What transformer could be used to power just a preamp?

[TheKT88KilledJFK]

Right now, I have built a couple of different guitar preamps but I am just using leftover transformers for actual amps.  Obviously I don't need 300mA for just the preamp tubes.  By my calculations, I'd assume 20mA would be plenty for 4 to 5 12ax7's, not including the heater filaments. I'd also like to keep around 400vdc over the rail for the tube, so a transformer like 360-0-360 is what i;d be looking for yet the only transformers I find at this voltage are really meant to power the power tubes as well and the cost reflects this.  I am hoping to find a preamp power transformer under $50 instead of putting a behemoth into my 1u preamps. 

Any ideas?

edit:

Also, since I just realized these transformers aren't getting heavily loaded, I should probably shoot for a voltage closer to the 400VDC, so a 285-0-285 transformer would probably be better but the point remains.

[labb]

Take a look at some of the Toroidal transformers..Antek Inc. comes to mind...Check the AS-05T320 or the AS-1T350


You might like to see this fellow's work:   http://www.frontiernet.net/~jff/SonOfAlembic/SonOfAlembicF2B.html

[drgonzonm]

check the hammond website for transformers.  There are several lower ma rated P/Ts that you might want to use. 

If you are willing to use a filament transformer and high voltage transformer, consider using an industrial 480v/120v lighting transformers.  Other somewhat common voltages include 385/120 (DIN), 277/120.  then select a filament transformer. 

I can thank Franco for this approach. 

[ernie_jr]

http://www.tedweber.com/wrvbpt




try this
Ernie

[Paul1453]

Another idea that may appeal to you is this.

A 12VDC laptop PS brick can power the heaters and this board.

Pros are low cost, small, and light.

Cons - some PS bricks, especially ones without a 3 prong plug, can create a horrible buzz.
I have some I pulled from the trash that are absolutely silent, though.   

[TheKT88KilledJFK]

Take a look at some of the Toroidal transformers..Antek Inc. comes to mind...Check the AS-05T320 or the AS-1T350


You might like to see this fellow's work:   http://www.frontiernet.net/~jff/SonOfAlembic/SonOfAlembicF2B.html

Oh wow that'll do it for sure.  I'll try out that 350 just for the head room.

[HotBluePlates]

The best approach is asking how much signal voltage/current must the preamp output? Then design the preamp back-to-front to deliver that output at/under whatever amount of distortion you think is acceptable.

Make a survey of available power transformers, noting Volts RMS and current capacity. Decide a rough power supply approach, as it will help you figure out how much B+ voltage you're likely to get from each available PT.

Once you do that, the tubes you choose to use will tend to imply how much supply voltage will be required. You design each stage, starting with the output stage. You figure out the output swing possible while staying at/under your target distortion level, and you'll find out whether the anticipated supply voltage is high enough for that stage. Doing this will also tell you how much drive signal that final stage will need to hit its output swing target.

Rinse & repeat for each earlier stage. The next earlier stage has to be able to swing a signal at least as big as your calculated drive for the final stage, plus any extra signal to make up for losses between stages. Also important because you can't accurately know how this stage will perform until you know everything about its load, which amounts to any circuitry between this stage & the final stage, as well as what the final stage's input circuit looks like.

You can build a preamp (or any amp) without designing in this way, by assuming higher voltage equals bigger, cleaner output. However, you may wind up paying for bigger/heavier/costly transformers, higher voltage caps, etc you didn't need to get the job done.

[HotBluePlates]

Different approach:

120v:240v transformers ought to be pretty common (it's a standard 1:2 step-up transformer). 240v RMS * 1.414 = 339vdc minus diode drop. You could use solid-state rectifier and get no less than 330vdc at your 1st filter cap.

How much current depends on the preamp particulars. Maybe as low as 8mA, maybe as high as 40-50mA. It probably depends heavily on the tube choice, output stage configuration and how much cable capacitance you expect to drive.

[kagliostro]

For preamp purpose (on U1 rack) the best I was able to find (price, space, current) was this

http://www.musikding.de/Toroidal-transformer-230V-270V-14V

(I've 3 waiting to be used in some U1 rack I've)

unfortunately the primary is 230v and I suppose you have 120v line

Franco

[PRR]

Triode Electronics had the Dynaco preamp PT good for four 12AX7 with DC heat.

http://www.triodeelectronics.com/dytr.html
PA211 Power Transformer Dyna PAS pa211 $34.95
PA522 120/240V Power Transformer for Dyna PAS PA522 $44.95

[TheKT88KilledJFK]

The best approach is asking how much signal voltage/current must the preamp output? Then design the preamp back-to-front to deliver that output at/under whatever amount of distortion you think is acceptable.

