Preamp Power SMPS

[GFR]

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!?

I think the circuit as shown in the project is OK for a pair of 12ax7's (300 mA each). The author has suggested the following:

http://www.handmades.com.br/forum/index.php?topic=3466.msg73255#msg73255
"Alterando-se o zener do circuito buck para 12V e ligando as válvulas entre os pinos 4 e 5, a corrente cai para metade! Como a limitação do indutor é devido a corrente, elevendo-se a tensão, a limitação pode ser contornada! Desta forma o circuito pode aceitar 4 12AX7."

By altering the zener to 12V and using pins 4,5 of the tubes, the current draw is 150mA / tube, and then it would be able to power 4 x 12x7's.

Another suggestion is to try to find an inductor with thicker wire. Maybe from a higher wattage bulb? 

The authors said that they tried this supply with a small single ended amplifier (one 6au6, one 6AQ5, based on the Harmony H400A) and after a while the inductor was smoking...

BTW, a great liitle amplifier:
http://www.youtube.com/watch?v=SxC5gzRiW5s&feature=related

[iccaros]

I have a small adjustable power supply board I can send you. It has space for three LM317 which would power 6 tubes heaters and with heat sinks your could power more.  I had them made, but I have a small mistake, the resistor holes are a little small, but its easy to work around.  They are adjustable voltage so you could run 6.3 or 12.6 depending on the size, or I could send you the Gerbers.

In anycase I have this extra board I don't have a use for, so I'll send it for free.

[PRR]

> 6.3v and 370ma, if I used a resistor of (15-12.6)*0.74 = 1.8k

? ? ?

These are somewhat higher current than 12AX7. How did you get a number 100 times higher than I got for 12AX7?

They are series. Current is 0.37A. (0.74A would be for a parallel 6V connection.)

Resistance is Voltage divided by Current.

(15-12.6)  / 0.74 = 3.243 ohms.

[iccaros]

> 6.3v and 370ma, if I used a resistor of (15-12.6)*0.74 = 1.8k

? ? ?

These are somewhat higher current than 12AX7. How did you get a number 100 times higher than I got for 12AX7?

They are series. Current is 0.37A. (0.74A would be for a parallel 6V connection.)

Resistance is Voltage divided by Current.

(15-12.6)  / 0.74 = 3.243 ohms.

he is not using 12ax7's but 6N1P's which only run at 6v at 370ma. so in series would be 12.6v @  740ma correct?

[craigmillard]

haha it was a little late last night when i sent and worked that out! Cheers PRR!

So 6n2p-ev's run at 6.3v with 340ma +-35 current so (15-12.6) / 0.68 = 3.5 ohms

Am i right in saying the wattage of the resistor should be 0.68*0.68*3.5 = 1.6 so 3 Watt to be safe?

Im still interested in taking a look at the regulators way too, would allow a board that can work with any supply over 12v!
Iccaros would you mind sending over the gerbers so i can take a look on how you have done it? I may be able to recreate in eagle?!
Im in the UK so posting the board is going to cost ya so dont worry! Thanks though!!

[Ripthorn]

If you do parallel the heaters, you could use a zener diode in place of a regulator to work with wider voltages.  A 5W zener would allow you to drop something like 7.3 V which would allow for up to 20V input (if the zener is properly heat sunk, sinked?). 

[iccaros]

here you go, very simple, I use DIP trace, so I added the project file if someone wants to mess with it.
http://dl.dropbox.com/u/14312589/Power2.zip

[PRR]

> 6v at 370ma. so in series would be 12.6v @  740ma correct?

No. Why are you doubling the current? Double-Voltage covers two tubes. Same current in each.

You have two boxes to store. Each box is 6 foot tall and 0.37 foot wide.

You have a 15 foot ceiling. How much floor space needed? How much wasted height?

You stack two 6' boxes to 12'. Being stacked, the pile is still 0.37' wide. There's 3' of wasted height.

The series-stack of two 6V 0.37A tubes is 12V 0.37A.

In electronics, we can't just leave "empty height", so you need a 3' high by 0.37' wide "filler". That's 1.11 square feet. Our "electronic filler" is a resistor which comes in sizes of 1 2 3 5 10 (Watts), and no "cutting charge" for using 1.11W of a 2W or 3W part. So use 2W or 3W.

[craigmillard]

Ha ok, good analogy PRR! Dont know why i thought i needed to double the current!?!

So actually i dont need very big wattage resistors for this at all a couple of paralleled 1/2 watt would do it.

