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Hey guys. I'm trying to build a headphone amp using the Aikido topology. In reviewing the schematic I'm not understanding heater power or heater connections. I learn better visually so I redrew the schematic showing all the pins for each tube.

So you can see that the B+ is 24v and comes in to some symbols which I'm assuming are the heater connections. I've circled them in red. Are these the heater connections? If so, which pins do I use for connecting, pin 4 or pin 5?

Also, in reviewing the heater requirements for the 6GM8 we need 6.3 volts. But the incoming B+ is 24v. How are we getting 6.3 out of this to the heaters?

Hi,

The heaters are in series, so 6.3 x 4 = 25.2V. 24V is well within the 5% tolerance specified for the tube heater.

Hmm, ok. Am I assuming correctly that the heaters can be thought of as resistors. So, resistors in series will have different voltages at each intersecting point.

Won't each heater then have a different voltage less than 6.3?
Or do I have this wrong.

It's the voltage DROP across each heater that is relevant. Four identical resistors in series will each have the same voltage drop.

Here's the secret... as long as ALL of the heaters in the string are designed to draw the same current, the voltage drop will be appropriate

You see in TV sets of yesteryear full of tubes, the heaters are all strung in a string and run from the AC line. The horizontal output was often a 33-40 volt (21V for a typical B&W unit) thing going all the way down to the 2-volt mini in the UHF tuner. Lots of 5-17 volt tubes in between. The secret was they all drew the same current at their rated voltages and the total tube voltage string always ended up at 117-120V.

The odd time they had a tube in there that drew less, usually a field-replacement, so the technician paralleled a resistor across it so that tube appeared to daw the same as the rest.

Cheers!

Thanks for everyones help! I think I understand now.