Les wrote:
Hi all!
I just wanted to pop in here and add my opinion on the AC vs DC vs SMPS on filaments. Now, I don't intend on arguing the possible sonic influences these various filament PS schemes may have. So don't try to get me to go there. I personally have never been able to discern any difference based solely on the filament PS. Most colouration I have heard has indeed been an inflection or reflection of the quality of the HV power supply, tubes in play, and other sources of inducted noise due to poor routing of signal wires. So no, I don't prescribe to the sonic argument in relation to filament PS. To me, the best argument for DC on a filament is to preserve filament (and therefore tube) life. Now one might argue that a SMPS is a pure DC, but it is not. It is a High Frequency modified and rectified AC signal. Some SMPS have very good final stage filtering, but they generally cost a little more and have garnered the "computer grade" label. For the purpose of filament preservation; any SMPS will do, but it is overkill. Do take caution if you're using a lower grade SMPS to keep the filament wiring separated from signal wires as much as possible. While you're not likely to actually hear the signal of the SMPS @ 30KHz + ( to my knowledge, no human posseses the capability to hear frequencies above 20Khz) you can indeed hear other noise that can be inflected from early stages - such as the primary rectification at mains in - should that noise be inducted in to the small signal lines of the amp. It is duely notable that there are some really cheap SMPS schemes out there that are in the human sonic range and they can naturally induce that high frequency signal in to amp signal lines. It will appear as a high pitch ringing or whine. If you want to use SMPS, go for it. I will use simple DC rectification and filtering. I have been able to acheive sonically quiet pure DC power supplies easily and cheaply. I don't use regulation unless VOLTAGE or CURRENT stability is an issue. Regulation should never be confused with noise rejection. The purpose of regulation is to maintane either a constant current or a constant voltage. Some regulators boast good noise rejection, but the simple fact is, they don't do it well. Again, in my opinion and for the purpose of filaments, it doesn't make a hill of beans difference, sonically speaking. So, since in my opinion power type on the filaments has no influence on sonic quality in an amplifier, why would I prefer DC over AC? To preserve the filament. That filament in a AC set-up is undergoing the same stress as a household lightbulb. It litereally is being turned on and off 120 times per second (at 60cycles, the voltage crosses 0v twice in every cycle [60 x 2 = 120] and changes current direction at every cross). It is a technically proven fact that a lightbulb will last far-far longer on a DC voltage than an AC voltage. The purer the DC the better the life span. So, for that purpose, an SMPS fits the bill just as well as a simple brute force AC-DC rectified PS. But if you want to believe that somehow that AC signal is inflecting some sort of pleasant colouration to your amplifier... Hey, its your tube and your money!

GOOD LISTENING!

Ps. Nice build sampleaccurite!

Thanks. The main reason I use SMPS is cost, ease of implementation, and weight. I just got some 12V 6A SMPS supplies on ebay for $9.99 and free shipping for a 6C33C OTL amp:
http://cgi.ebay.com/AC-110V-240V-Adapte ... 48370ff384I expect they will supply 6 amps DC at 12 volts in addition to a large amount of high frequency trash. But they will be shielded from the rest of the amp and the output of each will be filtered before it leaves the shielded area of the chassis. I've use them on KT88s with the Oddwatt as well as 300Bs and I've yet to have a desire to go back and replace them.
I've read some interesting claims about DC heater supplys used for directly heated cathodes and the signal current induced in the filament from one side to the other by the difference in signal potential having an effect on the sound, as well as the non-uniform distribution of current (electron) flow in the tube. Considering the very large anode to cathode potentials of the GM70 it seems to me that this effect would be negligible, but not having the opportunity to listen to ANY amps with AC vs DC I have only math to show the differences in potential between one side of a cathode and the other are tiny compared to the anode voltage. HOWEVER - consider that the DC voltage across the filament is NOT always tiny compared to, and in fact may be a significant portion of the grid to cathode voltage. If someone claimed that AC heating sounded better than DC with a particular tube in a particular circuit, I could believe that. However, if it does sound "different", I have to wonder what happens to the signal when the electrons flowing in the tube originate from a wire that changes its polarity 60 times a second. Add to that the only reason 60Hz is used is because it's the most convenient frequency.
The AC/DC debate will never be settled. Whatever pleases you the most is what you should use. I just want to get my amps up and running and tweak later. Use a bigger power tranny than needed, shoot for a little bit higher voltage than required, then drop it as needed with power resistors to fine tune the B+. That's my lazy a** method anyway, but it has served me well so far.