Modding a charge pump from 18V to 12V output

[MarkL]

Can anybody suggest a way to modify this little daughterboard so that it puts out 12V instead of the 18V?  I suspect it may just be a latter of choosing different diodes (and possibly doubling them up), but I'm a novice to futzing with this sort of thing, and don't want to make a really bad mistake!

I'm having trouble uploading a picture, so here's the URL link to the vero layout I'm hoping to modify:

http://4.bp.blogspot.com/-SHcE0P5tEao/U9rvGU589eI/AAAAAAAAIX0/hO5n62dQ2cs/s1600/Toneczar+Openhaur+charge+pump.png

Of course, if anybody has an existing vero layout they can share for a 9V to 12V charge pump, that would be great too.

Thanks in advance.

[Scruffie]

Adding a 78L12 regulator from the 18V output is probably the simplest most stable option.

[MarkL]

So, take the18V lead, feed it to the input pin on the regulator, take a wire from the output pin and feed it to the circuit, and then ground the GND pin?  Dumb question (the pinouts seem fairly intuitive) but I'd rather sound dumb than make a dumb error.

Note -- I'm doing this to try to run a Valvecaster circuit at 12V.  It just occurred to me...do I even need the 18V charge pump, or can I feed the 78L12 regulator the regular 9V source...that is, if I power the 78L12 from a 9V source, will it still put out 12V to feed the circuit?

[dbp512]

The 78L** series has an internal voltage drop of about 2V, meaning you need to supply a 12V regulator with at least 14V to work properly. I don't know if theres any easier way to power the circuit or if a charge pump and regulator is still the best.

[MarkL]

Okay...so if I understand correctly, then something like this would work:

1.  9V power supply to an 18V charge pump.
2.  18V out from the charge pump to the IN of the regulator; GND pin of the regulator grounded.
3. the OUT pin of the regular will provide 12V of power, which can then go to the circuit.

If I have that right (I hope I do!) would it be advisable to insert some power filtering caps between the regulator and the Power IN of the circuit, or should the power filtering be done before the current hits the regulator (or, for that matter, the charge pump)?

[EBRAddict]

Refer to the datasheet for the correct filtering capacitors for the regulator. Usually it's 220nF or 330nF in and 100nF out, but check the datasheet for your brand.

[Scruffie]

Powering a tube is different, a charge pump wont have sufficient current to power the heater so if you want to use a 9V supply you'd have to wire the heater for 6V operation (300mA) from a 7806 regulator and then the rest of the circuit would be fed by the charge pump, in which case you may as well omit the regulator and feed it the 18V.

[midwayfair]

Just use the 12V supply for the valvecaster.

The current required to power the heaters will literally melt most 6V regulators. You need a regulator with a heat sink tab, and a BIG heat sink. The heat sink you'll need with be about an inch wide and and inch and a half tall. You don't want it close to your hot glowing tube because then it won't be able to dissipate the heat. You can't let it touch the case, so you need to make sure it's a big pedal.

But the voltage is only half the problem. 300mA of current isn't a small number. It's absurdly high. The One Spot can supply almost 2A of power, but even it won't be happy about giving up 300mA to a single pedal, and would likely introduce a lot of noise into the daisy chain (you know how it's a bad idea to daisy chain Strymon/Eventide/etc. stuff? same reason: high current draw). Your average power brick (like the PP+) can't even supply the current necessary to power a heater. If you're not daisy chaining it and you're not powering it from a power brick, then it's getting its own power supply anyway. So get a 12V supply dedicated to your valvecaster. You can get one for like $10 on ebay that's intended for running LED arrays, so it'll have plenty of current.

[MarkL]

So Jon, what if I wanted to run the valvecaster plates at 18V, but run the heaters at 12V (which one of the other forums suggests is a good option for better tone/headroom)...could I use the RoadRage for that (and modify the LM78L15 to a LM78L12)?  Or are you saying that even the Road Rage wouldn't work in this scenario for all the reasons you noted regarding charge pumps?

[Scruffie]

As I said above, you cannot feed a tube heater from a charge pump.

You really do need to get a 12V supply, i've powered from a 6V regulator and as Jon alludes to, unless it's the only option it's not worth the hassle.

[MarkL]

Alright...I will move on from the charge pump option.  So my question now is...what must I do to feed 12V to the heaters but 18V to the plates?

[Scruffie]

That's much easier, just wire the heaters directly to the 12V (follow the 12AU7 datasheet) and also run the 12V in to a LT1054 charge pump input which off the top of my head is okay up to 15V but check its datasheet to be certain and have its output feed in to the plate resistors, that'll be 24V (minus losses so ~22V) so an extra bit of headroom over 18V.

[midwayfair]

Neither 18v nor 12v are full plate voltage, so why bother with the 18v which will still need the giant heat sink? The starved plate is basically the point of the design. If you want headroom you should really be looking at a different design, one with a real power supply that can give you at least 120v. (Grind customs has one and has instructions for safely building it.)

I'm not even totally sure you CAN dissipate the the 6V of waste at 300mA or more with any reasonable part.

I'm not trying to discourage experimentation, but it's a good idea if someone mentions an issue with a build (like current draw in this case) to understand the problem before suggesting a slightly different mod that will just recreate the problem under different circumstances. Spend a bit of time reading about Ohm's law and the power dissipation formula, and in the meantime you can breadboard the design as it exists and see if you don't like it or easily try other plate voltages.

[MarkL]

So Jon, since I am interested in using this primarily as an overdrive -- powering the entire circuit at 12V will work?  I want to avoid the problems people report with running it at 9V, which is why I thought 18V would provide better clarity and punch (while still yielding good drive characteristics).  But if I can get that with hitting both the plates and the heaters at 12V and avoiding having to use the heat sink...

[Scruffie]

If you're using a 12V DC Supply for the heaters you do not need a heat sync for anything, a charge pump will happily power the plates without getting warm.

A 12AU7 doesn't need full plate voltage to sound good, they're much better at lower voltages than a 12AX7 so 12V and 18V will make some difference or 22V as it would be with using a 12V supply in to the charge pump.

Neither 18v nor 12v are full plate voltage, so why bother with the 18v which will still need the giant heat sink? The starved plate is basically the point of the design. If you want headroom you should really be looking at a different design, one with a real power supply that can give you at least 120v. (Grind customs has one and has instructions for safely building it.)

I'm not even totally sure you CAN dissipate the the 6V of waste at 300mA or more with any reasonable part.

Who said anything about wasting 6V at 300mA? Even his original plan was to waste 3V at 300mA which is perfectly doable really, the 7806 is fine up to 1A but of course does need heat syncing.