1948 (TS-4H) Motorola VT71 restored!

[bandersen]

I had been using the crappy JFD metal ballast tube that came with this set, but no more!

I just scored some Amperite ballast tubes



These are glass enclosed balllast/regulator tubes filled with an inert gas. They really soak up the juice when the sets turned on and protect all those series wired tubes.

Technically, this tube is designed for the TS-18 chassis which specifies a 111 and 132 ohm element while this TS-4H calls for 105 ohm elements.

However, I've done the math and here's what I come up with. Yes, the 7JP4 draws 600ma and is in both strings.

    Series Filament #1                 Series Filament #2         
Tube    Type      V      A          Tube    Type     V      A
V-13    25L6      25    0.3         V-12    25L6     25    0.3
V-8     6SL7     6.3    0.3         V-6     6AU6    6.3    0.3
V-10    12SN7   12.6    0.3         V-12    6AG5    6.3    0.3
V-11    12SN7   12.6    0.3         V-17    12AT7   6.3    0.3
V-18    12SN7   12.6    0.3         V-2     6AG5    6.3    0.3
V-15    7JP4     6.3    0.3         V-3     6AG5    6.3    0.3
                75.4                V-4     6AG5    6.3    0.3
                                    V-5     6AU6    6.3    0.3
                                    V-16    6AL5    6.3    0.3
                                    V-15    7JP4    6.3    0.3
                                                   81.7

Assuming 117 VAC at the outlet, here's what I get which is not far off the TS-18 values.

(117 - 75.4) / 0.3 ~ 140 ohm
(117 - 81.7) / 0.3 ~ 120 ohm

That just leaves one issue - the pinout differs by one pin. I just needed to move the AC line from pin 5 to 6. No biggie.



Here it is in the set ready for testing.



Here's a crappy little video show it in action. One of these days I'll hook up my good digital video camera.

http://www.youtube.com/watch?v=HC86eiGhw_o

So, if you can find one, you shouldn't have any trouble using them in a VT71 set.

[zenithfan1]

Looking good! I can't wait until mine come! I hate the crappy metal ones.

[compucat]

I just ordered two of the glass ballasts from Ebay. When looking for these, search for amperite ballast tube. I still may make a resistive replacement as these are hard to find.

[bandersen]

Originally Posted by compucat

I just ordered two of the glass ballasts from Ebay. When looking for these, search for amperite ballast tube. I still may make a resistive replacement as these are hard to find.

They sure are hard to find and I have four sets that need one! I'd been researching technique to make resistive ones for weeks before coming across these and even ordered some resistors. So I make make up one or two as well.

[jeyurkon]

Weren't the original ones that you need the amperite 17A470303? Vacuumtubesinc has two of them.

John

[zenithfan1]

One of mine arrived with a broken element I hope I can get a replacement.

[bandersen]

Originally Posted by jeyurkon

Weren't the original ones that you need the amperite 17A470303? Vacuumtubesinc has two of them.

John

Ah yes, so they do and almost twice what I paid for these

That's why I did the math to see if the '459 was a good substitute. According to my calculations (which have been confirmed by others), this tube is actually better. The two filament strings in the TS-4* chassis are not equal and 105 ohms is a bit too low when using modern line voltages anyway. I'd also noticed that my filaments were running a little hotter than they should.

Now, there a nice even glow with this ballast tube

That being said, I imagine the '303 Amperite glass tube is superior to the '303 JFD metal ballast tube. I'm curious if someone has used the '303 and has any thoughts on it?

[bandersen]

Originally Posted by zenithfan1

One of mine arrived with a broken element I hope I can get a replacement.

Damn, that blows

Looks like he has a few left. I hope you can exchange it for a good one.

Bob

[jeyurkon]

In searching for this particular ballast for the past couple of years all I had ever found were posts from others also looking for them, going back quite a few years.

I'm surprised that the guy still has 8 left. I bought two, one as a spare. It was difficult to resist buying more.

John

[compucat]

Originally Posted by jeyurkon

In searching for this particular ballast for the past couple of years all I had ever found were posts from others also looking for them, going back quite a few years.

I'm surprised that the guy still has 8 left. I bought two, one as a spare. It was difficult to resist buying more.

John

My two 17A485459 ballast tubes arrived today and now my 9VT1 lives again. These glass tubes are so much better. I bought two so I would have a spare. I was tempted to buy more as well but I thought of the many others who have been looking for these. I am thinking of making a resistor replacement, mounting it in the set with an octal plug and if ballast tubes ever go extinct I can just plug in my resistor network and keep the set going. I want to refinish the cabinet, replace the torn grill cloth and add retrace line suppression to my set. It works very well of an Insignia DTV converter.

[jeyurkon]

Another good reason for making a resistor replacement is that if you have a problem with the set or need to do work on it you can test it with the replacement without worrying about blowing the easily destroyed ballast.

John

[Adam]

Ok, you all convinced me to try these. I ordered 2. Now do I understand correctly that these gas filled glass ballasts regulate current as the set comes on, so there would be no need to add a filament transformer to protect the 7JP4? - as I had been planning to do.

I have 3 ts-4j chassis: one just missing the ballast, and one missing the ballast and flyback. I figured I would try these tubes with those chassis (if I can find a flyback for that one). The third chassis was modified already for filament transformers, but whoever did it made a real mess of it with 3 separate filament transformers and a variety of series-parallel circuits. I got that one to work using that scheme eventually after I sorted it out, but it didn't work that well, and all those transformers didn't really fit under the chassis. On that one, given it's already modified, I thought I might as well go with making a ballast from resistors, and adding a filament transformer for the crt. Or maybe even try to make it into a transformer set entirely with all 6v tubes.

[jeyurkon]

Originally Posted by Adam

Ok, you all convinced me to try these. I ordered 2. Now do I understand correctly that these gas filled glass ballasts regulate current as the set comes on, so there would be no need to add a filament transformer to protect the 7JP4? - as I had been planning to do.
...

Based on tests that I did with a ballast that had only one bad filament I would have said no. That they behave just like a resistor. However, I was testing them under steady state conditions and as I raised the current the power dissipation got to a very high level before seeing any glow of the tube. I was afraid the Bakelite base was going to start burning.

Watching Bob's video of the tube as his set warm up changes my mind. The brief high current causes the filament to get hot enough to change resistance significantly and protect the 7JP4. The current when the tubes are cold must be much higher than what I tested the ballast to.

John

[Tubejunke]

[QUOTE=Adam;2907263]I have 3 ts-4j chassis: one just missing the ballast, and one missing the ballast and flyback. I figured I would try these tubes with those chassis (if I can find a flyback for that one). QUOTE]

I still have a small inventory of Thordarson and Merit NOS replacement flybacks and yokes. If you can get a part # cossreference for me I'll se if I have it.

[Adam]

Thanks, but the sams leaves all the cross-reference columns blank for the flyback. So I don't know if they even made a replacement for this one or not.