• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

filament AC vs DC

I am curious as to why i don't see much in the way of turning AC to DC and then filtering it for a low hum heater voltage? What if i just used a battery? I am wanting to build an amp and am tossing the idea around in my head, but I don't want to waste my time designing if it's not a good idea. Yes, I am new to this forum, but not new to electronics. I have seen filaments in amps using DC, but really no filtering. I wish my amp to be different than the rest, the reason for thought. It is too easy to just copy an amp, there are so many diagrams tested and tried, but I find more fun in experimenting.
 
Yourownfree-

I'm with you on that one. I don't see any point in not using regulated DC on the filiment when it is sooo easy to do. Especially if it removes a potential problem of hum injection. I too am new to valves and have never actually fired one up in anger, so just like you i'm a bit sceptical of whether or not building an amp based around valves is a good idea or not. But what the heck? i've built the other types so i may as well have a go. Who knows... i might even be converted :)

Leigh
 
It just isn't all that necessary, except for very low level stages, something like a phono stage, or the up front stages of a SW XCVR, or something like that.

Secondly, clean AC on the heaters is much better than DC unless filtered very well since the harmonic content is liable to impair the sonic performance. I did a project with DC heating voltage, and another with AC, and it really didn't make any difference (not low level).

With DHT types, however, constant current DC does seem to work better than AC. For IHT types, clean, balanced AC works just as well.
 
seven amps? :hot:

What you building? a computer! LOL

Erm.. in that case AC may be the best action, your obviously not talking small signal here. But if you want a crack at it, easiest way i can think of is rectify your 6.3v to give you approx 9V with some rough filtering. take a 7805 and use 3 diodes to raise its ref up to give about 6.8v, stuff that into a large NPN hung off the positive rail and that should do it, you should end up approx 6.2v. if you find you dont have enough drop to keep the 7805 going then using a low drop version should do the trick like a 2905. raising the rail voltage is not the best idea as the dissipation accross your NPN is gonna get high.

Maybe not good for such high current but definatly workable for smaller currents in the ss stages.

Leigh
 
#7

nitrate said:
But not in the signal path... It is after all 2008, you'd expect some silicon in there, it makes sense.

Leigh
Hello Leigh,
Yes, this is why I used a switched mode regulator in my RIAA preamp.
I am waiting for new manufactured ECC808. :D Than I'll make an AC
heated thing.;)

Kind regards,
Darius
 

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Shoog

Member
2002-08-15 10:16 pm
Eire
There are a number of extensive threads which address this very issue (use the search function) and will educate you in the technical reason why. There are very very few reasons to prefer DC over AC. I have many quiet projects with AC heaters (even a phono stage).

Shoog
 
#10

Hello Shoog,
I build up an AC heated RIAA with EF40, EF86 and EF800.
It was very difficult to find quiet tubes. Some don't have bifilar
heaters and so on. :mad: The ECC808 is very expensive.
ECC83 no chance because the g1 is not screened from the
heater and some are bifilar others not. :(
This is why I decided to make DC heating in the Triode RIAA. ;)

Kind regards,
Darius
 

Brett

Member
2002-01-07 6:02 pm
dsavitsk said:


If you have a good design to get 6.3VDC from a 6.3VAC winding that is sooo easy, is inexpensive, is not super hot, and that can supply several amps I'm keen to see it.
LV transformers are cheap and common, so why not use one unless you're just being a tightwad? It also gets any switching artifacts out of the HT transformer and as it's a constant current draw, it's easy to design a good clean supply. I typically only use AC in power tubes.
 

luvdunhill

Member
2006-07-09 6:59 pm
dsavitsk said:


If you have a good design to get 6.3VDC from a 6.3VAC winding that is sooo easy, is inexpensive, is not super hot, and that can supply several amps I'm keen to see it.

