It’s hard to believe that it’s taken about 5 hours over the last few days to wire and test the 12AT7 audio amp but it’s sad and true.
You can see the amp wiring at the bottom of this picture. I’ll admit, it looks a bit unusual, what with the size of components shrinking over the last 50 years….
Part of the fun was caused by my resolute commitment to testing each stage of the radio as it’s built. This is a great strategy, but sometimes it requires firing a few extra neurons.
In this case I was just dumb. I’d wired the B+ to the second triode in the 12AT7, fired the radio up, and heard nothing when I connected my little 6 dollar eBay audio generator to the preamp triode. This little generator is a great buy if you have nothing else. Otherwise, hard to say 🙂 But worth 6 bucks for sure. It’s like having a dime store compass when you are lost. It can on a good day get the job done.
After lunch, I looked again, and because the rest of the wiring wasn’t complete, there was no B+ on the preamp triode of the 12AT7. Adding the pesky little 100 K resistor resistor made a world of difference.
If you could load videos on a free WordPress page, you would be greeted here with the the semi-melodious of tone from the signal generator coming out of a speaker at a healthy semi-room-filling volume, and, I might add, with no evidence of hum or distortion (other than the distortion from the generator).
It is alive, and I’ve learned one more thing about step-wise testing of a vacuum tube radio. I’m sure you would have been impressed (not). But I’ve got power and audio.
Tomorrow I start the BFO and AGC section.
In retrospect I never had this problem before, because all my solid state builds have been done with modularized units, 50 ohms in and out, and each with it’s own 12 volt feed.