understanding tube heaters

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understanding tube heaters

Postby johnnyrocket » Mon Nov 16, 2015 12:17 am

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?

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Re: understanding tube heaters

Postby Geek » Mon Nov 16, 2015 5:07 am

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.
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Re: understanding tube heaters

Postby johnnyrocket » Mon Nov 16, 2015 12:30 pm

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.
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Re: understanding tube heaters

Postby dcriner » Mon Nov 16, 2015 2:25 pm

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

Postby Geek » Mon Nov 16, 2015 8:12 pm

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!
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Re: understanding tube heaters

Postby johnnyrocket » Tue Nov 17, 2015 8:50 pm

Thanks for everyones help! I think I understand now.
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