DC regulated heater supply ckt for preamp tubes to reduce hum.

[mikepukmel]

I read a comment by someone on factebook amp page, who said that replacing AC with regulated DC for preamp tubes helps reduce the noise a lot. Since we have lots of cheap regulator stuff these days, is anyone doing this on a commercial amp? Any reason not to try it (other than more components to fail)?

[Enzo]

SOme thoughts:

Running DC CAN reduce noise. Doesn't mean it is going to, that depends on whether there was any heater noise to start with.

The ONLY noise DC heaters will reduce is heater to cathode leakage noise - one source of hum. It will have no effect on things like hiss and other non-hum noise. Also it will have zero effect on other sources of hum like power supply ripple or poor grounding or lack of shielding.

It isn't necessary to regulate DC for heaters, after all, the AC was never regulated. Heater draw doesn't vary much anyway, so the voltage is fairly stable by itself.

DC heaters? TONS of amps have DC heater supplies. A bunch of Marshalls rectify and put DC on just the first two tubes. A Peavey 5150 runs ALL the preamp tubes on DC. It isn't rare at all.

[Chuck H]

That^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A pretty easy trick (that I haven't tried) is something that Mesa is doing in a few models. They rectify the supply for the first preamp tube only. This would absolutely be the most noise sensitive tube and I think most stock filament windings would handle it. So it might make a good "mod" for some of the old classic designs. Especially something like a Marshall type amp with a channel stack mod already. Since those amps weren't originally designed for such preamp gain it may be beneficial.?. And as Enzo said, it only applies where hum in the first preamp tube filament needs to be reduced and does nothing useful otherwise.

[mikepukmel]

Thanks Enzo, thanks Chuck. Sounds like, in general, if its working, don't over modify. I have a JTM45 type clone Ive been working on since cars had big fins, it seems, and the one thing Im worried about is noise. I realize it won't get rid of hiss, but hum is in some ways worse, since hiss is at least white noise. Probably worried more than I should, given the music I listen to and play is a little on the louder side, so probably wouldn't notice a little hum.

So, for a future "high gain" type build, maybe its worth it. Or a HI-FI type build, listening to those very low passages of a classical piece, where that added 60 or 120 don't quite fit.

The AC's weren't regulated but I hear lots of dire warnings about too high or too low heater supplies, and the line voltages are pretty crazy around here. Would it benefit at all from regulated heater supply, or is it a complete waste of $$$?

[mikepukmel]

That^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A pretty easy trick (that I haven't tried) is something that Mesa is doing in a few models. They rectify the supply for the first preamp tube only. This would absolutely be the most noise sensitive tube and I think most stock filament windings would handle it. So it might make a good "mod" for some of the old classic designs. Especially something like a Marshall type amp with a channel stack mod already. Since those amps weren't originally designed for such preamp gain it may be beneficial.?. And as Enzo said, it only applies where hum in the first preamp tube filament needs to be reduced and does nothing useful otherwise.

Chuck, I had not noticed the last quote in your quote list: ""If you build it, it will hum..." Justin Thomas" Heh. Oh boy.

[dstrat]

Hi Mike, Sometimes 12ax7 tubes hum in V1 right out of the box mostly not.
AC or DC on preamp tubes work. As was said above lots of high gainers do it but its not with out complication.
I built a clone of *ogne* xtc a few years ago, its high gain. I used AC 6.3 volt, or 3.15 x2 how ever you want to look at it.
No hum what so ever. so its not always a good idea to use dc heaters.
its simply a solution for a problem if one exists. IMO.

dave

[Chuck H]

+1 to that. When I do a build I buy extra preamp tubes so I can cull the noisy ones. Some hum, some just sort of futz and sputter and some have what sounds like a lot of cathode emission noise. A sort of low pitched hiss or high pitched rumble. I get pretty grumpy about buying two tubes to get one often times, but that's the brakes. So DO try a known quiet tube in the V1 slot before doing any circuit modification.

And (after trying a known quiet tube) if you want to know if the AC heater is responsible for hum you can just hook up a 6V battery to the V1 filament and see if it makes a difference. If it does then try the Mesa thing and rectify for the first preamp tube.

You can also try elevating the AC filament circuit. This is probably a good idea anyway since your amp uses a cathode follower. There was a time when many higher voltage Marshalls were eating the cathode follower tube due to the filament to cathode differential. Some of the Rusky tubes wouldn't handle it.

If your filament voltage is too high (over 6.8V I'd say) then you can use resistors or diodes in the filament circuit to reduce it.