Todd,
Thanks for your thoughts on my radio conversion to 12 volts. Since I did my first post on the subject, I rewired the tube filaments into series pairs. The output 6V6s now have their heaters in series, and the other 4 tubes all had a 300mA @6 volts spec, so I split them into series pairs for heater rewire. I did see the clutch on the tuner and measured the resistance at 2 ohms, so I added approx 2 ohms in series, wiring together two .56 ohm and a 1 ohm (it's what I had laying around), each with a 5 watt rating which should be enough, especially given the momentary, infrequent use when pushing tuner buttons. Someone had already put in a permanent magnet speaker. Then I wired in a 12 volt solid state vibrator from Antique Auto Radio.
When I put 12 volts to it and turned it on, the radio worked fine with the B+ supply going to 245 volts and then dropped down to 160 under load as the tubes heated up and started conducting (part of this drop, was when one side of the vibrator quit - read on). The radio worked OK for a short time, but the vibrator-induced square wave sinking the transformer to ground, viewed on a scope did have large spikes on the edges from the inductive collapse of the transformer at waveform switching transitions, and after a short time, first one output of the solid state vibrator died and then the other. I tore open the vibrator can and found the output FETs were in small, 8-pin DIP IC, and the plastic on one corner was fried. Whether the spikes killed it or the transformer I have was drawing too much current I don't know. The IC had no heat sink provision.
Running previously on 6 volts, before I converted the car to 12, the older solid state vibrator from the same company had worked fine with that transformer I cobbled in, and I used it for about 1500 miles of driving. Then I got thinking that since I had no further use for the 6 Volt unit, maybe the design was robust enough to withstand 12 volts, so I put that back in, applied 12 volts, and it promptly died. I opened up the failed 6 volt vibrator and being older (1998 date code) it's design was a circuit potted up inside 2/3 of the can with black potting compound so I couldn't see what was components were used. FYI - with this 6 volt setup, I was getting a B+ of 115 volts under load, so either the load was too much to sustain a higher voltage (which I doubt), or the transformer core was in partial saturation, or there were just some inefficiencies of driving the transformer backwards, as the now-primary low volt winding was not very close to the core, being wound over the high voltage winding.
So next. I plan on building my own solid state vibrator, with robust transistors, and will add some reverse diodes, to shunt the reverse inductive spikes. I can easily come up with a design, but also have done a websearch to see what circuits others have used and their comments. One suggested off times at the vibrator transition points of 10 % to 20%, which sounded like a good idea to clean up the switching transition. Since, by adding another wire, I can have access to the 12 volt battery to power a vibrator circuit, I have more circuit options than having to derive a supply from the transformer terminals I'm switching to ground. I'm thinking of using a 40106 CMOS hex Schmidt trigger as a building block. I have used Schmidt trigger stages in these 40106s in the past in medical product designs (before retiring) to very simply create oscillators and pulse width timing circuits with a minimum of components. With the one Schmidt trigger, I can set my base frequency and then build in the 10% delay windows using two other Schmidt triggers, and, of course, their are 6 Schmidt Triggers in the one IC. I'm thinking I will use 2N3055s for outputs to switch the transformer windings to ground, or, I am open to suggestions of FETS or other bipolar transistors. I'm thinking 2N3055s because I have some.
I'm putting more time into all this than most people would think it is worth, but this kind of stuff is fun for me, especially since I retired and no longer do this for a living.
Further thoughts and critiques are welcome! Also welcome are solid state vibrator designs you or others may have used.
Thanks!
Dave