I just finished one. Using a pair of Hammond 183K12 and two Linear LT1083 regulators. For 6.3V operation I had 14.74VAC at the transformer, 8.57VDC after FWCT rectification and smoothing and regulated to 6.3VDC. I was using a pair of IXYS Schottky diodes. I measured 2.2mV ripple with the load connected. This was pulling 2.4A. At 12.6V I was getting 14.53VAC at the transformer, 16.67VDC after full-wave rectification regulated down to 12.6VDC. Ripple was around 1.8mV and I was pulling 1.74A. Heat was very manageable, I think I calculated something like 5.86 C/W needed to be within the limits for the power section and control section of the LT1083. I'm going to try another few tubes and see how much current I can draw in the 6.3V case, without reaching the drop-out voltage. I have plenty of room in the 12.6V case.
 
Brett said:
unless you're just being a tightwad?

You seem to not be in tune with the spirit of this forum ;) If I'm here, of course I'm a tightwad.

luvdunhill said:


I just finished one. Using a pair of Hammond 183K12 and two Linear LT1083 regulators.

That's $26 in regulators, $50 in transformers, plus the added cost of the capacitors resistors, circuit boards, shipping, etc. and you have spent $100 to get zero benefit.

I'm with Shoog here. I run everything I can AC, which means most things (the exception being a DAC, and even there I'm not convinced it's necessary -- I'm just not going to tear it apart now that it is working). If your tubes won't allow AC heaters due to hum, find some better tubes. I've had hum caused by any number of things, but never from a heater.

-d
 

Brett

Member
2002-01-07 6:02 pm
dsavitsk said:
You seem to not be in tune with the spirit of this forum ;) If I'm here, of course I'm a tightwad.
Been here a long time, and never has "the spirit" been soley for cheap: if that were the case, then go and buy a Behringer etc chinese generic part. You can't beat them for the money, new. I prefer the best I can and am tired of compromising. With a large cache of parts on hand, and very high effiiency speakers (>100dB) it's simply easier for me to do DC.
 
luvdunhill said:


3 out of 4 ain't bad from where I come from :)

Well, when the difference could have paid to upgrad from a steel core to a nickel core in the OPT, or from a Solen to a AN cap and made a real difference, then I'd say it's a poor trade off. I think it's always worth the effort to try AC, and if it doesn't work, then buy the extra parts.


Brett said:

Been here a long time, and never has "the spirit" been soley for cheap: if that were the case, then go and buy a Behringer etc chinese generic part. You can't beat them for the money, new. I prefer the best I can and am tired of compromising. With a large cache of parts on hand, and very high effiiency speakers (>100dB) it's simply easier for me to do DC.

Oh please, it was clearly a joke. Cheap is not for cheap's sake, but maximizing the bang for the buck makes sense to me. You can heat your tubes however you like. I'm just saying, my money is better spent elsewhere. My speaker are 98dB @ 1mW, and AC heaters are fine here.
 
DC is great for phono stages and for filaments in directly heated tubes (2A3, 300B, etc). Using a common mode choke, you can minimize many of the complaints about DC filaments on directly heated tubes. People who complain about having transistors or solid state diodes in their tube gear should probably be ignored. Diodes work great and are really the only option besides batteries for getting DC on your filaments. Also, this tosses out the implementation of current sources completely, which would be pretty sad and limit some design options.
 

luvdunhill

Member
2006-07-09 6:59 pm
dsavitsk said:


Well, when the difference could have paid to upgrad from a steel core to a nickel core in the OPT, or from a Solen to a AN cap and made a real difference, then I'd say it's a poor trade off. I think it's always worth the effort to try AC, and if it doesn't work, then buy the extra parts.

man, I just got the thing working, don't rain on my parade! plus, my project is DC coupled with no caps or transformers, so nothing to spend extra money on there :)
 
I think it was a fun challenge making a high current, low voltage DC supply. I probably won't ever do it again, but I have 6.3V @ 9A. I had never designed such a high current linear supply before.

Plus, I could make some small compromises on layout to show off the tubes better since I didn't have to be as afraid of the heater wires. Also, fluctuating line voltages are not a concern for my heaters.

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