PAia 12ax7 valve circuit advice needed

[Ice-9]

Hi, I'm looking into building a valve distortion pedal which i intend to build more or less to get the sounds i want, so it will be an ongoing design as far as the actual effect circuit is concerened. So i have been checking out as many different schematics as i could find. (just for ideas etc)

The question i need to address first is psu. I intend to use an AC adapter to power the unit but want to be able to use any adapter between 12v and 20v.

I have breadboarded the psu in the circuit linked below with good results but as i want to be able to use 12- 20 volt adapter then i will need to add voltage regulators to keep the outputs at +/-15v.

http://www.paia.com/prodimages/siabsch.pdf

Will there be any disadvantage to adding 7815 and 7915 regulators to this circuit apart from the obvious one of having to sort out the valve  heater voltage. (as it is taken directly from the 12v ac adapter in the schematic)

And my second psu question applies to the B+ . Would it be an advantage to get more volts to the B+ than the 45v this circuit has.

[Mike Burgundy]

As far as electronics is concerned, no problem at all.
There's a few catches: you'll need at least 17.7 V going into a 7815 to get a nice clean 15V. Rectifying 12VAC will get you *close* to that, but not really there.
You could get away fine with 12V regulators and an accordingly lower tube plate voltage (Tube Driver and others work on 12V or even 9V) but it might (probably will, slightly) alter the sound, or accept that a 12V adapter won't do.
Also, it's a good idea to keep the filaments separate, so with a different AC they will need their own supply and regulator. 7812 should do fine.

Changing/increasing plate voltage will alter sound and response from the tube, the original Stack in a Box article goes into the hows and whys for a bit. It's almost like it's excaggerating tube characteristics. Search for "starved plate" here and on google to learn more. On the one hand they were designed to run much, much higher, on the other....

That said, if you don't really *have* to have it (such as when youre forced to run it off a driving car's dynamo when it's actually in traffic, erm... never mind) I'd say stick with one and be done with it. Any particular reason you want the ability to vary the AC adapter?

It is great fun to tinker with starved tube circuits. There's lots of info here, and on the net...
hih

[Ice-9]

Thanks for that Mike,

The effect circuit will be custom designed to add whatever controls and extras that i want to add to it but i was just working with that schmatic for the psu part. The reason i want it to work with a range of ac adapters is that the only one i have at the minute is an 16v ac one from my blackstar HT dist, and when i breadboarded the the psu i noticed that the split rails from 16v is giving me +/- 27v (way to high for the 4049 hex inverter as the datasheet says max  + / - 18v.  I don't want the scenario at a gig that my one and only correct ac adapter is lost / broke etc, but i do have a spare 20v one (BANG). I want this pedal to be bullet proof so to speak or at least to a certain extent idiot proof.

I think i might follow the idea of the +/- 12v regulators for the split rail as you suggest. Once i'm happy with the psu for this project i will only then start on the signal part of the design. This one i'm really looing forward to doing as it will be the first vavle preamp i will have done.

Now to do some research on starved plate.