Anyone ever gain anything from running amp filaments from DC?

[eleventeen]

I've often wondered if hum could be reduced by using a 6.3 VDC regulated supply inside a Fender (or other) tube amp. Anybody ever done this to advantage? This wouldn't exactly be a walk in the park, in a Twin, say, you'd need maybe 6 amps of filament current. That implies a fair sized heat sink, in a non-generous amount of space, already running pretty warm/hot with poor ventilation for components mounted under the chassis. As for format, I'm imagining a conventional simple solid-state regulated supply using a 2N3055 (or equiv) pass element. If you went to a solid state rectifier with a stock PT, theoretically you could series-connect the 6.3 volt and 5 volt windings to give an AC input to a bridge rectifier of nearly 12 volts (no load) but the 5 volt winding couldn't take any kind of 6 amps. So can't go that way. So the implication would be that an add'l or auxiliary filament transformer would be required. Better, I suppose one could more intelligently use a Condor or a Power-One open-frame 5 volt logic-type power supply but you'd have to defeat the OVP and quite possibly the range could not be easily tweaked to 6.3 volts. The whole thing as a thought exercise rapidly approaches mechanical ugliness.

I would anticipate the consensus answer would be: Not worth the considerable trouble; Plus, you're just as likely to induce & thus add to hum (via ground loop) as you are reducing hum by balancing the two AC legs of a conventional filament supply.

Anyway, just asking if anyone has done this. The idea quickly gets to be more than what would appear to be worth it for the potential gain.

[FYL]

DC heaters can be useful with some non-humbucking tubes - the 6Sx7 family for instance - or in high gain amps using 3 or more cascaded stages. But only at the preamp level, and only with an optimized/regulated DC supply.

In most cases heater elevation or virtual ground, proper phasing, good layout/lead dress and tightly twisted heater wires can give as good or sometimes better results, so it's not really worth the bother.

BTW, going DC for power tubes heaters in a PP amp is useless: the two legs are out of phase and any common mode induced hum will cancel thru the PT.

[tubeswell]

FWIW several PV amps use DC heater pre-amp rail. You need to tote up all the voltages but the current draw stays the same for the whole lot.  Say you have 3 x 12AX7 in series - that's 12.6 x 3 = 37.8VDC @ 150mA, which you can get off a 26.7VAC - 0 winding with a SS bridge rectifier. You can then use that same winding to drive 4 x EL84s in series straight off the winding (6.3VAC x 4 = 25.2VAC give or take @ 750mA), so in that case you'd make your winding about 26VAC @ 1A minimum. You'd then probably run a mains voltage lamp off the mains.

[RicharD]

DC filaments are almost never seen in guitar amps and are seldom the cause of hum unless there is a layout or grounding problem.  The $9.99 test is to disconnect the heaters from the PT filament winding and temp in a 6V lantern battery.

[alerich]

My Marshall Studio 15 employs DC heaters for the first two gain stages (V1). Interestingly, it utilizes the 2x100ohm artificial ground for the DC part but not for the AC part.

[PRR]

> need maybe 6 amps of filament current.

No.

You ask if 6V AC near circuits can add hum. You have 0.020V signal at first tube, 40V signal at final tube. Which one will be most affected?

Generally there are three hum-critical stages: first stage, stage after volume pot, and reverb recovery. See Alerich's Marshall plan: only V1 preamp and post volume/tone network gets DC, the driver is AC heat and the EL34 sure are AC heat.

Generally in hand-wired, you can twist and route heater lines well away from audio paths and get hum way far down.

Often on PCB amps, when budget won't allow flying heater lines, DC heat is "necessary"; but only on the little tubes, not the big ones.

> That implies a fair sized heat sink

No. You need clean DC (rippled DC is worse than pure AC), but you do not need Regulation on DC any more than on AC.

In general, the ~~1V drop per rectifier adds-up on 6V DC circuits; that's why Peavey generally wires their small tube heaters in series at ~~25V DC. 2V drop on 6V useful is 25% waste; 2V drop on 25V useful is 8% waste. (It also makes less traces on the PCB.)

> artificial ground for the DC part but not for the AC part.

The AC part is effectively centered around ground via the rectifier and DC CT. (If the DC were any good, it would not need a CT.... either lazy thinking or some empiracal design here.)

[HotBluePlates]

I've often wondered if hum could be reduced by using a 6.3 VDC regulated supply inside a Fender ...

Who's got a humming Fender? Every one that has been functioning properly that I've owned or built (and I've owned a LOT of vintage amps at one timeor another) has had no obvious hum.

The only item I've ever owned that used d.c. heat for tubes is a tube mic preamp that I have. That's cause input levels can be vanishingly low compared to guitar signals, and input transformers could pick up hum. Yeah, the transformers were nice shielded units, but the preamp builder opted for d.c. supplies (including for heaters), and located all supplies and regulators in a separate chassis connected by a shielded umbilical. Possibly overkill, but there is zero hum at any level.

[LooseChange]

I can't hear it. Never noticed a difference.
You are better off tweaking your B+ and Bias supply filters first.

[The_Gaz]

Never tried it, and I think I found this schem on this forum, but this is ostensibly the way that Alessandro Amps does it. Seems very simple to implement if you've got an extra 5V winding lying around.

[murrayatuptown]

Ulrich Neumann used a mix of ac, dc & dc-elevation in his Deluxe Plus DRRI build. He added a separate preamp channel with two stages of 6922 (6DJ8/ECC88) and cathode followers. There are some broken links in his stringsandfrets site right now...I can only access the thumbnail of the Ver. 2 schematic. some stuff broke with his last re-org. Can't read it on my phone. I am pretty sure he used both DC and DC elevation on one of the 6922's.

See if you can read it. There are two versions of the schematic plus a color-coded on...hopefully one works.
Also, the text description explains most of what he did & why...about 30 pages worth, which may be useful before studying the schematic.

https://sites.google.com/site/stringsandfrets/deluxe-plus?authuser=0