advantage that the output voltage is 2.828 RMS instead of only 1.414 RMS.  So, your secondary can be 177VRMS instead of 354VRMS. Practically speaking, because of resistance losses, the voltage would be somewhat lower under load.  And of course, you'd still need whatever VA rating, so the transformer size/weight would be the same.

With either topology, you'll have a significantly lower low-B+, and should you decide to use a regulator, will not have to dissipate as much power in the series resistor."

815 Transmitter Power Supply

I keep a couple of generic brute force power supplies around to use with my homebrew projects. This one grew out of a much modified and cannibalized WRL transmitter that I found at a swapmeet several years ago. From the back, on the right side is the high voltage section including transformer, choke, filter capacitor and rectifier tube. It will easily deliver the 500 VDC at about 200mA that I need for this project. On the left side is the bias supply. This section has a VR-90 with a pot across it. The exact bias voltage is set and then left alone. The six foot cable shown will plug into an octal socket on the back of the 815 transmitter.

QRP Operations from W0IBM shack

Today was a great day to stay inside in SE MN: windy, overcast, temp in the 30s and snow in the forecast.

The first Tuesday of the month is informal meeting day at W0IBM, Rochester, MN. ( see http://www.w0ibm.org/ ) I took along my K1 as show and tell. 20 wasn't real active but I hooked up to the W0IBM three element yagi and managed a contact with George, KF4UCC, in Chesapeake, VA.

On the 815 transmitter front I've done a rough layout to help orient tube sockets and studied the dimensions a bit more. It still looks like it will fit. I'm just about ready put the drill to that virgin chassis.

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.

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.

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.