AC Power for tube filaments, surprisingly good

[aerius]

So I've been on vacation for the last few days and decided to go play with my tube amp since I had nothing better to do. After reading a whole bunch of stuff on the diyaudio forums I got a bunch of stuff together and played around with various power supplies for the heaters in my tubes.

First off was a generic DC power supply I got as a kit in the local electronics store way back when. It sounds decent, with Grados there's more background hiss than I'd like but other than that I didn't notice anything wrong with it until I got the good stuff.

Next was a benchtop power supply I found in the surplus shop. This is the quietest power supply, with Grados there's just the slightest background hiss from the tubes which I can hear only in a fairly quiet room with no music going. If I leave the headphones on my head for about 15-20 minutes it's almost impossible to notice the hiss unless I unplug & replug the headphones into the amp. As for sound, well, it makes my amp seem even more detailed and easier to listen to. It was like getting a nice tube upgrade, everything sounds more real.

Then I went to AC heating with some transformers I have from past projects. It was nearly as good as the benchtop PSU, and much better than the generic DC unit. There's just the tiniest bit more hiss and that was it, no buzz, no hum, nothing. This is the setup which I'm now using since I have no room and no way to mount the benchtop PSU to my amp. For all effective purposes, at least with the gear I have, it sounds just as good as the benchtop supply.

Last, and worst by far is a switchmode PSU similar to the one in the amp which we love to hate. Even though mine was grounded properly unlike you know who, it still coupled a shitload of noise into the tubes. Completely unlistenable with Grados, and the noise was noticeable even with my 400 ohm K340's. It hissed, buzzed, and hummed, and made my ears hurt. It killed the details in music and made it very fatiguing to listen to. Absolutely horrible.

What's next? I don't know. I like the idea of an amp that has 4 power transformers, it seems manly in a way. It also has no electrolytic capacitors anywhere on the amp and that's kinda cool too. I might get a new HV transformer so I can get rid of the last 2 pieces of silicon in my amp, go to a CT full-wave rectifier instead of the current hybrid bridge rectifier which uses silicon and tubes.

[Guest sacd lover]

Good post .... and some interesting conclusions. I wish I could get my transformer coupled supra done soon. I am dying to try that out.

[JBLoudG20]

I definately feel that a good DC power supply is better than AC heating the tubes, but not worth the extra cost in most of my builds. My past 2 amps were AC heated, and they sounded damn good to me.

[aerius]

I suddenly realized I had a bunch of lead-acid batteries which could be used for powering the heaters. I charged them up and hooked them into the amp, and it sounded just like the benchtop DC unit. Unfortunately 6BL7's suck a fair bit of juice so I have to pull the batteries out and charge them every couple days. It's an interesting idea and sounds quite nice but unfortunately it's not practical for me. Oh well, back to AC power.

[grawk]

How would it sound if you left the batteries trickle charging while listening?

[JBLoudG20]

I would imagine it would sound about as good as the DC regulator for charging the batteries

[aerius]

After reading through John Atwood's DC filament supply test, I did some thinking and decided to give DC heating another try using the parts I have at my disposal. I went with a variation of circuit #4 from the report, but with a couple extra RC stages and different parts values. Damn, now we're talking, this little simple circuit is as good as the big benchtop power supply, it works, and it's so simple it's not even funny. I only converted the input tubes to DC since I don't have enough room to do all the tubes, but yeah, this was worth it.

[NotoriousBIG_PJ]

Nice find. Now that I think of it, my amp was converted to use DC heaters. Before the mods (hooking the headphones up to the speaker posts using resistors) the amp was noticeably more noisy.

Biggie.

[cotdt]

Clean DC power should be better than AC. I tried battery powering the filaments on my indirectly heated triode, and the 60Hz hum dropped by 20dB compared to AC. Unfortunately, I had other noises coming from my B+ supply that was a greater issue, so I went back to AC for convenience. In the future, I will revisit DC battery powering the entire amp.