Preamp Power SMPS

[craigmillard]

Hi Guys,

I posted this over the other forum as couldnt get on the site earlier, wierd DNS issues after the server change:(

Anyways...

Working on a layout for a tube preamp i want to build and am laying out the power section at the moment!  I want to go down the SMPS route and have built various 555 based SMPS designs, Cheers RICK:)

For this one i wanted to try out the max1771 as it can deliver higher current with less noise, anyhow i have designed a single layer board based on Nick de Smiths ideas:

http://www.desmith.net/NMdS/Electronics/NixiePSU.html

But also included a lm338 voltage regulator for the heater supply to allow upto 5a supply for the heaters. Hopefully this board can then supply 300v for 8 tubes with heaters too 

Before i pull the trigger and etch this board anyone fancy taking a scan over it?! pretty please 





Cheers
Craig

[iccaros]

did you say 8 tubes?
so with 12ax7 you have 3ma x 8 which is 24ma.
If you have 12au7 that is 7ma x 8 which is 56ma.

I believe that SMPS is rated for 18ma - 55ma depending on the materials and starting voltage, see experiments on that page, could go higher with some changes, like bigger inductor and a fet with a lower turn on resistance. They never got to 300v though, I would think this would lower your output current.

AS for heaters, at 150ma per tube @ 12v  your @ 1.2 amps, which that regulator is good for 1 amp with heat-sink  sorry missed that 5 amp regulator.. you show am LM317

It does not matter how much current the MAx can sink as current is sourced through the fet. as for less noise, Max 1771 SMPS have given me more issues in that area than any other type.

Rsense needs to be big, 2A according to the notes, so expect 2watts of heat, but the resistor should be 4watts to be safe.


these are just my observations, if this works for you, I would be willing to buy one or two from you.

[craigmillard]

yup 8 tubes:) want to try a slo100 based preamp so all 12ax7's/6n2p-ev tubes!

According to nicks stats his results show 120ma with certain parts which i have replicated, so i have a vishay IHB-3 inductor and the 2SK3772-01 FET to try.

The Inductor space on the board is fitted to the IHB-3 which is pretty large!:)

I have never used a max1771 before but have read they are quite picky so in the board design i have followed all of Nicks suggestions. Also left quite a bit of space around the rsense resistor for large resistors!:)

Yer the voltage regulator is the lm338 so should be able to handle upto 5a so for the 6n2p-ev tubes need to run at 6.3v and 300ma current each!!

If it all works i will certainly put up the toner transfer for all

Did you notice anything major up with the board design iccaros?

[iccaros]

I am assuming you did a copper pour on the back attached to ground as a ground plane. or are you etching this your self?
I see no errors, but somethings are small with my eyes. What did you make this in, if you do a gerber export with drill file they can be easily made.

[craigmillard]

I made this for home etching so is single sided, the disadvantage is no groundplane although the ground plane on the front is pretty substantial.

I didn't want to wait/pay to get the boards made to test the design, rick has made and had success with max1771 designs on perf before so hopefully the layout i have created will be good enough to get a few more ma's. Im not looking for Nicks 120ma's but enough to drive the tubes in a preamp.

Although would be cool to get enough to power an output stage:)

The board was made in eagle so gerber exports are possible and if worth it (they work ) they could be produced for sale on the forum?

[iccaros]

cool Good luck, I would buy two from you, if you get them made.

[craigmillard]

Cheers Iccaros, will keep this thread updated with the progress, hopefully over the next couple of weeks... 

[GFR]

I've posted this at the topic about light bulbs:

http://www.handmades.com.br/conteudo/projetos/pdfs/Bile%20Nervoso%201.0.pdf

from http://www.handmades.com.br/forum/index.php?topic=3466.0

based on this:

http://www.romanblack.com/smps/smps.htm

A smps to power tube preamps or pedals, can give over 200V for the plates and 6V for the filaments from 9-30V, AC or DC.  There are two converters, the filaments are fed by a converter too so you don't waste power heating a LM317.

The inductors are salvaged from light bulbs.

[craigmillard]

looks interesting GFR, how much current can that supply? can it do 2-3 amps for the heaters? looked as though it was up to 600ma or so...

I like the idea of a buck converter for the heaters though, def better efficiency, but can it get enough current through!?

