Is that still a problem? Back in the early 90's I heard that they were hard to find so if you blew one, you had a problem. But with all the R-390a comments I've read in modern times I haven't heard a thing about them.
Printable View
Is that still a problem? Back in the early 90's I heard that they were hard to find so if you blew one, you had a problem. But with all the R-390a comments I've read in modern times I haven't heard a thing about them.
Got a tube number?Quote:
Originally Posted by W1GUH
I may have one or two spares.
Thanks, but I don't need one. This was a curiosity question. That tube had been a problem and since I haven't heard anything about it I was wondering if it was still a problem. Don't have an R-390a -- don't have the room. Pity -- they're great pieces of equipment.
There are a couple friends in the immediate area who own them. I'll run this question by the gang next time we're on 10M SSB or 2M FM simplex.Quote:
Originally Posted by W1GUH
Thanks!
Hello.
I decided to study what the ballast tube does.
It acts as a light bulb, much as the indicator lamp on a 5 tube AC/DC radio.
On a normal light bulb the resistance goes up as the voltage increases, nothing wrong with that, except that a light bulb has a very low resistance when cold and thus a very high inrush current.
Replacing the ballast tube with a constant current regulator will not only serve the function of the ballast tube but also provide a soft start function, extending the life of the tube lineup.
The 3TF7 was lousy at it's job but there was nothing else at the time available. In one of the older issues of Electric Radio is an article on building a little voltage regulator/current limiter that does a marvelous job. There is also an eBay seller in Germany (I think?) that offers a plugin replacement for the 3TF7, I don't know if it does both functions or if it is only a voltage regualtor?
A lot of people use the 42 ohm resistor fix from "pearls" and I've also seen even more with just a wire strap from pin 2 to 7 of the socket. In field use running on portable generators the regulator is badly needed but it's questionable if it's needed at all with the normal stabile AC voltages we have today.
Shooting for the longest possible tube life, I use the Electric Radio voltage regualtor/current limiter setup.