I had two much-awaited parcels arrive today by USPS — my three new tubes for the Mon-Key electronic keyer project I'm working on.
Because the filaments in the three tubes in the Mon-Key aren't lighting, I assumed it was a case of an open filament. I had checked the continuity of the line cord, and it was good … or was it?
Until I looked over the schematic again (and did more research on resistive line cords), I didn't realize the line cord is actually a three-conductor cord. The hot side goes to the on/off switch, the other side goes to the plate of the rectifier tube. The third conductor I hadn't figured on was the resistor element in the line cord itself.
The resistor element is simply insulated nicrome wire that heats up to dissipate some of the line voltage. The Mon-Key's filament voltages add up to approximately 60 volts, so the line cord has to drop about that much voltage on its own. These cords were called “curtain burners” by servicemen. They were used at one time in the 5-tube classic AM radio until they finally started manufacturing tubes with filament voltages that added up to the 120 volts (by doing so they could manufacture a radio without the need for a power supply transformer. The majority of tabletop AM radios from the postwar period thru the late 1960s were of similar ilk).
Today I again fired up the Mon-Key and found that the resistive element is open in the cord. My continuity checks confirmed that the resistive element was open.
How do you replace a resistive line cord? Simple answer — you can't. I've been researching this issue, and they don't have replacements for these cords. Using the line cord to drop line voltages wasn't that safe an idea 60 years ago. When it opens up, you're out of luck. There is a work around, and I'm going to try this.
One Web site by a guy who restores the classic 'All American” 5 tube rigs says you can replace the cord and insert a rectifier diode — a 1N4004 or 1N4005 in the line. The diode serves as a half-wave rectifier and cut the voltage essentially in half, or close to the 60 VAC the tube filaments require.
Another way is to use high wattage resistor(s) to drop the voltage. This generates a lot of heat, and isn't the best solution — though this essentially is what a ballast tube is in the old AC/DC radios. These were designed specifically to generate heat and drop the line voltage, and were a safer way to do it.
Another Web site by a guy who restores old broadcast radios said the best cure for a bad resistive cord is to pick another radio to restore. Resistive cords and ballast tubes are inferior designs, says he.
Since the Mon-Key doesn't have 5 tubes like the old All-American, there's no way it could have easily dropped the voltage without adding a transformer. I'm going to research the rectifier method, though another site suggests the resultage voltage (RMS) is closer to 85 voltes. My filaments may also light the room at this voltage.
It may be a case of simply trying it and standing back. I'm going to drop in the new filter caps before I do this, its probably a safe bet the orginals are shot, or the application of voltage will make them explode.
There's another alternative I could try — a variac. I could simply use the variac to drop the AC voltage to an acceptable level. Not exactly the preferred fix, but it would work. If I ever forgot to run it on the variac, it would make lots of smoke.
With Field Day preparations under way, I suspect this will be a project for the first of next week.
G'nite to all, and 73.