Running a treble booster on 12v

[Threefish]

HI all,

I'm a bit of a noob, so be easy on me.
I've built a mini-pentode preamp/chip (LM1875) power amp combo for practice, and while I like the sound, it could do with a bit more treble. All cap mods I can figure out to the preamp have been tried, with me now falling back on a smaller value cap in my AMZ booster to give me a bit more jingle. I'd like to wire in something permanent in my combo cabinet, and the preamp runs on 12v, so I'm thinking a screaming bird/Vox treble boost type thing on 12v will be ideal, with just a bit of gain, set fairly low by a trimpot - I'd like to keep the sound very clean. I also have 36v and 5v available to me through the preamp, but I thought they would be too high and too low respectively. So, mentors - what do you think? What mods would i have to do to the SB/VTB (I'm still trying to understand transistor biasing), or should I try something like a Fetzer valve modded for more treble?

[nelson]

You could regulate the 12v to 9v using a voltage regulator. Having the treble boost circuit foot switchable would be a good idea. I havent heard good things about the screaming bird, so I would breadboard a couple of circuits and try them in the preamp, before commiting to a final design.

I am a bit of a newb too, but I think that would work.

[Threefish]

Thanks Nelson, I was thinking about the same thing last night. Everything I'd read about the Screaming Bird was fairly negative, but as the difference between it and the LPB1 is just in the input and output cap values, I thought I could get a balance between the two. I put together a 12v-9v regulator circuit with a couple of caps last night (off of the LM7809 data sheet) and at least now I have 9v to work with. Emailed ROG and got a reply about how I can mod the Fetzer valve :
"You can reduce the output cap value to 10n or so to start.  Too small of a value will begin to reduce the output level. Or, you could use a  
smaller source bypass cap, like 4.7 or 10uF, to tighten up the bass.  
Finally, using an MPF102 will provide a brighter tone than a J201, at the expense of gain."
Thanks again to ROG.
So I'll have a go at running the Fetzer at 9v and 12v, see if there is any difference for how I want to use it.
I'll post my results.

[aron]

I think you could run it off of 12VDC without any problems. You can try it and if the biasing is off, adjust the biasing resistor.

[Steben]

I think you could run it off of 12VDC without any problems. You can try it and if the biasing is off, adjust the biasing resistor.

I concur

[Threefish]

Thank you all for your input.  
I ended up just using the 12v supply with everything else stock (except for the 2n5088 - couldn't get one, so used a 2n5089), and it didn't work that great. The sound was quiet and really broken up, and I noticed that as I turned off the power, in the milliseconds between background noise and silence, the sound level rose suddenly. It sounded like the trannie was completely overloaded or something.
I'm sure someone could give me an explanation for this, but I'm not that concerned, because :
Before starting to fiddle with biasing, I tried a few other transistors, and got a great sound with a 2n3904 in the socket. As much gain as I wanted, and good volume. The others I tried were a 2n2222, and a BC109, probably neither that suitable, but I had them both lying around. Got sound with both, but very low volume.
I've got the setup now as input-pentodepreamp-pot-SB-pot-chipamp. The pot following the pentode varies the volume going into the Screaming Bird, and the pot following the SB varies the volume into the chipamp. I've also got a toggle switch on the panel to change the Screaming Bird output cap from .0022 to .0047. Not much difference, but enough to get a bit of overdrive with the .0047, and hardly any with the .0022.
I'm now really happy with my amp and it's sound.
In case you're wondering, I got the pentode preamp as a kit from Oatley Electronics (www.oatleyelectronics.com) for about AU$33. It's for general audio (hifi?) and doesn't really have the treble boosting qualities that guitar preamps have, hence the treble boost circuit I've grafted on. It doesn't have a lot of gain (according to their instructions, about a gain factor of 8, and that's not a typo), but it has a nice sound to it.
It would probably make a good input stage for an overdrive stompbox. I'm thinking I might give that a shot sometime in the future.

[Threefish]

Sorry everyone, I just realised that in my previous post I was going on about using a Fetzer valve circuit. I actually ended up using a Screaming Bird circuit for more treble.
Sorry for my confusing ramble above.  :lol: