Need opinions! Does a tube filament sound better being heated up with AC or with DC ?

[plasmod3]

What do ppl think?

 

[Atmasphere]

DC, and regulated if you can.

A lot depends on what sort of circuit. In a preamp if you want the best out of it its a no-brainer.

For a power amplifier it may not seem as important, but having done it both ways even in a higher power amp it makes a difference.

SETs which usually use Directly Heated Triodes (DHT) benefit quite a lot since hum is otherwise endemic. The problem there is the heater can develop hot spots so the polarity should be reversed on occasion.

 

[plasmod3]

DC, and regulated if you can.

A lot depends on what sort of circuit. In a preamp if you want the best out of it its a no-brainer.

For a power amplifier it may not seem as important, but having done it both ways even in a higher power amp it makes a difference.

SETs which usually use Directly Heated Triodes (DHT) benefit quite a lot since hum is otherwise endemic. The problem there is the heater can develop hot spots so the polarity should be reversed on occasion.

Most interesting. I imagine in set you are referring to a DC component on the pine if AC is used causing the hum? Same for pentodes?

What about for Dacs ?AC or DC you think?

 

[acg]

I use both.

Preamp is 10Y filament bias using CCS.

Power amps are six channel SETs, all cathode bias AC heated, even the DHT channel, with zero hum with your head inside the 109dB/w/m horns. Got them hum free at first attempt...it is all geometry so you do need a bit of room in there for careful wiring layout.

EDIT: Discrete LCR phonostage is DC heated with simple regulated supplies for IDHT tubes...that is not the place to use AC.

 

[Salectric]

I have compared AC vs DC on heaters for linestages and phono stages, with somewhat inconsistent results. With two linestages, AC was the clear winner sonically. The DC supply sounded sharper and crisper, perhaps slightly more detailed but recessed in the midrange, not as natural to my ears. Based on that experience, I have used AC heaters in all of my homemade linestages, DACs and power amps.

The outlier experience was with a phono preamp. Common wisdom says you need DC to avoid noise in a phono stage, but that’s not entirely true. In my case, my phono is a 2-stage affair with a high-gain input tube (D3a) followed by passive RIAA eq and then a lower gain 5687 and output transformer. The input tube needs DC but the second tube can use AC without noise problems. The outlier part occurred when I was trying a Type 27 output tube instead of the 5687. The 27 had some noise with AC on the heaters; DC eliminated the noise and also sounded better.

By the way, Ypsilon phono stages follow a similar approach to what I tried. They use DC on the input tube and AC on the second tube.

 

[Atmasphere]

I have compared AC vs DC on heaters for linestages and phono stages, with somewhat inconsistent results. With two linestages, AC was the clear winner sonically. The DC supply sounded sharper and crisper, perhaps slightly more detailed but recessed in the midrange, not as natural to my ears. Based on that experience, I have used AC heaters in all of my homemade linestages, DACs and power amps.

The outlier experience was with a phono preamp. Common wisdom says you need DC to avoid noise in a phono stage, but that’s not entirely true. In my case, my phono is a 2-stage affair with a high-gain input tube (D3a) followed by passive RIAA eq and then a lower gain 5687 and output transformer. The input tube needs DC but the second tube can use AC without noise problems. The outlier part occurred when I was trying a Type 27 output tube instead of the 5687. The 27 had some noise with AC on the heaters; DC eliminated the noise and also sounded better.

By the way, Ypsilon phono stages follow a similar approach to what I tried. They use DC on the input tube and AC on the second tube.

One problem with DC filament supplies, in particular if they are regulated, is the high Voltage supplies must be regulated as well to prevent changes in the AC line Voltage from affecting how the circuit performs.

 

[dennis h]

DC, and regulated if you can.

A lot depends on what sort of circuit. In a preamp if you want the best out of it its a no-brainer.

For a power amplifier it may not seem as important, but having done it both ways even in a higher power amp it makes a difference.

SETs which usually use Directly Heated Triodes (DHT) benefit quite a lot since hum is otherwise endemic. The problem there is the heater can develop hot spots so the polarity should be reversed on occasion.

Could you supply a reference for "The problem there is the heater can develop hot spots so the polarity should be reversed on occasion."

No hands on here like you have, just haven't seen or heard that one yet and was wondering if all DHT tubes or just a select few.

 

[Atmasphere]

Could you supply a reference for "The problem there is the heater can develop hot spots so the polarity should be reversed on occasion."

No hands on here like you have, just haven't seen or heard that one yet and was wondering if all DHT tubes or just a select few.

That was something I was taught in school. I didn't find anything searching on it. A hot spot would cause filament failure FWIW.

Its for that reason we've always used AC filaments in our amps, although one amp, the Nirvana, did used regulated filaments. It was a good 10dB lower noise and seemed to also sound smoother, which should be no surprise since no tube is really linear. That means that AC line noise can mix in the tube with the audio signal producing intermodulations.