Preamp PS - 270v 0.08A / 14v 2A - Which circuit is best for Heaters AC or DC ??

[kagliostro]

I'm waiting for three low profile Thoroidal Transfromers that I ordered for (guitar) preamps to be mounted in U1 rack that I want to build

the higher voltage with low profile transformer I've find is HT 270v 0.8A 0.08A - Heaters 14v 2A

I think the HT is good for this purpose (270v * 1.4 = 378v) I can lower it with resistors, the tubes need not very much current so I've learned (here in the forum) that this can be easily done

the question is about Heaters PS, the windings are rated 14v 2A, I think the preamps I'll build will have from two till max four tubes with 6.3v and/or 12.6v filaments

if the filaments require 12.6v I've think to use a resistor or two diodes in series (with paralleled anti series) to reduce the 14v to 12.6v

but I can also use a DC PS with 7812 stabilizer and a diode connected as to lift the voltage to 12.7v

if the filaments require 6.3v I think to use an LM317 or a 7805 with two diodes to lift the voltage to 6.4v

if I require both voltages a mix of the two solutions

But I'm not sure about all this, also someone told me (if I decide to use DC) is better to have a Constant Current PS instead than a Constant Voltage PS

many way to do this thing, I need your councils about

Thanks

Kagliostro

[triode]

Just my $0.02...

With 0.8A (800mA) you are going to see a bit more voltage than 1.414*270=378V. I bet you see 400+V
if you have a few preamp tubes pulling say 0.040mA...

With 14V at 2A, I would do DC, not for noise, just for the whole "the windings are 14V 2A"... you seem to have
little choice but to go DC. How many tubes pulling how much current? Two to four you say? If you need both
just do a 6.3V and 12.6V DC regulator after rectification. I would just do a linear regulator as you suggest...
You should have plenty of current in that winding for that, just get regulators that can sink enough current.

Constant current vs constant voltage... Eh, just put a linear _voltage_ regulator in there that can sink
enough current for your tubes.

[HotBluePlates]

I agree with Triode.

Further, if you use 12.6v wiring, four tubes pulls ~150mA*4 = 600mA. 6.3v wiring will pull double current. You have to rectify the heater voltage, then drop the excess across a regulator. For 12.6v wiring, that's 14v *1.414 = ~19.8v - 12.6 = ~7.2v (deduct another 0.6v or 1.2v, as appropriate for diode drop). 7.2v * 600mA = 4.32w, and this power must be dissipated by the regulator.

For 6.3v operation, the current doubles, and the voltage wasted by the regulator increases to 13.5v. Now, the regulator must dissipate 16.2w (still assuming four 12A_7 tubes).

[kagliostro]

Thanks Triode & HBP

With 0.8A (800mA) you are going to see a bit more voltage than 1.414*270=378V. I bet you see 400+V

My fault I wrote 0.8A instead of 0.08A

Your council is about use DC, thinking about, that way there is the advantage that I can act also relays

I was thinking about the voltage in excess the regulator need (1.25v - 2v) but as you told the winding has a 2A current so I can think that starting from 14V AC + rectifier and capacitor the regulator will be able to work regularly

if I've two different tubes that require both voltages 6.3v and 12.6v

as the regulators are not very expensive do you think is better to drop the voltage in one time

(~19.8v - ~13.5v) = 6.3v

or is better (~19.8v - 7v) = ~12.8v - ~6.5v = 6.3v as to distribute the dissipation in two regulator instead that only in one ?

to drop in one way to 6.3v there are some options, an LM317 or 7805 or better LM338T that is rated for 5A instead than 1 - 1.5A for the other two (depending on the brand)

what would you choose ?

Kagliostro

[Willabe]

If you have 19.8v you can still use 2 regulators to get down to 12. Don't they make a 16v or 15v 3 pin regulator?

Plus when you wire up the filaments you don't have to run the wires across the tube, just pins 4 and 5, instead of 4/5 and 9.


           Brad        

[sluckey]

Here's a bench supply that I built. It has served me well for 30 years. The 5v output is good for 3 amps. The + 12 volt outputs are good for more than 1 amp. All regulator chips are TO-3 style mounted on proper heatsinks. The bridge delivers +24vdc to the regulators. Since you only need +6 and +12, your PT with a standard bridge would work very well with this circuit and you would eliminate one big filter cap and the negative regulator.

This supply is overkill for your needs but it may give you some ideas. Your idea is fine too. Don't worry about feeding unreg 19vdc into a 7812 or 7806.

[Willabe]

LM7815 at Digikey, $0.70.

http://search.digikey.com/us/en/products/LM7815CT/LM7815CTFS-ND/1923053


     Brad     

[kagliostro]

Thanks Willaby & Sluckey

the 78xx & LM317 (rated 1A or 1.5A - TO-220) are very very cheap

a bit more expensive the LM338T (but rated 5A - TO-220) is € 2.38

only LM338K (rated 5A - TO-3)  is expensive, from € 10.00 till € 70.00

so the use of that regulators in TO-220 is not expensive at all

Kagliostro