Being new to this hobby I have a very basic question. I am checking the wiring to a 6U5 eye tube in an AWA model 163 radio. The circuit can be found on this link: http://www.kevinchant.com/uploads/7/1/0/8/7108231/52_163.pdf.

I know that pins 1 and 6 are the heater pins and they operate on 6.3VAC. Why do you get continuity from both pins to ground? I don't understand why both pins get connected to ground? I checked the heater pins of the other tubes in the chassis and safe thing. Why is this? Doesn't this mean that there is 6.3V running through the chassis?

My apologies if this is a very basic question.

Finally, can anyone tell me if a 6E5 tube is a good substitute for the 6U5/6G5. I seem to find contradictory info on the US Radio forum site - may have something to do with the voltage requirements for the two different tubes. The 6U5 is much more expensive and hard to find. There are several 6E5's around. The one I have has a broken internal heater filament.
Regards, Angelo

Hi Angelo
One side of the 6.3V heater winding on the transformer is connected to ground, so only one wire needs to be run from the transformer to each valve heater. The other heater pin is also connected to ground, completing the circuit. This is quite normal.
The reason both pins appear to be connected to the chassis ground is that the DC resistance of the transformer winding will be very low and in parallel with all the valve heaters.
Regards
Harold

The 6E5 is more sensitive than the 6U5/6G5, so would fully open with a smaller signal. Have a look at

http://www.magiceyetubes.com/.

Substitutions page

"The 6G5, 6H5, 6U5, CV504, and VT98 are exact substitutes. Replacing with a 6E5 would result in higher sensitivity and may close even with weak signals. Adjustment may be necessary. "

Here is a site describing how to substitute the cheaper 1629 (12v filament) for 6U5 & 6E5 (6V filament) using a voltage doubler circuit.
http://www.nostalgiaair.org/References/Articles/TheFlash/Flash04.htm.

Just been through that with a Bandmaster D184E that had a 6G5 (faulty) instead of 6E5.

6E5 is a much more sensitive tube and requires -7.5 V on its grid vs -22V for the others. To get zero angle. Adjustment is required unless you use the correct part.

It is quite normal for one of the heater pins to be grounded. It is a seperate circuit by way of isolation due to the transformer. Resistance will be low due to the transformer winding, all the 6.3V heaters, & globes being in parallel.

On most of the Indicators there is a 1Meg resistor on the triode plate. It is nearly always faulty.

There is an article on them in the current (Dec) "Antique Radio Classified" (Port Washington) http://www.antiqueradio.com/.

Also refer http://www.magiceyetubes.com/.

Marc

Thanks for the explanations. I thought there was a risk of a shock because the heater current was running through the chassis. I am assuming the heater pins of all tubes are wired this way? Is this why the schematic shows two arrows going down from each tube - is that to ground?

You would be pushing your luck, or have jumped out of the shower to get a kick out of 6.3V. Although several have died from licking 9 V batteries??????

The filament wires of a 5Y3 and it's cousins, are a different matter, and carry not only the 5 VAC, but have all the B+ DC that can be mustered, on them.

Emphasised in the Bandmaster, whose filament winding shorted the DC, due to a large paper filter cap in a metal box, crushing it's insulation.

Only for the fact that I was monotoring the HT and had one hand on the kill switch, did it not cook the transformer.

It was playing up beforehand & was put onto my isolation transformer, as it has a kill switch, just for things like that.

Marc

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Something I missed:-- With heater tubes they normally wander along either with one wire grounded, or both floating.

It really is a waste & clutter to put heater wires in. Unless there is something odd, like multiple voltages. or AC filaments.

Eg Midwest with four 45's with 2.5V filaments, a 2A5, 2.5V heater tube. A 5Z3. and the other ten are 6.3V

One of the crankiest radio's I have ever worked on. I would like to think drastic measures were taken so he who "designed" it (sic), could never make the same mistakes again.

Marc