Ge 12t4

[cameronflyer]

I have been working on this GE 12T4 of an on for several months now. I am almost ready to fire her up for the first time. I have ordered a replacement 6sq7 for one with a cathode short.it should arrive this week.
I have one question though? The Sam's I have has some rather glaring mistakes. There are 2 , 10watt resistors inside the high voltage cage. The Sam's list them as 75ohm but both read 2.5k. There condition is far to bad to try and risk using them as is.
Does anyone have a set like this, or a different schematic to help identify the correct value of these resistors? Any help would be appreciated. Thanks

[Kevin Kuehn]

I believe they are in series with tube filament strings? Riders lists them as R454,R455 - Glowbar, 75 ohm, GE #RRW041. So they are thermistors and will drop in resistance as they warm up. 75 ohm would be their hot rating.

[Kevin Kuehn]

oops double posted.

[Kevin Kuehn]

I have a NOS Workman 041, 75 ohm Glowbar that is listed as replacement for GE RRW041. This one only reads around 850 ohms cold.

[cameronflyer]

Looking at the Sams thermistor makes sense. I wish the parts list , listed them that way. I don't encounter thermistors looking like that in my day to day work. I wonder if I could replace them with 2, 75ohm 10 watt resistors and 1 CL90 thermistor?

[Username1]

Well, if they are to be 75 ohms, and they read 2.5K ohms, and they are in series
with the filaments, then the current will be very low, and should not be a problem
at all if you fire it up..... tubes will not lite.... If it's a thermister and they drop
like said, when they get hot, then at 2.5k they are a pretty good safety cushion
for the tubes.... Better than 850..... might be smart to leave them alone.....?

Looks like a tv I just got done working on for my brother.... Bad electrolytics,
I saw those 2 resistors in the string, didn't even give them a thought....
Filaments come on slow, so I guess it's working as it should.....

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[Kevin Kuehn]

Yeah, I think I'd give them a try as is. Then verify some tube filament voltages after they warm up. I'm not sure if the CL90/resistor combination would give you anywhere near the desired warm up delay.

[Kevin Kuehn]

Just for fun here's a picture of the Workman sub, looks like a dead ringer for those in your set. I'm not sure how these hold up with age setting on the self. This must be pretty old stock, considering it has the GE number, as listed in Riders. You can have this one, but who knows how hard it will be to find another?

[cameronflyer]

Originally Posted by Kevin Kuehn

Just for fun here's a picture of the Workman sub, looks like a dead ringer for those in your set. I'm not sure how these hold up with age setting on the self. This must be pretty old stock, considering it has the GE number, as listed in Riders. You can have this one, but who knows how hard it will be to find another?

Thanks for the offer, but you are correct it would be very difficult to find a second one.

[Gregb]

When I checked mine they read 400 ohms each when cold.

Gregb

[cameronflyer]

I fired her up for the first time tonight. She has a pretty good picture considering I only re-capped her and replaced the seleniums. She doesn't however have any sound yet and one of the 2 thermistor is much slower than the other so half the tubes take a minute longer to come up to temperature. I will get back on it in the AM and start on the sound issue as well as try to come up with a thermistor solution.

[Electronic M]

Why not get kevin's thermistor and replace the slower one with it?

[cameronflyer]

Originally Posted by Electronic M

Why not get kevin's thermistor and replace the slower one with it?

Believe it or not I repaired the thermistors. I remembered back in my early days in the TV repair business we used to use a high tin solder to repair the leads on thermistors. I heated them to above 500 degrees with a heat gun and re-soldered the leads to restore good contact. They now work perfectly!

[cameronflyer]

I finally got the sound going. The alignment is just about perfect. A little tweaking and I can start on the cabinet.

http://youtu.be/u2wkcm1b58E

[Username1]

That's the way ! Good Job !

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