The AM Forum

Planning my next homebrew big rig. I’ve pretty much got it planned out except for the audio circuitry ahead of the driver tubes. The driver tubes will either be 6A3’s or 6B4’s. The audio input to the transmitter will be processed audio from an RE-27 mic. I want to use balanced audio input to PP grids. I am trying to decide what tubes and transformers to use ahead of the driver grids.
I was looking at the schematic of the Collins 300G which uses PP 6J5’s resistance coupled to 6A5 driver grids, pretty simple. I think I can adapt this circuit to my project. But the input transformer to the 6J5 grids is specified as having a 500 ohm primary and a 15000CT ohm secondary. My UTC LS-10 is 500pri/60000sec per split side. Didn’t know if this transformer would be suitable or not.
What input transformer would you guys recommend to drive the 6J5 grids? Or perhaps I can drive the 6A3 grids directly from the line through an input transformer. I have built several speech amps using unbalanced input to a single grid, so I could always do that again, but I wanted to do balanced audio input to take full advantage of my outboard processed audio.
The tube lineup will be PP254W’s or 100TH’s running 1500V@300mA modulated by zero bias 838’s at 1250V. I have a UTC LS66 mod transformer and an LS-102 mod reactor.
Just gotta figure out how to get the audio to the driver grids. 🤨

Thanks, Ron W8ACR

Hi Ron,

I have an input transformer (pictured) but, unfortunately it is not CT'd. You can use the whole secondary to go grid to grid and create a CT with resistors though. At 40K on this secondary it'll put you closer to the 15K side to side you're looking for.

Just a thought! Ifn you want it, it's yours!


buddly WD8BIL

Your LS-10 is just about the best thing you could use for any line-to-grids application, Dr. Ron.

Still trying to remember where I put all my 838's...

73DG