Question about tube heaters ???

[mountianjustice]

   I would like to know if a hotter heater sounds better than one that is barely glowing? As in if running just the heaters at 12vdc or 9vdc sounds better the plates are running on a separate supply.

[amptramp]

There is information here:

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/dht.html

on directly heated triodes but the claim is made that other than the type 27 (an old tube from the 1920's), indirectly heated cathodes do not show the same changes in characteristics.  I do have an amp where a tone control stage is run at 5 volts on the heater of a 12AX7 with the heaters in parallel so the nominal voltage would be 6.3 volts.  Supposedly, this reduces the distortion by going into the temperature limited rather than space charge limited part of the tube characteristics.

[Transmogrifox]

Probably the best way to answer that question is to make a variable power supply that you can tweak with a pot and see if changing heater voltages in your particular circuit has any drastic effect.

Generally going too low or to high on heaters reduces the life of the tube, so if you value your tubes it's best to stick really near the rated voltage unless you have a reason to change it, and you have proven to yourself that a different heater voltage does something to make your particular circuit sound better.

Also, you may be able to get to the same end as for the sound by measuring the bias shift at a certain heater voltage, then re-bias your tubes to land in the same spot as you did with the heater voltage that sounded best.  It may be as simple as that.

There is no doubt that what you supply to the heaters will have an effect on how the tube performs, but whether that is better or worse from a distortion or clean amplification perspective needs to be tested with a specific circuit.

See the "inherent truths" thread, and I think you will see that most experienced DIY'ers in this forum seem to have come to the conclusion that tweaking subtle things usually ends up in a lot of time spent coming to the conclusion that the sound you want comes from tweaking R's and C's in the pre and post EQ/tone circuits.  To me messing with heater voltages is a similar pursuit as changing between Ge and Si diodes of different sorts, where the same effect could have been obtained simply turning the drive pot and output volume pot to compensate the level change -- but no drastically awesome change in sound, and nothing that would make somebody in the audience think "oohhh, stunning guitar tone!"

That to say, I wouldn't take that as a reason not to play with it -- in fact what I am saying is DO experiment with it, but I don't think anybody here can tell you whether starved heaters sound better in your circuit.  We can only tell you that it will make some kind of difference, but it may be possible to obtain the same effect in a different way than reducing the life of your tubes.

[mountianjustice]

   Thanks Transmogrifox great info. I never though of a pot on the heaters to see where they sound best. I do have a pot for cathode bias, so one more cant hurt right. As long as there is no magic smoke.

[Transmogrifox]

As long as there is no magic smoke.

With tubes there isn't likely to be any magic smoke.  It will come to a magic nothing if you get it too hot.

You might get smoke out of your pot if power rating on the pot is too low.  Remember, you're going to supply power to heaters, so anything you take from the heaters in the tube makes a heater outside the tube.  If using a linear supply (simple pot, or pot on transistor) you should expect to burn off about 1 Watt per tube you supply (assuming the 12ba6 which I am guessing since you have other threads referring to the triple ba6tard and these tubes).  The ideal would be a pot with the wiper connected to a transistor in a TO-220 package with the tab screwed down to a piece of metal for a heat sink.  Especially if you're regulating down from the 18V supply you were talking about in another thread -- in that case you probably want one TO-220 packaged transistor per tube heater, then just wire the pot wiper to all 3 Gates/Bases (FET or BJT should work fine in this application).

Without a heat sink of some sort this will probably get to "burn your finger" hot.  Might get hot enough to smoke.