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.