Converting 9v pedal schematic to 18v model

[Guitarfreak]

If I had a schematic for a pedal build that ran in 9v and I wanted to run the pedal using 18v power source, what would have to be done to the component values in the pedal and which sections of the schematic?  I don't really have much experience with this stuff, but the experience that I DO have tells me that it's not going to be as simple as 2x values across the board.

[mth5044]

You don't double the values or anything like that...

All you really have to do is make sure that all of the components are rated for the higher voltage. Caps need be to higher than 18.. usually you can get them in 25V.. transistors, IC's, etc need to be checked for the max voltage they can take.

That's pretty much it. That's why people can take pedals like the OCD and run it on 18v without any changes.

[Guitarfreak]

The OCD is actually what gave me the idea haha.  I assumed that the pedal was designed in 18v and running it at 9v is like underpowering it.  I guess that makes sense though, thanks for the insight.

Which brings me to my next question.  Just how many volts would be needed to properly run a 12ay7 tube?  I don't want to design a crappy sounding starved plate setup or anything along the lines of that.  And how difficult would it be to design a schematic around one?

[anchovie]

The idea behind running a pedal at 18v rather than 9v is to increase headroom before distortion. With the OCD, the MOSFETs-as-diodes setup will provide clipping at either supply voltage but an 18v PSU will reduce the amount of clipping from hitting the rails of the op-amp in the gain stage.

If you look up a datasheet for a 12AY7 it will most likely give you 250V as a typical plate voltage. How difficult would it be to design a schematic around one? It depends what you want it to do! It's not much different to designing around a 12AX7 and there are plenty of schematics of amplifiers that use those for preamp stages.

[Guitarfreak]

The idea behind running a pedal at 18v rather than 9v is to increase headroom before distortion. With the OCD, the MOSFETs-as-diodes setup will provide clipping at either supply voltage but an 18v PSU will reduce the amount of clipping from hitting the rails of the op-amp in the gain stage.

If you look up a datasheet for a 12AY7 it will most likely give you 250V as a typical plate voltage. How difficult would it be to design a schematic around one? It depends what you want it to do! It's not much different to designing around a 12AX7 and there are plenty of schematics of amplifiers that use those for preamp stages.

Didn't understand what you said about the clipping reaching the rails of the op-amp, but it sounded cool anyway.

Since the tube wants to see 250v, is it safe to say that within the realm of a stompbox and not a rack unit or amp that it is an unrealistic goal?  I actually looked up a PDF file on the tube and it said something along the lines of it runs on 12v.  I had high hopes for about 30 seconds.  The main reason that I am asking is that I thought it would be cool to build a pedal that was meant to go in front of a clean channel to not give distortion, but only a fatter clean sound without any EQing.  I thought that pushing a low gain tube to saturation before the amp would be a cool way to accomplish this.  I am booked right now, but in a month or so my schedule will be free for the most part, so any help in doing this would be greatly appreciated.

[PRR]

> it runs on 12v

Uh.....

A cold tube is a dead tube. We have to heat-up a Cathode.

It is convenient to do this with a low voltage, often 6V or 12V.

Now that it is hot, we can suck electrons from Cathode to Plate and induce an output signal.

It is NOT easy to suck electrons through bare vacuum. And the more electrons loose in there, the less they want to flow.

It is possible to "run" these tubes on voltages of 10V and less. The available output at low voltage is VERY small. That might be OK, since guitar-cord work is all small stuff. But the really frustrating thing is that the tube materials were selected assuming 50V-300V. When working with 10V on output and maybe 1V at input, stray "contact potential" makes it very hard to find a workable operating condition. Also these small-volt issues give a different "overload tone" than the same tube working at more "normal" voltages.

Finding "good tube tone" is more a lifetime search than something you can slot onto a calendar.

[Guitarfreak]

Finding "good tube tone" is more a lifetime search than something you can slot onto a calendar.

That's alright.  I've spent the last year or so getting a rig together and testing my mettle in the recording world getting good recorded tones, might as well move on to building pedals right? 

With a wall current of 120v, I would need a transformer to get the voltage up to ~240v right?  Would this be possible in a power supply unit?  I am down for experimentation, I just need some guidelines so I don't end up killing myself, or worse... ending up with bad tone 

[PRR]

> I would need ...  Would this be possible ... some guidelines so ...

Merlin's site may be a good place to start.

http://www.freewebs.com/valvewizard/

[merlinb]

With a wall current of 120v, I would need a transformer to get the voltage up to ~240v right?  Would this be possible in a power supply unit? 

Perhaps the easiest way to start fooling around with high voltage tube pedals is the Real McTube.
http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm

All you need is a 12Vac wall wart, which provides heater voltage. You also feed into a 'backwards' 12V transformer (it can be very small, say 3VA) and rectify to get about 250Vdc or 120Vdc depending on the transformer you use.

The supply is not super high voltage, and fairly high impedance, so it won't kill you (providing you don't have a pace maker...). You're then free to mess about all you like with a couple of preamp valves. Under such conditions, they're extremely difficult to damage!

[Guitarfreak]

Thanks for the support.  Looks like I've got some homework to do for this project.

[petemoore]

  Tubes...the schematics look easy and simple when the power supply is assumed.
  Assuming power supply is fine, assume that this is the circuit section with the 'higher hurdles'...not too bad with small signal stuff though, since the current requirements can allow small-ish transformer to be sufficient.