One 3TF7 is used in the R-390A. It is a current regulator or "ballast" tube.
The Amperite part number for the ballast tube is 3TF7.
Amperite also carries an equivalent, the TJ311M01.
- RT510 is located on the IF subassembly, where it provides a regulated
6 volt filament voltage to V505 and V701, which are wired in series. RT510
is fed from the 26.5 volt filament supply.
RT510 appears in figure 77 of TM-11-856 and in figure 85 of TM-11-5820-358-35.
Of all of the tubes in the R-390A, the ballast tube has garnered the most
controversy, discussion and creativity regarding a replacement. Replacement tubes
cost between $15 and $25, generally.
Some replacement suggestions that have been tried:
Nolan Lee provided some additional background on the 3TF7:
- Jumper pins 2 and 7 of RT510 with a piece of wire, Replace V505
(The BFO oscillator tube) and
V701 (the PTO oscillator tube) with 12BA6 tubes.
- Jumper RT510 with a resistor of 40-50 ohms rated at 5-10 watts
- Install a 3-terminal Solid state voltage regulator to provide 12.6 volts
- Rewire the sockets of V505 and V701 to use the 6 volt filament string
available to other tubes.
"OK, after listening to all of the hype and BS about the ballast
tubes in the R390A, I figured I'd research it a bit an post my
findings. Put your boots on bubba, it's gonna get deep...
If one of you guys is saving stuff for an R390A FAQ, the info below
would go well in it.
Digging thru a 1982 Amperite AM-82 application guide, I found a few
interesting things that I'll pass on to you guys. If you deal with
a distributor that handles Amperite, get them to get you a copy,
it's an interesting book.
The resistance wire is usually iron, and the glass envelope is
filled with either hydrogen or helium gas for heat conductivity.
The glass envelope runs about 160 degrees. Since I'm one of those
people that refuses to use the metric system, you know WHICH
160 degrees I'm talking about. It ain't Kelvin either.
One of the posts I read today mentioned a shelf
life with ballast tubes. I suspect that it's related to ballast
tubes that use helium as the filler gas. Helium is famous for
it's ability to pass thru the wall of sealed steel high pressure
cylinders. I ain't no engineer or chemist but have had some
experience with high pressure gases and have see firsthand that
helium will "disappear" from sealed bottles. If I'm not mistaken,
the 3TF7 ballast tube is filled with hydrogen rather than helium.
OK, back to my original post...
Current regulation is usually within plus or minus 1%.
They work with either AC, DC, or pulsating current.
When the current in the circuit is increased to a high enough
level for the regulating function to start working, only a
small portion of the filament will glow. As the voltage across
the ballast increases, more and more of the filament will glow.
When the entire filament is glowing, you're at "max" and any
additional increase will overheat the tube and shorten it's life.
The rated life expectancy when operated as recommended within it's
ratings is 2000 hours. Run it at "max" all of the time and it's
only 1000 hours. Run it at 80% of max and it's 5000 hours.
Here's a direct quote from Amperite AM-82 that you'll really
DUTY CYCLE DEPENDANT
If a steady voltage of a value in the middle of the operating range is
applied to the tube continuously, it's life will be tens of thousands
of hours. Opening and closing the circuit with the resulting expanding
and contracting of the filament greatly reduces the life of the tube.
Also, as in incandescent lamps, turning the unit on and off many times
will reduce it's life especially if the unit if operated near it's
maximum voltage. If full voltage is applied to the tube, the circuit
may be opened and closed only a few hundred times before the current
is outside of the limits or the filament is burned out. Thus the life
of the tube will be determined entirely by it's duty cycle.
I figure that over the last 23+ years that I've had the old Collins,
it's been on for "24 and 7" for at least 15 of those years. 15 years
is 131,400 hours. That original 3TF7 is still going just fine. I'm
not saying that it won't puke when I finish the overhaul of the
receiver and power it up, but even if it did, it gave pretty damn
I finished my OH of my 67 EAC back in the middle
of October of 1998. It's been running 24 hours a day and seven days
a week since then. That's about 18 and a half months or more than
13,300 hours on the very same ballast tube that was installed in
it when it was assembled back in 1968. If the gas hasn't leaked out
yet, I suspect that it won't. Back to my original post...
The folks at Amperite that I've dealt with have been a hell of a
nice bunch. I needed some information on some odd "non standard"
numbered ballast tubes. They transferred me to an engineer and I
received all of the answers that I needed. Very sharp and friendly
bunch of people.
For what it's worth, there's another part number for the 3TF7
that was used for tubes that had different testing requirements
than the standard mil-spec and was for a Govt contract in 1978,
and not for civilian or commercial sales. After I corner the market
on them I'll post the number. Just joking...a friend of
mine found a stash of them and sent me three of them last week or
so to research and experiment with. After talking to the engineer
at Amperite a few hours ago, there's no need to experiment. I now
know exactly what they are.
The end flap of the boxes is labeled as follows:
The side panel is labeled as follows:
Resistor Current Regulating
The tubes themselves are labeled as follows:
(circled Amperite "A" with lightening bolt)
So, if you spot any of these TJ311M01 marked ballast tubes, grab
a few, they'll work just fine in your R390A."