[craigmillard]

Another Quick Q,

The LM338 is going to require a large heatsink as its going to need to drop alot of voltage say 6v for 12 down to 6.3 so 6*3A = 18watts!
Any suggestions on heatsink?
Do you think attaching it to the chassis (insulated) be sufficient?

[iccaros]

I do not think the case by its self will dissipate that much heat quick enough.  If you had a case with fins running length with the sides enforced like a heat sink then maybe. I would contemplate a small fan. or break up the heaters. what is your starting voltage? 15v?  you could get ride of the voltage regulators and run two tubes in series with a dropping resistor. 

This is an example of a heat sink for 20watts,

[craigmillard]

that's what i thought, the case wouldn't be good enough and the heat-sink is going to be massive to deal with the voltage drop. My starting voltage is likely to be either 12 or 15v, prob 15.
So you are saying i could run two tubes in series on 15v and then parallel another 2 tubes at 15 v in series etc..

[Ripthorn]

I would probably drop in a high wattage zener diode to dump off some of the voltage.  For most regulators you only need about 2V more than the output voltage (less with a LDO regulator), so that would get you down to about 6W to dissipate (still quite a lot).  I would probably do multiple parallel regulation stages, say 2 tube heaters per regulator.  OF course, board space and component count goes up.  Or, if you really wanted you could just get an uber big zener diode to do your regulation, just have dump the excess voltage off to ground. 

On another note, this looks like a cool project, I'll eagerly await results.  Also, do you already have the 6n2p's?  I know they are cheap and all, but maybe 12ax7's would be a better choice for the heater issue.

[iccaros]

@craigmillard yes, but you would need a dropping resistor per sets of tubes, which is calculated using ohms law. If the heaters are treated like a resistor and not the inductor they really are. so we need to drop 2.4v @ 600ma. 2.4 / .6 =  4R.  with a 2watt resistor.
check my math, I have been wrong

Edit
@Ripthorn, A zener is a good Ideal, and has been used in many designs. I think one big one will produce the same heat would it not?

[craigmillard]

I think it would actually be easier to separate the heaters from the HV supply now and create a separate board section. Im not sure which approach to take, i quite like the idea of the regulated heaters but using as ripthorn said multiple regulators in parallel for 2 tubes running in series. so the reg outputs 12.6 to be given in series to the heaters of 2 tubes? If my math is right: (15-12.6)*0.770 =1.848 watts to be disipated?
This would also be a relatively easy switch to create a regulated feed from the board for 12ax7;s too!!

If i gave board space for 4 LM3176T's say instead of the LM338 then they would need a smaller heatsink individually allowing fitting in a tighter chassis!?
Whats the Heatsink Thermal Resistance calculation again? i cant remeber:)

I would have just gone for the 12v approach on the tubes but i have a supply of 6n2p's already, hence the direction

[PRR]

> 12 down to 6.3 so 6*3A = 18watts!

The plan shows "9V-30V". Dropping 30V to 6V in a linear regulator is VERY wasteful. 80% of your electric bill is just heating a hunk of metal.

If you have good steady 12VDC (10V-14V), just feed it to the heaters! 12AX7 12AU7 have a 12V connection. The few 6V-only tubes can be paired-up and run series.

[MetalGuy]

If you have good steady 12VDC (10V-14V), just feed it to the heaters! 12AX7 12AU7 have a 12V connection.

+1. This is the way to go. Get a 12VDC regulated linear supply and feed the rest from it.

[craigmillard]

So if I use the 4 regulators to get 12v and then series 2 tubes each I should be good?
Or you thinking supplying the heaters direct from the power supply? Only thing is I wanted to use a higher voltage for the HV smps at 15v!?

[PRR]

> the 4 regulators

Why 4 regulators??

> 15v!?

Then put a resistor in series with each 12V tube heater string. 12AX7 12.6V 0.15 Amps, you need to lose 15V-12.6V= 2.4V, and 2.4V/0.15A= 16 ohms 0.36W. 15 ohm 1W is fine. Two 33 ohm 0.5W in parallel is fine.

[craigmillard]

So I'm using 6n2p's which run at 6.3v and 370ma, if I used a resistor of (15-12.6)*0.74 = 1.8k and run 2 of them in series off each heater string?