[Radioroslyn]

Was looking over the diagram for my 116X and noticed that it uses a hum balance pot to null the 60cy AC on the directly heated output tubes (6A3's). This is a common practice with audio tubes that don't have a cathode. But wait. One of the 6A3's is tied across the pot and the driver tube heater and the RF tubes. The other 6A3 filament is tied across the IF amps,det,and 1st audio. It's filament winding has a ct which is grounded. Pretty Odd!!

http://www.nostalgiaair.org/PagesByModel...013123.pdf
Terry

[codefox1]

Works does, sort of by phase.

[Mondial]

As was mentioned, the hum balance pot nulls the 60 cycle component in the output caused by the AC on the directly heated filament of the output tubes. When properly adjusted, the in phase hum components of each output tube cancel in the push pull output circuit, which only responds to signals 180 degrees out of phase.

I guess to save money, Philco did not use separate windings for the output tubes alone, but combined the heater load of the other tubes, putting half of the total filaments on each output tube filament winding. Since the rest of the tubes have indirectly heated cathodes, they are not affected by the phase of the heater AC set by the balance pot.To the remaining tubes on the winding with the balance pot, it looks just as if they were fed by a center tapped filament winding.

[ipwizard]

The hum pot seems to work fine in the circuit. The 116x I am working on now had a little hum after the recap and adjusting the hum pot made the hum all but disappear.

[Radioroslyn]

Mondial
The pot is only across one of the 6a3's the other has a grounded ct.
Was just thinking that it would be cheaper to just have one 6.3v winding with the pot across it than two. I don't think that filament and the heater tube would interfere with each other. Just seems counter intuitive.
Terry

[Mondial]

I think you are better able to compensate for mismatch in output tube characteristics with two individual filament windings. If the two 6A3's were perfectly matched, then a single winding with a hum balance pot would be fine. The pot just serves to find the exact electrical center point of the AC winding for balance.

But if the tubes characteristics differ slightly, you may be able to get a better hum cancellation with individual windings. Effectively each has its own center tapped filament supply, one being variable to achieve exact balance and hum cancellation.

Too bad Philco didn't use 2A3's instead of 6A3's. The lower the filament voltage the less hum induced in the output, but then they would need a separate 2.5V winding.

[Radioroslyn]

I guess if I was really nutz I would just pop in a si diode & cap and run the filaments on dc. By 1935 the older 2.5 volt tubes where falling out of favor for the 6v jobs you could run off of a car battery.
Tnx for all your comments.
Terry

[Radioroslyn]

Mondial
Not trying to belabor the point but would there be a difference between the 2A3 and the 6A3? As they both require about the same amount of power the operate the filament. 2.5v at a higher current or 6.3v at a lower current. I'm not much of a rocket scientist, some times the simplest things stump me!!
Terry

[Mondial]

Hi Terry,

Yes, there is a subtle difference in operation between the 2A3 and the 6A3. Although as you say, the filament power used is the same (6.25 watts) the voltage along the length of the filament differs more than twice as much with the 6A3. Its a little bit difficult to explain but I will give it a try.


First think about a tube with an indirectly heated cathode with a heater inside. The voltage along the length of the cathode is the same at any point, which is why it is referred to as a unipotential cathode There is no change in cathode voltage caused by the heater voltage, the filament just raises the temperature and nothing else.

Now take the directly heated filament of the 6A3. Lets assume that we are using DC to light the filament, which in this case is also the cathode. Now we have a cathode which is not unipotential, the voltage varies across its length from 0 to +6.3 v. So at one end of the cathode you have 0 V and at the opposite end you have +6.3 V. This affects the bias of the tube since you effectively have 6.3 V more bias at the positive end of the filament than the negative end. So one side of the tube conducts more than the other. The usual bias of a 6A3 is about 45 v, so 6.3 is a significant difference. With a 2A3, the filament voltage and difference in bias is only 2.5 v and therefore has much less of an effect.

Now consider what happens when you light the filament with AC. Instead of the filament always being 0 and 6.3. the polarity reverses 60 times per second. So there is an AC component of the bias which creates the hum in the output. If you provide the ground return for the filament at the exact center tap point of the filament supply, the AC variation is minimized, now being + and - 3.15 v and it somewhat balances out. But the lower the filament voltage, the lower the AC variation and the resulting hum. So the 2A3 will always have a lower hum output than a 6A3. The lower the directly heated filament voltage, the closer you get to the ideal case of the unipotential cathode of a indirectly heated tube.

Sylvania, the creator of the 6A3 and the later octal equivalent 6B4G, must have realized the problem because they finally came up with the 6A5G. This tube is built just like its predecessors, but has tiny isolated cathodes built around the filament, which are connected to one of the filament pins. By grounding this filament terminal, there is no voltage difference across the cathode, no AC and therefore no hum. But by the time this tube came out, the 6L6 had already been developed which became much more popular as a high power output tube.

Mike, WA2YGA

[Radioroslyn]

Tnx Mike
For your in depth description of how that all works! Didn't know about the 6A5G very interesting. Get on the air much? I haven't. Going to be fixing up a 40m and 10m dipole soon. Looking for a deal on some RG-58X
Terry N3GTE

[Mondial]

No, have not been on the air lately either. My 80M dipole came down a while back and haven't had a chance to put it back up.

The 6A5G is a really interesting tube and was the ultimate audio power triode design. Hi Fi amps were still being made with it into the 1950's. Here is a link to the data sheet:

http://www.bunkerofdoom.com/tubes/syl43/...43/p71.gif

[chris25260880]

A hum null potentiometer is still very commonly used on high power
tubes used in FM Broadcast transmitters though disappearing fast as
solid state transmitters are now efficient and cost effective up to
the 50 KW AM and 40 KW FM sizes