The preamp is just going to  be fed into a mesa 400+. 

After the last cathode follower, the output signal voltage is still around 100v+ in the simulation, which I suppose actually makes sense. I didn't realize that the effects loop signal level could be such a high voltage.  I need to step that down to something around a volt haha. 

[tubeswell]

Given that most pre-amps are dual triodes (and occasionally small signal pentodes, or even more rarely, some triode/pentode combos), then any little old PT should do the trick, with the main limiting factor being the heater juice for the pre-amp tube filaments.


So, a Tweed Champ PT (that is good for 50-60mA on the HT, and ~2A on the filaments) will be sufficient for virtually any type of typical guitar pre-amp with up to 6 dual triodes, with say SS rectification (or a Tweed Deluxe PT with SS rectification if you want a stiffer supply). You can use the 5V winding for supplying a fixed bias rail down to -7V (or lower if you use a -ve voltage doubler)


Even a little Hammond PT for a Fender stand alone reverb tank (with a maximum heater draw of 1.05A) will work - if you only want to heat up to 3 dual triodes.


If you want relay/opto switching capability or a separate fixed bias supply, or ability to run +/- power rails for BJT, JFet or MosFET etc, then you need something with extra secondary windings

[sluckey]

The preamp is just going to  be fed into a mesa 400+. 

After the last cathode follower, the output signal voltage is still around 100v+ in the simulation, which I suppose actually makes sense. I didn't realize that the effects loop signal level could be such a high voltage.  I need to step that down to something around a volt haha.

Maybe you don't really need to pump it up that high to begin with!

[PRR]

> don't really need to pump it up that high

But if 1V is good, isn't 100V a hundred times better?

[TheKT88KilledJFK]

> don't really need to pump it up that high

But if 1V is good, isn't 100V a hundred times better?

Well you know what they say about if it's too loud...

The preamp is just going to  be fed into a mesa 400+. 

After the last cathode follower, the output signal voltage is still around 100v+ in the simulation, which I suppose actually makes sense. I didn't realize that the effects loop signal level could be such a high voltage.  I need to step that down to something around a volt haha.

Maybe you don't really need to pump it up that high to begin with!

When looking at the characteristic diagrams, it appears that the only way to keep the tubes at the same Q point while maintaining the same amount of gain was to run a pretty high plate voltage.  We are trying to achieve the same distortion characteristic of dual rec, but for bass.  There were lots of other changes made but the actual clipping characteristics will be maintained as that's the whole point of the preamp. 

[labb]

Just for info: the 50VA Antek Toroidal  transformer family will fit inside a 1U enclosure. Height is 1.6"

[vibrolax]

Just for info: the 50VA Antek Toroidal  transformer family will fit inside a 1U enclosure. Height is 1.6"

Not trying to be a nit-picker, but whether the 50VA Antek transformer will fit in a 1U enclosure very much depends on which enclosure you choose.
It does "fit" in the Par-Metal 1U enclosure, but only if you omit the rubber mounting pads and supplied retaining bolt and hub.
There are other 1U enclosures with less vertical space than Par-Metal's.

I hold them in place in my builds by dropping them over a ~1" high slice of ~1.125" delrin rod screwed to the enclosure bottom.
The enclosure top cover will pretty much rest on the top of the toroid.

Do not run a retaining bolt through the toroid between the top and bottom of the chassis.  This will create a "shorted-turn", and things will get hot.

I consider using these toroids in a 1U box a mechanical hack that barely works.  But I like the price and quality.

[kagliostro]

AnTek transformers are very cool (and cheap)

I really would like to find it available on this side of the pond

to get it from the USA is a large amount of customs fee and shipping

Franco

[Ritchie200]

Do you really need to fit all this in a 1U space?  This means your preamp tubes will be cramped horizontal which may limit accessibility - obviously depending on your intended layout.  Have you thought about using Dougs Stereo Preamp project for inspiration?  He even sells the tranny for $52.28, which only blew your intended budget by a couple of bucks.  If you can make it a little taller you will also have room to work and play while you tweak it to taste and for maintenance.  Just a thought.

Jim

http://el34world.com/projects/StereoPreAmp1.htm

[MakerDP]

I recently used an Antek in a stand-alone preamp build. It was perfect. Voltages came out almost exactly to what was spec'd and I paid like $28 for it shipped.

Another good choice is the P-TF47609 which is the replacement tranny for a Fender stand-alone reverb unit.