filament AC vs DC

[oldeurope]

no semiconductors

Tatatata... proudly present an AC heated 300B SE amp:
http://rl12t15.blogspot.com/2007/11/direkte-heizung-300b.html

Kind regards,
Darius

 

[yourownfree]

filaments

Ok looks like i have a challenge to make an inexpensive 6.3 volt regulated supply. Then again it looks like its not necessary. It would be a neat idea though. If i was to do it i think i would use... well I'll give it some thought. What i get is basically you can do it but why do it. In my mind I'm thinking hum is coming from somewhere. Power supply, nearby transformer etc. Seems to me the power transformer and filtering should be done separately from the main unit and enclose it to prevent radiation. Get the purest DC voltage going to the amp, guard amp against any outside interference and then hum should almost be gone. I also understand power transformers become more efficient when operated at a higher frequency other than 60 Hz, say about 400Hz. That has crossed my thinking. But even thinking about tubes, i need to go back and study up on the theory again, but what if polarity plays a part in it, to the filament that is. Ok i would use mosfets for the filament power supply. To me they are inexpensive power.

 

[Shoog]

In my experience almost all hum is down to poor layout. I have a few hum free PP amps where the +B has volts of ripple on it. (and I have 100db+ speakers). Still if I start dicking around with the Single point star grounding I can almost guarantee that hum will rear its ugly head.

My rules are;
Single point star grounding.
Current balanced differential stages.
Tightly twisted AC heaters with a good earth reference/elevated reference. Pay careful attention to routing on the valve socket.
Grid and screen stoppers are essential. Anode stoppers are sometimes desirable.

Spend as much time reading up on all of the tricks of correct layout and less on pure DC and your time will be much better spent. It is unlikely that you will implement a good DC heater supply initially and this is likely to have a negative effect on the end result (think ringing and RF interference).

Hope that helps.

Shoog

 

[zigzagflux]

I won't do DC on power tubes ever again; its too wasteful on power.

In complete agreement with you on this point.

It is unlikely that you will implement a good DC heater supply initially and this is likely to have a negative effect on the end result (think ringing and RF interference).

Add me to the list of those making that very mistake. I have built DC supplies that I suspect contaminated my amp with RF. The more I replaced the DC supplies with AC, the more clean the RF spectrum became. And with each step, 60/120 Hz hum did NOT increase.

AC for me. I don't do vinyl, though, so can't add to the phono stage opinion.

 

[yourownfree]

thanks, makes sense about the rf, sounds like a vicious circle. Create the DC now have to get rid of rf that i created, gaining nothing more than another project, then if i tried to get rid of RF, I'd end up causing something else in the process. Maybe simpler is better, and like you said balancing and twisting, but I still have to wonder and dream.

 

[Brett]

I have built DC supplies that I suspect contaminated my amp with RF.

There are no RF issues with correct design, which is neither expensive, nor difficult.

 

[Sheldon]

In my experience almost all hum is down to poor layout. I have a few hum free PP amps where the +B has volts of ripple on it. (and I have 100db+ speakers). Still if I start dicking around with the Single point star grounding I can almost guarantee that hum will rear its ugly head.

My rules are;
Single point star grounding.
Current balanced differential stages.
Tightly twisted AC heaters with a good earth reference/elevated reference. Pay careful attention to routing on the valve socket.
Grid and screen stoppers are essential. Anode stoppers are sometimes desirable.

Spend as much time reading up on all of the tricks of correct layout and less on pure DC and your time will be much better spent. It is unlikely that you will implement a good DC heater supply initially and this is likely to have a negative effect on the end result (think ringing and RF interference).

Hope that helps.

Shoog

I take the same approach, and it works well. I have found that I needed DC on my 801A amp, but that has a relatively high filament voltage, so is a little harder to get completely quiet for 100+dB speakers. I've played with 3A5' and they are designed for battery power. They definitely need DC. I've also used DC on my phono stage, but didn't try it without, so I can't say for sure there. Were I to redo it, I'd try AC first - too hard to optimize the filament wiring now. All other tubes are AC, and I like my amps quiet, as in no noise until my ear is right on the driver.

Sheldon