Voltage divider power supply?

[thermionix]

Unwind the secondary until you get into the 9V range..

That's not a bad idea, but would take a lot of trial and error, if I could even get the endbells off in the first place, AND assuming that the primary wind isn't on top of (or interleaved with) the secondary.  I've done that with a hot humbucker before, just using DC resistance as a rough guide.  I think I went a bit too far, not really considering that it was likely wound with 43AWG instead of 42.  I ended up giving it to a buddy of mine and he thinks it sounds cool.  Probably a good neck 'bucker.

[thermionix]

Would make a great benchtop supply I bet, tho!!

This comment has been bouncing around in my skull (like the mythical .22LR slug) for a couple days now.  If you were to build a benchtop power supply, ideally it would be adjustable, right?  For pedal work, you'd probably want a range of about 9-18VDC, right?  How would you go about accomplishing that?  Some kind of rheostat setup on the primary maybe?

[maiko]

Would make a great benchtop supply I bet, tho!!

This comment has been bouncing around in my skull (like the mythical .22LR slug) for a couple days now.  If you were to build a benchtop power supply, ideally it would be adjustable, right?  For pedal work, you'd probably want a range of about 9-18VDC, right?  How would you go about accomplishing that?  Some kind of rheostat setup on the primary maybe?

An lm350 Avr ic with a pot.  say 2-5k range.   oh and on the secondary side.  youll need a rectifier before it.   

[antonis]

For a well stabilized benchtop PS you will need more than maiko proposed...


P.S.
Plz Thermionix Plz -  Plz Thermionix Plz - Plz Thermionix Plz :

WE DON'T ADD ANY RHEOSTAT (or anything similar touchable device) ON THE PRIMARY SIDE..!!!

[GibsonGM]

Never never never never anything on the primary side other than a fuse in the hot wire and a terminal switch!   Everything of the primary must be isolated, sealed up, no contact, do not touch even with another object...


A benchtop power supply could be made using an LM317 adjustable voltage regulator on the secondary side.  It would need to be heatsink-mounted and isolated from the chassis.  You would develop heat; in a larger enclosure this may not be a big issue.  OR, you may need a dropping resistor and have 2 settings...one for 9 to 12V, one for 12-15V, some such thing.  Design question: is the transformer/PS beefy enough to make the effort worthwhile, or will it burn up soon if you make some error, cause a short?

If you're not comfortable with these ideas...if they are not things you've worked with before, you might want to take it slow and figure out the consequences of dropping 18V to 9V (or below) with a regulator.    It is safe to do, but you need to understand what's happening in terms of heating.   There are projects out there that do this in a well-explained tutorial manner, you might want to go look for them and study a bit!    You COULD light something on fire if you weren't careful....

[thermionix]

I was just thinking about light dimmers when I said rheostat.  I know a variac would do it, although rather large, heavy, and expensive, lol.  And there would be nothing controlled about the output voltage.

I'm comfortable with "simple" power supplies, transformer > rectifier > filtering.  I build and repair tube amps.  The solid state stuff, like voltage regulators, is still alien technology to me.

I'm back to chucking this transformer aside.  The "benchtop power supply" comment just made me think there was maybe a possibility I hadn't yet considered.

[GibsonGM]

Well, there is, like I mentioned.  You'd have to grab an LM317 regulator and read the design notes at the end of the data sheet.  It shows you how to connect it....if you were doing 15V, and plug in 18V - dropping like 3V....no problem.   

As you wish to drop more, tho, you will need to 'put it somewhere', and that dissipates heat.  So you'd get a whopper heat sink and mount the thing to it.   Some designs even switch in some honking resistors to dump some before the regulator.

It's not a big deal, if you have a junkbox full o'crud, you can whip that out in a couple hours.  As long as you know where the trouble lies and how to overcome it.   

There was a radio shack design (back when they put out books) for a very similar benchtop supply, LOL...I think the trafo was 25V, but the concept is similar and that would only limit your upper voltage.   Probably right on the internet, "18V variable power supply using LM317"....

Here is one such discussion:  http://www.electronics-tutorials.ws/blog/variable-voltage-power-supply.html

If you don't have a benchtop supply, it's a good project if you're so inclined.

[antonis]

I build and repair tube amps.  The solid state stuff, like voltage regulators, is still alien technology to me.

No intention for irony or insult, honestly, but HOW do you make repairs without a benchtop (or portable) power supply..??

I thought that the FIRST step in any mains powered device was the after stepped down DC circuit "isolation" for testing..

[bool]

...

I'm comfortable with "simple" power supplies, transformer > rectifier > filtering.  I build and repair tube amps.  The solid state stuff, like voltage regulators, is still alien technology to me.

