This transmitter , the one you see fading in and out on the site front page, came about when cleaning the shack and finding a box of tubes to good to throw away but probably having little market value, Both tubes chosen were used although each check out within specs on my Mil surplus tube checker. I decided to build the transmitter using ordinary materials for the chassis and front panel much like a ham might have used as the country came out of the depression and when these tubes were fairly new on the market.. The Chassis is constructed on a wooden rectangular frame covered in aluminum from siding used in window installation, flashing aluminum could have been used as well. The front panel is made from a well used 8 by 10 inch piece of aluminum covered by a thin panel of siding attached with spray adhesive. The design which is based loosely on circuits published by the ARRL and on 6AG/6L6 articles found on the Internet. The basic circuit goes back to the late 1930s possibly as early as 36 but definitely by 38. After the end of WWII in 1945 all the hand books for several years featured a crystal controlled version of the 6AG7/6L6 tramsmitter The design calls for plug-in coils and all of the examples I found used crystal control.
Being woefully short of suitable crystals I opted for solid state VFO control, again loosely based on the one published in the handbook for several issues during the sixties and seventies. My VFO uses a dual gang capacitor and is switchable between 80 and 40 meters by means of a panel mounted toggle switch. A reduction drive (a Johnson item I believe) offers a suitable tuning rate. The VFO is housed in a small metal box which probably housed cookies at some point I get them for 99 cents each at the local Goodwill Store. So far I have only wound coils for 80 meters. The coil forms are pill bottles fitted to tube bases taken from bad octal tubes. The coil form are strengthened by having a cardboard tube epoxied inside the form.
A coat of black satin finish paint (Walmart's Fusion brand ) applied to the front panel and sides, the chassis itself is left with the grey paint finish it came with. I am looking for a sourse of white lettering to label the controls. So far no joy! Once I locate some lettering I will apply a coat of clear finsih to the front panel.
As for operations, the transmitter tunes well on 80 and produces about 7 watts into a fifty ohm dummy load.However at this level the tone is a bit harsh for my ears and sounds much better when tuned to about five watts out. Although I don't hear any Key clicks the keying is hard and I plan to do the research and soften the keying at a later time.
I have made several on air contacts with the rig and the signal reports have been satisfactory and include of course, the 599ers, (as if I believed that) I plan to use this rig next straight key night coupled with a mating 40 and 80 meter Three tube superheterodyne using a filter made from surplus xtals at about 456 kHz. I will describe in a future blurb. UPDATE: After being disapointed in my ability to find white dri-Transgfer lettering I finally threw in the towel and used a Dymo labeler and labeled the front panel, here are the results:
There appearing to be considerable interest in these old homebrew type transmitters I am adding the following information. To start, The 80 meter coils are wound with resonance near the full mesh of the tuning capacitors. I have learned that I can also tune the forty meter band resonating near the top of the capacitor mesh. Although a dedicated set of coils might be better I will use these for now,
Your milage may vary depending on the characteristics of the coil you wind and the value of your tuning capacitors. The schematics for both the VFO for the transmitter are shown below. The schematic for the tube portion is not my original work, it is a copy of a circuit I found on the internet (and there are many) that I redrew to reflect the changes I made in the circuit. The VFO circuit is hand drawn by me and offers the component values I used. I did not list the band switching arrangement nor the power supply for the vfo but in this rig it ,is a front panel mounted switch that switches the relay voltage selection to either 80 meter or 40 meter output. The power supply taps into the 6.1 volt AC , rectifies iand doubles it before filtering and regulating it with 7812 and 7805 VR devices..The schematic is not intended as a "How To" item but as a starting reference since any prospective builder will have his or her own junkbox parts that differ from mine. As an example, the tank circuit capacitors that act as the band set for each band , are seperate sections of a compression trimmer and I have no idea what their actual ranges were originally. I simply adjusted them for the band set I needed, real cut and try style. The Caps in series with the band spread capacitors were likewise chosen to produce the band spread i wanted. again cut and try. All of the bypass caps in the VFO are ceramic discs and with the exception of the two feed back caps which are ploystyrene caps the rest of the fixed caps are NP0 units. Here are the schematics