using DC to power heaters?

[renshen1957]

Something I've heard but never experienced (probably because I haven't been using tubes for 20+ years) is that when using DC heaters, some tubes will develop hot spots in certain areas of the heater when powering with DC. These hot spots will eventually weaken and the heater will fail at that point. Again something I've never experienced but I've heard before. Maybe it's only an issue with thoriated tungsten or barium oxide but I'm not sure as I'm yet to find any reliable sources on this yet.

Also I have a similar notion to Roberto in that DC heaters are a band aid fix to a whole multitude of problems (bad building technique, too much gain too early/or just too much, noisy tubes, non-elevated heaters, etc). I've found in practice that there is NO substitute for a quality layout when it comes to heater noise introduced from lead dress issues. There is also a capacitive coupling effect between the heaters and cathode that the DC-elevated heaters will alleviate. Using purely DC heaters is usually a last ditch effort in my book and IME is not necessary for all but the most demanding applications (in which case, why the hell are you using tubes?!? ).

Hi,

All Chris said and might I add, I have in a special cases added DC voltage regulation to DC powered tube heaters. (Some specialized recording equipment or Hi Fi)

This combination is not necessary for guitar amps in general. One can produce a dead quiet tube amp by all means at your disposal.

I haven't heard of DC heating problems on preamp tubes nor have I heard of DHT power tubes being a problem. In fact it's a solution to how full of hash (noise) the AC power supply has become.

The DHT power triodes is more of a hi fi thing and frankly most guitarist couldn't afford to replace a match pair of 300B power tubes let alone a quad. (People balk at the Psvane EL34 Phillips metal base prices.)

Here is some information on the subject, http://www.meta-gizmo.net/Tri/July/filadht.html .

Quite frankly if I wanted that extreme levels of quiet in heaters (or power supplies), I'd go with active regulation (MOS-Fets). But who would want to pay for such in either a hand made or production amp? The majority of guitarist are going to add vintage FXs (think 4558 IC's) that will add considerable noise (enough to require a noise gate) It's not needed in an amp to be played out.

Best regards

Steve

PS edit PP power tubes do not require DC heaters, and are basically noise canceling. My old fisher x 202 c has dc heaters in the preamp tubes.

[Cliff Schecht]

Hey Cliff, have you tried any of the new production 6SL7's if so what did you think of they.

No I haven't, I still buy really nice old 6SL7's for $3 a pop so why waste money on more expensive new manufacture variants? I'd probably buy the Tung-Sol's if I was going with new manufacture though.

[ampdoc1]

I've wondered why those Peavey amps with DC heaters used series wiring instead of parallel. They weren't dummies, so I suspect there must be a
logical reason.


??????

[Ken Moon]

If I'm going for ultimate silence, I use a separate 6.3V CT heater transformer (I like the Hammond ones with the enclosed windings like the 167N6), elevate to 50V or so, put in a 250R humdinger, and install a series 0.1R resistor on each leg (right next to the first power tube that the heaters are wired to). I usually use the Mills MRA-12 resistors.

All of these concepts are covered on Merlin's Valve Wizard website, except the 0.1R series resistors. These lower the transformer output down to 6.3V or so using transformers that are spec'd for 115VAC input, using today's higher wall voltages, which can reach as high as 125V.

Make sure you follow Merlin's advise on lead dress also (unless you're building a TW replica, where the loopy heater wiring has been proven to work if copied exactly).

[Leo_Gnardo]

I've wondered why those Peavey amps with DC heaters used series wiring instead of parallel. They weren't dummies, so I suspect there must be a logical reason.??????

Got a guess for those 5150, JSX & similar - less current therefore skinnier circuit board traces & less chance of melting a wire in their jumpers between main and preamp boards. The power supply is diametrically opposite the tube pre board in the chassis, so they would have had to put long fat traces if they ran all that distance with a 6.3V supply.

Question for Cliff - where do you find old 6SL7 at $3? Even used old stock. It's been a headache for me to fix B-15's & similar for years. I've even made my own adapters - like GT's Substitubes - so I can use standard pre tubes. What a hassle. Sure would be nice to have the authentic item. That is, if you would be kind enough to let me know. Thanx! LG

[Cliff Schecht]

If I'm going for ultimate silence, I use a separate 6.3V CT heater transformer (I like the Hammond ones with the enclosed windings like the 167N6), elevate to 50V or so, put in a 250R humdinger, and install a series 0.1R resistor on each leg (right next to the first power tube that the heaters are wired to). I usually use the Mills MRA-12 resistors.

All of these concepts are covered on Merlin's Valve Wizard website, except the 0.1R series resistors. These lower the transformer output down to 6.3V or so using transformers that are spec'd for 115VAC input, using today's higher wall voltages, which can reach as high as 125V.

Make sure you follow Merlin's advise on lead dress also (unless you're building a TW replica, where the loopy heater wiring has been proven to work if copied exactly).

Ahh yes, I also run my heater wires in a very specific way. I'll try to get a pic tomorrow. It's hard to describe without a picture..

[Ken Moon]

I've had good luck doing mine this way. It's not exactly like Merlin's "up the middle" approach on the preamp tubes, and I have to mount the tubes in the right orientation to do it this way, but I've never had a problem with grund loops or noise that supposedly can happen by wiring the preamp tubes from the outside, like I do.

I also like to use 2 shades of red wire that are very close, just so I can keep track more easily of which wire is which, but from a distance, it looks like I did it all with one color

[img519]http://i514.photobucket.com/albums/t346 ... eaters.jpg[/img]