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Saturday, January 26, 2019

Push-Pull Colpitts Transmitter - 3

I'm making good progress on my new transmitter.  With a lot of help from my friend KE0EXE and his table saw I now have a nice looking oak base. I went ahead and shellacked it so that it looks pretty good. 3/16" copper tubing for the antenna link and 80 meter coil was not available locally so I have to go to Amazon for that but I did find 1/4" tubing for the 40 mtr coil at my local big box hardware store. I added a National type A vernier dial to the Cardwell tuning capacitor. This should help a lot getting on frequency.

The next task is to find all of the screws, nuts and bolts that I need and then start fastening everything in place.

Monday, January 14, 2019

Push-Pull Colpitts Transmitter - 2

I think I have enough to get started....

The tough part to find was the dual 500 pf Cardwell capacitor. I found one by putting my need out on the AWA Group Yahoo group. What I now have isn't exactly what QST called for but should be close enough. A trip to Menards got me wood for a base. I'll need to pay a visit to a friend with a table saw. Copper tuning I should be able to find locally at Menards, Lowes or Home Depot. The small
parts under the "chassis" I should have on the shelf.

Next step is to cut and varnish the base.

Sunday, January 13, 2019

Push-Pull Colpitts Transmitter - 1

I’ve started on a new transmitter for the next AWA Bruce Kelley 1929 QSO Party. The “BK” requires non-crystal-controlled transmitters that are of 1920s design and use tube types that were available in 1929. I found a transmitter described in the January 1934 issue of QST that meets these requirements.

For a good over view of the transmitters built to be used in the BK take a look at VE7SL’s gallery page at . Of the 65 transmitters 48 are either Hartleys or TNTs. The next one in the  list was the TPTG with 9. Only 2 were Colpitts. I have TNT and Hartley transmitters for the BK and a TPTG is a lot like a TNT so building a Colpitts for the BK sounds like a good next choice. This will round out my 1920s “big three”.

The 1934 QST article lists several features of this design that sound attractive:

Antenna coupling to the non-plate portion of the tank coil. The more common link coupling to the plate ends of the tank coil results in more second harmonic output and less frequency stability when the antenna is tightly coupled for maximum output. Center/swinging link coupling should be an improvement.

Push-pull tube capacitance in series. Temperature changes within the tubes that impact the interelectrode capacities will have less impact on frequency stability. This configuration also reduces the amount of current through the tubes.

Symmetric layout. A symmetric layout is less prone to exhibit signal instability

Grounded tuning capacitor body. In this design the tank tuning capacitor body provides shielding so that hand capacity has less of an impact on frequency. This transmitter should be easier to get on frequency.

Easily converted to an amplifier. If I decide to move on to a MOPA BK rig this will be one section that I already have.

In addition to information about this particular transmitter design, this QST article also gives hints that are useful for any 20s transmitter:

Use a well-regulated/stiff power supply

Route power and antenna cables away from the transmitter

Do not place the power supply close to the transmitter

Now, may the parts hunt begin!