Sound Discriminator Question

[Cory]

Hey Guys,

I'm working on an RCA 8T241 and have most of the problems ironed out but have an interesting question. It uses a 6AL5 as the sound discr, with a 5.1 ohm resistor in series with the filament. If I jump the resistor I get good sound and the plate voltages are where they should be, if not, all that passes to the output is a very quiet hum.

The resistor checks spot on, and I've swapped a couple 6AL5s thinking heater characteristics may vary. Still, what I find is that the majority of the heater voltage is dropped across that resistor and the tube doesn't warm up enough to conduct.

The easy thing to do would be eliminate that resistor but I'm pretty sure it's there for a reason Any suggestions?

Cory

[Eric H]

Cory, I don't know the reason for the resistor but I can't see the harm in giving a 6.3 volt tube 6.3 volts?

Does this resistor show up in the schematic?

I'll have a look at this 241 chassis I have here and see if it uses a resistor.

Eric

[ChuckA]

Cory,

What voltage drop to you measure across the resistor? The 6AL5 draws .3 amps @6.3v so you should be dropping about 1.5v across the resistor. Is your filament voltage correct?

Why it's there I can only guess, there is also a small resistor in the filament lead of the horizontal sync discriminator 6AL5.

There is a filament spec for the 6AL5 with the cathodes at equal potential, maybe because they are not in this circuit the designers lowered the filament voltage. (??)

Chuck

[Cory]

Well...problem solved

I must have had too many Smirnoffs last night as that 5.1 ohm resistor just checked open. Doh!

A subbed a 3.9 I had on hand and figure that's close enough.

Sometimes it's the easy ones that get you. Nothing compared to last night when I had to fix an open outer winding in the yoke

But I'll say, the picture sure looks good now and it even gave me an excuse to use my tele-check.

Thanks guys!

Cory

[ChuckA]

Cory,

Don't feel to bad, I had the same type problem with an RCA 8TV41 that drove me crazy. The resistor in the horiz sync 6AL5 was open and because that tube is inside the HV cage I didn't see it wasn't lit. Worked for hours on no horiz sync problem, until I finally started taking everything apart and saw the tube was dark.

Chuck