[600MRG] Homebrew Tube Transmitter

[Michael Mussler]

I should have noted that my 6146B amplifier runs class AB linear. The linear was desirable for running digital modes. The total tube line up is: 6AU6 -> 5763 -> 6146B pair. It uses a regulated screen supply and about 900 VDC on the PA plates. 

I’ve driven the amplifier with a keyed DDS VFO for strictly CW operation and a home brew transmit converter and commercial HF transceiver (Elecraft K2 or K3 radios with the transverter options) for other modes. The converter follows the same strategy others have noted: A Mini Circuits double balanced mixer using a 10 MHz LO and 10 - 10.5 MHz IF. The LO can be a simple TTL 10 MHz oscillator in a DIP package. Narrow band modes like WSPR work best using an oscillator with good long term stability. A GPS disciplined reference oscillator works well for those modes. I use the same conversion scheme on the receive side with a sharp cutoff 7 pole low pass filter between the antenna and mixer. It prevents products caused by strong broadcast band stations from showing up in the desired spectrum below 500 kHz. I do not use a gain stage at the input to the mixer. I found the ambient noise level is always higher than the noise floor set by the mixer loss and resulting noise figure. 

Others have noted the necessity of good filtering to prevent the second and third harmonics from showing up in the broadcast band. I use a similar 7 pole filter between the amplifier output and the antenna but with appropriately rated components. I’ve never had any complaints of RFI to AM broadcast band listeners but drove around the neighborhood with the car radio tuned to those frequencies just to make sure. 

I’d be glad to share details of any of these items if you’d like to try what I’ve been using.