...

That's why I mentioned the simple zener regulator (with a 5W zener; 1N5346). That should be "simple" enough and if made well; reliable in long-term.

[thermionix]

I build and repair tube amps.  The solid state stuff, like voltage regulators, is still alien technology to me.

No intention for irony or insult, honestly, but HOW do you make repairs without a benchtop (or portable) power supply..??

I thought that the FIRST step in any mains powered device was the after stepped down DC circuit "isolation" for testing..

The amps have their own power supplies, why use a seperate one?

[antonis]

The amps have their own power supplies, why use a seperate one?

For not working with an open enclosure mains wires - in case of compact amp..

For not carrying to your bench an extra load - in case of separate PS..

For establishment of good PS and pass to rest of the circuit troubleshooting..

For implementation of Good Laboratory Practice..

[GibsonGM]

You should have a bench power supply just to be cool, and to test things you make.  Perhaps to isolate a section of some equipment you're working on, and powering that alone...or to run a fan while you work, maybe a radio.  To power a piece of test equipment, maybe...

I've NEVER used a separate power supply to diagnose/repair a tube amp.  I see no reason why some people would not, that is up to them, but it's never been anything I would NEED to do the job.    If one was needed, I'm picturing something that has several windings, can put out high voltage, with a separate rectifier and filters...you COULD power heaters off a benchtop supply, I s'pose.   

I was talking about 9-18V variable for pedal work, or other circuits in the realm of 9V.  Batteries suck to change all the time.   

[thermionix]

For not working with an open enclosure mains wires - in case of compact amp..

The mains wires aren't the scary ones in a tube amp.

For not carrying to your bench an extra load - in case of separate PS..

Never encountered a tube amp with a separate PS (not claiming they don't exist)

For establishment of good PS and pass to rest of the circuit troubleshooting..

If the amp's PS isn't good, I fix it.  Troubleshooting usually takes minutes.  At least half of the problems are found just by looking!

For implementation of Good Laboratory Practice..

Laboratory?  It's rock-n-roll, man!

[Les Turnbull]

I'm using a Xbox 360 power brick which was coppers from a carboot sale , 12v out into a little home made break out box with just one 7809 9v regulator inside , Gives me 9v 1.5 amp , runs my 7 pedals with over a amp to spare . Total cost under 4 quid .
edit - Forgot the daisey chain I purchased so total cost around £6 .

[GibsonGM]

I'm using a Xbox 360 power brick which was coppers from a carboot sale , 12v out into a little home made break out box with just one 7809 9v regulator inside , Gives me 9v 1.5 amp , runs my 7 pedals with over a amp to spare . Total cost under 4 quid .

That is a good one.    Some people take an old PC power supply and figure out which wires are 3.3v, 5v, 12V.....put them on a rotary switch.  Add a power switch, appropriate fuse if there isn't one, and isolated banana jacks and off you go.

It is just useful to have a variable (or stepped...) power supply around.  Least, to me it is! 

Example: I took my 0-30V 3A supply down in a crawlspace once, used it to power up a mini car tire air compressor...pressurized a hyrdronic heating line I had repaired to test for leaks.  I put a schrader valve in one of the bleeders.   Prevented my having to fill it with water and then drain it again if it did in fact leak.       

[antonis]

You forgot the - all time classic - battery charging, Sir... 

[EBK]

There was a radio shack design (back when they put out books) for a very similar benchtop supply, LOL...I think the trafo was 25V, but the concept is similar and that would only limit your upper voltage.   Probably right on the internet, "18V variable power supply using LM317"....

Are you talking about this one (from Radio Shack's book Building Power Supplies, published in 1991)?

[GibsonGM]

EBK - Yes!!  I have that little book somewhere, ha ha.  Good circuit to build.   Easily scaleable to a smaller transformer.   

You forgot the - all time classic - battery charging, Sir... 

Of course! If you limit the current properly, because just connecting it might cause the battery to explode, no?   I do that outside, with a real charger...

[antonis]

Of course! If you limit the current properly, because just connecting it might cause the battery to explode, no?

But of course, Sir...!!

A power supply without current limiting adjustment doesn't worth to be named "Benchtop PS"... 

[GibsonGM]

Of course! If you limit the current properly, because just connecting it might cause the battery to explode, no?

But of course, Sir...!!

A power supply without current limiting adjustment doesn't worth to be named "Benchtop PS"... 

In the Northern Realm here, we use a fuse to limit current being drawn from MY benchtop power supply     I have no other special device...although mine will provide no more than 3A, so it IS limited....but 3A will probably light on fire almost any circuit that I make for audio, ha ha....