Thanks for the zip iccaros, im looking into the reg version too! I have the toner transfer in hand to make the main board up this weekend hopefully:)

A bit of etching on sat morn

[craigmillard]

Just an update:



Worked straight of the bat!

How could i test the current capability of it? What would you guys suggest for a load?? not sure how to do it!?

[iccaros]

for a simple test, you can take an EL84 or some power tube, build  a simple design around the datasheet for a SE amp.  and see where it idles at, you could then double up those. and put in a signal.  When your done, you would not only know what this is capable of, but you would have the start of a good tube tester.
In the real world we would use a calibrated load tester, but those are expensive and you want to make sure it will do your needs (3ma * which is 24 ma. But It would be cool to get 65ma..

Nice Work by the way, it look compact.
the power tube with a simple preamp, would also let you judge noise levels.

[craigmillard]

Cheers iccaros, I have some el84's kicking about but no se output transformer? Can it be tested without? I assume it's needed for the load??

[PRR]

> How could i test the current capability of it?

What current do you need?

Eight(!) 12AX7 tubes probably won't suck more than 16mA.

Other small tubes may be run a lot like typical audio 12AX7 stages, or maybe more; but there's little reason to go over 2mA per triode in audio modification stages.

Assume you can live with 320V at 16mA.

> What would you guys suggest for a load??

Resistor, of course.

320V/16mA= 20K.

320V*16mA= 320V*0.016A= 5.12 Watts.

You probably have half-watt resistors. You need ten (for short-term tests).

Ten 1/2W resistors some 1.8K some 2.2K in series is close-enuff 20K 5 Watts.

If you can hold over 300V with that load, you're cool (and the resistors will be hot).

[iccaros]

Cheers iccaros, I have some el84's kicking about but no se output transformer? Can it be tested without? I assume it's needed for the load??

Paul has a good plan, but if you go the tube route, a 5K resistor, note heat, would work but you can not test for noise that way.

As Paul wrote, giving a 20K load would work well, and then you can raise it  to see where you start dropping voltage.

[craigmillard]

Tested with the 20k load (Cheers Paul!) and seems to hold at the 320:) only thing is my digital multimeter picks up a small movement in the v's normally fluctuating around 3-5v?

Is this a problem, will it create noise? 

Iccaros, i want to test it with tubes but dont have a single ended transformer:( i think i will build a basic preamp at some point and try it and see:)

Also going to get some higher wattage resistors so i can test a higher loads!:)

[GFR]

Another idea for the filaments, "electronic transformers" for halogen lamps output 12V and are used to drive 50W lamps...

[iccaros]

Tested with the 20k load (Cheers Paul!) and seems to hold at the 320:) only thing is my digital multimeter picks up a small movement in the v's normally fluctuating around 3-5v?

Is this a problem, will it create noise? 

Iccaros, i want to test it with tubes but dont have a single ended transformer:( i think i will build a basic preamp at some point and try it and see:)

Also going to get some higher wattage resistors so i can test a higher loads!:)

3-5v could indicate noise, Like in this video of a 555 based SMPS, http://dl.dropbox.com/u/14312589/MVI_0256.AVI
I ended up replacing the output capacitor with one twice its size I think from 2uf to 4uf. But on another build, that did not help at all.

as for OT. I use a 70v line transformer for testing, you can use it for low wattage SE or as a Push pull. 
Since you are building a preamp, go ahead and build it to test with.. From expericnce you may have to lower the voltage  alittle and it will clean up, but even if you lose 20v I do not see that making a world of difference on the preamp.

[defaced]

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

The SLO only has 5 pre amp tubes.  If you're doing a preamp, you won't need a phase inverter, so you're down to four tubes.

[craigmillard]

Hi Guys,

Been a while but i have been busy beavering away at this and have a preamp up and running based on the SLO100:)
Sounds mean!! Although im getting crazy amounts of hiss!!! take a listen!

https://www.dropbox.com/s/qc5dy2gmh5qt9be/Hiss_Track1.mp3

What do you think? does that sound like a ground issue? I thought amplified resistor hiss but i have swapped to 2watt metal films for plates and .6watt metal films everywhere else?

The hiss is extremely loud when not playing!! I have removed both the first 2 tubes and the hiss is at an acceptable level so i think its within that area but as this amp has so much gain in the first section im not sure where to go?

Any ideas!?

[craigmillard]

Just a thought....

I have put the smothing caps for each stage on the power board instead of with each preamp section as per merlins grounding guide!

Do you guys think this will cause this much hiss?