AC filament supply?

[Enzo]

And with four or five 12 volt tubes, you could probably use the same winding as the bias supply.

[d95err]

It seems that DC heaters often creates more problems than it solves, at least for fairly inexperienced builders.

And so do reversed phase OT leads, grounding of all sorts, improper bias circuits, bad layout, bad lead dress, backwards lytics, etc.

What I mean is that lots of newbies (I'm quite a newbie myself) tend to mess things up with DC heaters when trying to fix something that wasn't really broken to begin with.

If the amp hums, it's probably better to start checking the basics like grounding and lead dress. Once you've corrected these basic problems you could start looking at DC heaters. (Heaters on PCB traces being the exception of course, as discussed earlier in this thread.)

[Enzo]

Well, I certainly will grant that a lot of people enter into something with the idea it needs to be redesigned instead of simply fixed. But i am thinking more in terms of designing DC heaters into an amp to further hum reduction goals as opposed to trying to fix new-found hum by adding it retroactively, or to cover over bad design.

[cminor9]

Wow, that generated an interesting discussion. Thanks for all the insight, guys!

[6267]

be careful how you rectify and filter--it's easy to get more than 6.3v DC after it's been through some diodes and a cap! I do have an amp that I recovered from the junkyard that's got the heaters run DC off the bias tap. It worked fairly well, and I always wondered if there was some interaction there that gave it such good sustain. One other thing to worry about with DC though is long-term filament damage, I've heard that the DC doesn't heat the wire as evenly. The only filament I have ever lost was on a DC heated amp (anecdotes!).

[jrfrond]

be careful how you rectify and filter--it's easy to get more than 6.3v DC after it's been through some diodes and a cap! I do have an amp that I recovered from the junkyard that's got the heaters run DC off the bias tap. It worked fairly well, and I always wondered if there was some interaction there that gave it such good sustain. One other thing to worry about with DC though is long-term filament damage, I've heard that the DC doesn't heat the wire as evenly. The only filament I have ever lost was on a DC heated amp (anecdotes!).

I've always questioned the effect of DC on heaters. Because it is a constant level, and there is no zero-crossing as the sine wave shifts polarity, I've theorized that DC might wear out heaters at a faster rate. Of course, I might be full of hot air, but I am just thinking about basic electrical principles.

[6267]

I have heard exactly that, although it was something about the constant polarity causing the heater filament's molecules to migrate toward one end of the filament, causing weak spots...it might be a directly heated thing.

[kg]

I've always questioned the effect of DC on heaters. Because it is a constant level, and there is no zero-crossing as the sine wave shifts polarity, I've theorized that DC might wear out heaters at a faster rate.

sure, there's no zero crossing, but there's also a higher ac voltage impressed across the filament, which is theoretically just as damaging to longevity (if not more).

in general the warnings about dc heating and tube life DO NOT apply to IDH tubes with cathodes. with DH filament tubes, there can be premature "wear," especially to those with oxide coatings, since one end of the filament will be emitting a greater proportion of the total cathode current than the other end. this is because the filament itself has a voltage drop across it which effectively varies Vgk along its length. this can be offset by periodically reversing filament polarity.

there also seems to be a sonic penalty associated with DC filaments, something i tossed around with steve bench a few years ago. a portion of that conversation can be read here.

ken