[AMRadio] Valiant Audio Driver Stage Problem Resolved !

w0ng at comcast.net w0ng at comcast.net
Tue Oct 16 19:28:44 EDT 2007

Just wanted to let everyone know that thanks to your combined expert advice and suggestions it appears that the problems with the audio driver stage in my 1957 Valiant transmitter are resolved.

When you mentioned about how the grid leak resistor functions properly (or improperly) it got me to thinking. I didn't remember seeing a 150k grid leak resistor in the circuit. Sure enough it was missing!  Not sure how or why but it was missing from the circuit for sure. Needless to say I wired a new one in promptly.

Based on your advice I also decided to replace the aging (and overheated from too many soldering irons) V15 tube socket and the resistors, caps and shielded audio lines at V15 with new ones. The tube socket pins were a real mess (solder everywhere) and cleaning them seemed like a huge waste of time.

Well, I'm happy to report that the "gremlins" have left the audio driver stage. Checked the power-off resistances and power-on voltages at each tube socket in the audio chain and everything is in spec as compared to the charts in the Valiant manual. Powered up the rig, loaded it up, connected an audio oscillator to the mike input and the modulation pattern looks good on my scope. No smoke anywhere!

Thanks again to everyone who offered advice and especially for putting me on the right track. I really appreciate the help.

Bill Pancake
Fountain, CO

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