245 TPTG / 245 Amp

TPTG Oscillator on the right, Amp on the left.

This transmitter began as a single tube 245 TPTG which later became known as a TNT transmitter.  The oscillator was built following the instructions given by George Grammer in the December 1929 issue of QST.   I attempted to reproduce the set as exactly as possible, using vintage components where possible.  Although he used a 210 tube, with the cost of those tubes way out of sight, I decided to use a 245, which is lower in cost and were available in 1929.  The tubes were in a 1929 Victor floor model broadcast set, which were replaced with a pair of ST shaped bulbs.

The amplifier was built, patterned after the amplifier section of an earlier MOPA.  That article used a 171 driving a 210 tube.  (That article used a Hartley oscillator).  Again, I swapped the 210 and replaced it with the other globe 245 from the same broadcast radio.  It is a neutralized triode amp.  With the increased output from the 245 oscillator, I was able to loosen the coupling slightly between stages resulting in a very stable set.  The 245 neutralizes easily, and provides good isolation between the antenna and oscillator.  The transmitter runs off of 300V and the PA pulls about 50 MA fully loaded, so the max input is about 15 watts.  By unloading the amp somewhat, it will qualify to run the 1929 Bruce Kelley QSO party at 10W input.

The TPTG oscillator is VERY stable with very little hand capacitance effect due to the fact that the end of the tank coil facing the front of the TX is at RF ground potential.  It makes for a fairly respectable transmitter in itself and have worked a number of stations with it alone.

Metering provides plate current for the amp as well as a 0-1A antenna current meter.  I use the antenna current meter mainly as a reference, as it barely moves when using my single wire fed Windom antenna on this transmitter.  I calculated the impedance of this antenna to be about 125 ohms at 3550 khz.

I had thought about using a 4:1 broadband impedance transformer to step down this impedance to something that would show a little more antenna current, but that hasnt been done at this time.  Currently, the meters are loose, sitting on the tabletop in front of the transmitter.  The milliammeter is housed in a nice square bakelite case with 2 thumbscrews on top.  Will be building a similar case, probably out of lacquered presdwood for the RF ammeter.