The AM Forum

I've been working on the RF section of my vintage Phone transmitter. I had some stability issues but I solved them by just using some parasitic suppression resistors in the grids of my power tubes.

i've yet to get the modulator or internal power supply built, but at least for now I have the tricky part (the RF part) up and running well!

Please feel free to browse the following directory to see photos and a schematic for this!

http://99.239.146.96/Projects/Transmitter/Transmitter_V2/

Included here is a schematic - EDIT: I realized that I labeled the final tank tuning cap incorrectly, I've updated the schematic and have included the update here. I must admit a sense of pride in making such old technology STILL work so well! I had a few stumbling blocks, namely parasitics, but I sorted them through one by one the old-fashioned way and got this as a result!

The rated limit of these old tubes is 120V, but out of curiousity I wanted to see if I could juice things up a little....I managed to feed it 180V and it didn't blow up! In fact I got more output, no spurs! But I had to up the final bias to around 20 volts. Still, these tubes don't grow on trees so I won't make a habit out of this.

Later on I'll isolate final PA B+ from the rest of the circuit and probably do Heising modulation, maybe feed the final PA 150V or something, those are power tubes and I think they can do it.

Very impressive....I like it!!!  Gud luk.....Steve  KL7OF

Yea, I'm getting about 4 watts out when I crank it up to 180 volts, continuous at 120 volts I get about 2 1/2 watts out, perfect for QRP operation!

An interesting footnote.....I put the design into a computer simulation program here. My simulation program has vacuum tubes in among it's device-models.

According to the simulation, the design doesn't work! ::)

That is why I'll never have much respect for this darned computer-generated nonsense and design, I think the software in my brain is better than any bit of digital rubbish! ;D