DC Pre-Amp Tube Filaments on vintage Fender - pros and cons?

[wizard333]

Have had questions about doing this as a noise reducer. Not hard to wire up, but some questions:

1) Anyone tried it? How much of a noise reduction do you get in the real world? Rivera does this on a number of their amps so I'm guessing it has an affect.

2) Can the filament tap handle the extra requirements of this? I'd think that adding the filter cap for the bridge rectifier would strain the system somewhat. I've seen people mention that as well in web articles but none talk in depth about it. Any ideas there?

Thanks!

[pdf64]

All it can possibly do is reduce line frequency hum - not hiss, shot noise ripple etc. If hum's a problem on a vintage fender circuit, it can be solved more simply by improving lead dress. Which will yield benefits with stability and other induced noise anyway. Peter.

[guitician]

If your talking about converting an old fender amp to DC filaments I would say that your better off just using spiral filament tubes selected for low noise. DC filaments are used were the high gain pre-amp stages in phono inputs require it. A normal fender amp will see very little benefit.

[wizard333]

All the other things - lead dress, etc etc. have been done.

Its not a noisy amp. The question is, can it be even more quiet?

Anyone tried it and know for sure, and have some comment on any possible technical issues?

[guitician]

All I know is that rectifying the filament winding will produce a higher voltage with filtering, and you have to use dropping resistance in series to get within tolerance.

[Enzo]

Hum is not monolithic. In other words it is not an overall thing that the more "cures" you add the less there is. Hum is a mixture of sounds. There is hum from power supply ripple, hum from poor grounding, hum from poor wiring dress, and sometimes hum from within the tubes.

If you have power supply ripple, all the "improved" grounding and shielding in the world won't reduce it. And if you have poor grounds at the input jack causing hum, then adding filter caps will have zero effect.

SOmetimes, not always, sometimes in a preamp tube, the heater AC can influence the signal amplification. This results in hum. Making the heaters DC will eliminated THIS source of hum. it will have zero effect on any other source of hum.

SO IF you are getting hum from the heaters in the first stage tube, then you MIGHT see somewhat less if you run the heater on DC, but if that is NOT the source of your hum, it will do nothing.

[Don Moose]

Let me point you back to pdf64's comment - DC heaters can reduce AC power hum. **No other noise reduction benefits.** And this is at some risk that the extra wiring doesn't end up getting AC power hum picked up by a grid wire or something.

The extra current demand is slight, except at startup when the heater resistance is lowest. You could add one of these inrush current limiter devices to moot that, if you like.

Hope this helps!

[tedmich]

AC heaters have their fans too, and many amps have some AC and some DC heaters. Its a relatively easy mod, even getting 6.4VDC from 6.4VAC with or without a SS regulator, but seldom worth it unless there is a real problem to solve. (Some suggest a 6V lantern battery test to see if DC is worth it)